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Planet Earth/3d. Radiometric dating: Using chemistry to tell time

Radiometric dating to determine how old something is – the hour glass analogy
The radioactive decay of isotopes and use of excited electron energy states have come to dominate how we tell time from the quartz crystals in your wrist watch and computer, to atomic clocks onboard satellites in space. Measuring radioactive isotopes and electron energy states is the major way we tell time in the modern age. It also enables scientists to determine the age of an old manuscript a few thousand years old, as well as uncovering the age of the Earth itself at 4.6 billion years old. Radioactive decay of isotopes has revolutionized how we measure time, from milliseconds up to billions of years, but how is this done?

First, imagine an hour glass filled with sand which drops from two glass filled spheres connected by a narrow tube. When turned over, sand from the top portion of the hour glass will fall down to the bottom. This rate of sand falling is a linear rate, which means only sand positioned near the opening between the glass spheres will fall. Over time the ratio of sand in the top and bottom of the hour glass will change, so that after 1 hour all the sand will have fallen to the bottom. Note that an hour glass cannot be used to measure years, nor can it be used to measure milliseconds, since in the case of years, all the sand will have fallen, and in measuring milliseconds, not enough sand would have fallen in that short length of time. This ratio is measured by determining the amount of sand in the top of the hour glass, and the amount of sand in the bottom of the hour glass. In chemistry dealing with radioactive decay, we call the top sand the parent element and the bottom sand the daughter element of decay.

Radiometric dating to determine how old something is – the microwave popcorn analogy
Radiometric decay does not work like an hour glass, since each atom has the same probability to decay, whereas in an hour glass, only the sand near the opening will fall. So a better analogy than an hour glass is to think about popcorn, in particular microwave popcorn. A bag of popcorn will have a ratio of kernels to popped corn, such that the longer the bag is in the microwave oven, the more popped corn will be in the bag. You can determine how long the bag was cooked by measuring the ratio of kernels and popped corn. If most of the bag is still kernels, the bag was not cooked long enough, while if most of the bag is popped corn, then it was cooked for a longer time.

The point in which half of the kernels have popped is referred to as half-life. Half-life is the time it takes for half of the parent atoms to decay to the daughter atoms. After 1 half-life the ratio of parent to daughter will be 0.5, after 2 half-lives, the ratio of parent to daughter will be 0.25, after 3 half-lives the ratio will be 0.125, and so on. Each half-life the amount of parent atoms is halved. In a bag of popcorn, if the half-life is 2 minutes, you will have half un-popped kernels and half popped popcorn, and after 4 minutes the ratio will be 25% kernels and 75% popcorn, after 6 minutes, only 12.5% of the kernels will remain. Each 2 minutes the number of kernels will be reduced by one half.

You can leave the bag in the microwave longer, but the amount of kernels will drop by only half for each additional minute, and likely burn the popcorn, leaving a few kernels still left un-popped. Radiometric dating works the same way.

What can you date?
The first thing to consider in dating Earth materials is what precisely you are actually dating. There are four basics moments that determine the start of the clock in measuring the age of Earth materials:

 * 1) A phase transition from a liquid to a solid, such as the moment liquid lava or magma cools into a solid rock or crystal.
 * 2) The death of a biological organism, the moment an organism (plant or animal) stops taking in new carbon atoms from the atmosphere or food sources.
 * 3) The burial of an artifact or rock, and how long it has remained in the ground.
 * 4) The exhumation of an artifact or rock, so how long it has been exposed to sunlight.

Radiocarbon dating or C-14 dating
There are two stable isotopes of carbon (carbon-12 and carbon-13), and one radioactive isotope of carbon (Carbon-14), a radioactive carbon with 6 protons and 8 neutrons. Carbon-14 decays, while carbon-12 and carbon-13 are stable and do not decay. The decay of carbon-14 to nitrogen-14 involves the loss of a proton. For any sample of carbon-14 half of the atoms will decay to nitrogen-14 in 5,730 years. This is the half-life, which is when half of the atoms in a sample have decayed. This means carbon-14 dating works well with materials that are between 500 to about 25,000 years old.

Radiocarbon dating was first developed in the 1940s, and pioneered by Willard Libby, who had worked on the Manhattan Project in developing the atomic bomb during World War II. After the war, Libby worked at the University of Chicago developing carbon radiometric dating, for which he won the Nobel Prize in Chemistry in 1960. The science of radiocarbon dating has been around for a long time!

Radiocarbon dating measures the amount of time since the death of a biological organism, the moment an organism (plant or animal) stopped taking in new carbon atoms from the atmosphere or food sources. It can only be used to date organic materials that contain carbon, such as wood, plants, un-fossilized bones, charcoal from fire pits, and other material derived from organic material. Since the half-life of carbon-14 is 5,730 years, this method is great for material that is only few hundred or thousand years old, with an upper limit of about 100,000 years. Radiocarbon dating is mostly used in archeology, particularly in dating materials during the Holocene Epoch, or the last 11,650 years. The first step is to collect a small piece of organic material to date, being very careful not to contaminate the sample with organic material, such as the oils on your own hands. The sample is typically wrapped in aluminum foil to prevent contamination. In the early days of radiometric dating before the 1980s, labs would count the decay in the sample measuring the radioactivity, the more radioactivity, the younger the material was. However, a new class of mass spectrometers were developed in the 1980s giving the ability to directly measure the atomic mass of atoms in these samples. The steps are complex, but yield a more precise estimate of age. The steps involve determining the amount of carbon-14, as well as the two stable types of carbon-13 and carbon-12. Since the amount will depend on the amount of material, scientists look at the ratio of carbon-14 to carbon-12, and carbon-13 to carbon-12. The higher the ratio of carbon-14 to carbon-12 the younger the material is, while the carbon-13 to carbon-12 ratio is used to make sure there is not an excess of carbon-12 in the first measurement, and provide a correction if there is.

One of the technical problems that needed to be overcome was that traditional mass spectrometers measure only the atomic mass of atoms, and carbon-14 has the same atomic mass as nitrogen-14. Nitrogen-14 is a very common component of the atmosphere, and air that surrounds us. And this is a problem for labs. In the 1980s a new method was developed called the Accelerated Mass Spectrometry method, which deals with this problem.

The first step of the process is to take your sample and combust the carbon in a stream of pure oxygen in a special furnace or react the organic carbon with copper oxide, both of which produces carbon dioxide, a gas. The gas of carbon dioxide (which is often cryogenically cleaned) is reacted with hydrogen at 550 to 650 degrees Celsius, with a cobalt catalyst, which produces pure carbon in the form of power graphite from the sample, and water. The graphite is held in a vacuum to prevent contamination from the nitrogen-14 in the air. The vacuumed graphite powder is then purged with ultra-pure argon gas to remove any lingering nitrogen-14 which would ruin any measurement, in a glass vial. This graphite, or pure carbon is ionized, adding electrons to the carbon and making it negatively charged. Any lingering nitrogen-14 will not be negatively charged in the process, because it has an additional positive charged proton. An accelerated mass spectrometer spins the negatively charged atoms passing them through the machine at high speeds as a beam. This beam will have carbon-14, but also ions of carbon-12 bonded to 2 hydrogen, as well as carbon-13 bonded to 1 hydrogen all of which have an atomic mass of 14. To get rid of these carbon atoms bonded with hydrogen, the beam of molecules and atoms with atomic mass of 14 is passed through a stripper that removes the hydrogen bonds, and then through a second magnet, resulting in a spread of atomic mass of carbon-12, carbon-13 and carbon-14 on the detector for each mass. The ratio of carbon-14/carbon-12 is calculated as well as the ratio of carbon-13/carbon-12 and compared to lab standards. The carbon-13/carbon-12 ratio is used to correct the ratio of carbon-14/carbon-12 in the lab and to see if there is an excess of carbon-12 in the sample, due to fractionation. To find the actual age in years, we need to find out the initial amount of carbon-14 that existed at the moment that the organism died. Now carbon-14 is made naturally in the atmosphere from nitrogen-14 in the air. In the stratosphere these atoms of nitrogen-14 are hit by cosmic rays from the sun, which bombards the nitrogen-14 with thermal neutrons, producing a carbon-14 and an extra proton, or a hydrogen atom. This process is dependent on the magnetic field from the Earth and solar energy, which vary slightly in each hemisphere, and when solar anomalies happen, such as solar flares. Using tree ring 14-carbon/12-carbon ratios, where we know the year of each tree ring, we can calibrate 14-carbon/12-carbon ratios to absolute years for the last 10,000 years.

There are two ways to report the age of materials dated this way: One is to apply these corrections, which is called the radiocarbon calendar age or you can report the raw date determined solely from the ratio, called Carbon-14 dates. Radiocarbon calendar ages will be more precise than simple carbon-14 dates, especially for older dates.

There is one fascinating thing about determining the initial 14-carbon/12-carbon ratios for materials during the last hundred years. Because of the detonation of atomic weapons in the 1940s and 1950s, the amount of 14-carbon increased in the atmosphere dramatically after World War II, as seen in tree ring data and measurements of isotopes of carbon in carbon-dioxide of the atmosphere.

This fact was used by neurologist studying brain cells, leading to the medical discovery that new brain cells are not formed after birth, as people born before the 1940s have lower levels of 14-carbon in their brain cells in old age, than brain cells of people born after the advent of the nuclear age, which have much higher levels of 14-carbon in their cells. However, over the past few decades, neuroscientists have found two brain regions, the olfactory bulbs (where you get the sense of smell) and the hippocampus (where the storage of memories happen) that do grow new neuron cells throughout life, but the majority of your brain is composed of the same cells throughout your life.

Radiocarbon dating works great, but like a stop watch, it is not going to tell us about things much older than 100,000 years. For dinosaurs and older fossils, or rocks themselves the next method is more widely used.

Potassium-argon (K-Ar) Dating
Potassium-argon dating is a great method for measuring ages of materials that are millions of years old, but not great if you are looking to measure something only a few thousand years old, since it has a very long half-life.

Potassium argon dating measures the time since a phase transition from a liquid to a solid took place, such as the moment liquid lava or magma cools into a solid rock or crystal. It also requires that the material contain potassium in a crystal lattice structure. The most common minerals sampled for this method are biotite, muscovite, and the potassium feldspar group of minerals, such as orthoclase. These minerals are common in volcanic rocks and ash layers, making this method ideal for measuring the time when volcanic eruptions occurred.

If a volcanic ash containing these minerals are found deposited within or near the occurrence of fossils, a precise date can often be found for the fossils, or a range of dates, depending on how far stratigraphically that ash layer is found from the fossils. Potassium-40 is radioactive, but with a very long half-life of 1.26 billion years, making it ideal for determining ages in most geologic time ranges measured in millions of years. Potassium-40 decays to argon-40, as well as calcium-40, argon-40 is a gas, while calcium-40 is a solid, and very common, hence we want to look at the amount of argon-40 trapped in the crystal and compare that amount to potassium-40 contained in the crystal (both of which are fairly rare). This requires two steps, first to find out how much potassium-40 is contained within the crystal, and second how much argon-40 gas is trapped in the crystal. One of the beautiful things about potassium-argon dating is that the initial amount of argon-40 in the crystal can be assumed to be 0, since it is a gas. Argon-40 was not present when the crystal was a liquid and cooled into a solid. The only argon-40 found within the crystal would be formed by radioactive decay of potassium-40 and become trapped inside the solid crystal after this point. One of the problems with potassium-argon dating is that you have to do two different lab methods to measure the amount of potassium-40 and the amount of argon-40, and within a single crystal and not destroy the crystal in the process of running those two separate tests. Ideally, we want to sample the exact spot on a crystal for both measurements with a single analysis. And while potassium-argon dating came about in the 1950s, it has become less common compared to another method, which is easier and more precise, and only requires a single test.

40Ar/39Ar dating method
This method uses the potassium-argon dating technique but makes it possible to do a single lab analysis on a single point on a crystal grain, making it much more precise than the older potassium-argon method. The way it works is that a crystal containing potassium is isolated, and studied under a microscope, making sure it is not cracked or fractured in any way. The selected crystal is subjected to neutron irradiation, which converts any of the potassium-39 isotopes, to argon-39 isotopes a gas that will be trapped within the crystal (this is similar to what the sun does to nitrogen-14 to change it to carbon-14). These argon-39 isotopes join any of the radiogenic argon-40 isotopes in the crystal as trapped gases, so we just have to measure the amount of argon-39 to argon-40. The argon-39 number will determine about how much potassium was in the crystal. After being subjected to neutron irradiation the sample crystal will be zapped with a laser, that will release both types of argon gas trapped in the crystal. This gas is sucked up within a vacuum into a mass spectrometer to measure atomic masses of 40 and 39. Note that Argon-39 is radioactive, and decays with a half-life of 269 years, so any argon-39 measured was generated by the irradiation done in the lab. Often this method requires large, unfractured and well-preserved crystals to yield good results. The edges of the crystal and near cracks within the crystal, may have let some of the argon-40 gas to leak out, and will yield too young of a date. Both potassium-argon and argon-argon dating tend to give minimum ages, so if a sample yields 30 million years within a 1-million-year error, the actual age is more likely to be 31 million years, than 29 million years. Often potassium-argon and argon-argon dates are younger than other evidence suggests, and likely were determined from fractured crystals with some leakage of argon-40 gas. Studies will often show the crystal sampled and where the laser points are, and the dates calculated from each point in the crystal. The maximum age is often found near the center and far from any edge or crack within the crystal. Often this will be carried out with multiple crystals in a single rock, to get a good range, and taking the best resulting maximum ages. While potassium-argon and argon-argon are widely used it does require nicely preserved crystals of such fragile minerals grains as biotite, which means that the older the rock, the less likely good crystals can be found. It also does not work well with transported volcanic ash layers in sedimentary rocks, because the crystals are damaged in the process. Geologists were eager to use other minerals, more rugged minerals, that could last billions of years yet preserve the chemistry of radioactive decay—the mineral that meets those requirements is zircon.

Zircon Fission track dating
Zircons are tough and rugged minerals, found in many igneous and metamorphic rocks, and are composed of zirconium silicate (ZrSiO4), which form small diamond-like crystals. Because these crystals are fairly rugged and can survive transport they are also found in many sandstones. These transported zircons in sedimentary rocks are called detrital zircons. With most zircon dating, you are measuring the time since the phase transition from a liquid to a solid, when magma cooled into a solid zircon crystal.

Zircon fission track dating is more specifically measuring the time since the crystal was cooled to 230 to 250 °C, which is called the annealing temperature. Between 900 °C to 250 °C the zircons are somewhat mushy. Zircon fission dating dates the cooler temperature when the crystal became hard, while another method dates the hotter temperature when the crystal became a solid. Zircons are composed of a crystal lattice of zirconium bonded to silicate (silica and oxygen tetrahedrals), the zirconium is often replaced in the crystal with atoms of similar size and bonding properties, including some of the rare earth elements, but what we are interested in is that zircon crystals contain trace amounts of uranium and thorium. Uranium and thorium are two of the largest naturally occurring atoms on the periodic table. Uranium has 92 protons, while thorium has 90. Both elements are radioactive and decay, with long half-lives. These atoms of uranium and thorium act like mini-bombs inside the crystal, and when one of these high atomic mass atoms decay, it sets off a long chain reaction of decaying atoms, the fission of which causes damage to the internal crystal structure.

The older the zircon crystal is the more damage it will exhibit. Fission track dating was developed as an independent test of potassium-argon and argon-argon dating, as it does not require an expensive mass spectrometer, but simply looking at the crystal under a powerful microscope and measuring the damage caused by the radioactive decay of uranium and thorium. Zircon fission track dating is also used to determine the thermal history of rocks, as they rose up through the geothermal gradient, recording the length of time it took to cool to 250° C.

Uranium–Lead dating of Zircons
Uranium-lead dating is the most common way to date rocks and used to determine the age of the Earth, meteorites, and even rocks from the Moon and on Mars. It has become the standard method for radiometric dating, as new technology has made this method much easier. In the 1950s and 1960s, geologists were eager to figure out a way to use the uranium and lead inside zircons to get a specific date, more precise than estimates based on fission track dating, which was somewhat subjective. The problem was that all those radioactive tiny atomic bombs causing the damage to the zircon crystals over millions of years was also causing the loss of daughter product that would escape during those decay events, such as the gas Radon. The decay of the two most common isotopes of uranium (Uranium-235 and Uranium-238) is a complex chain of events, during which the radioactive gas radon is produced as one of the steps. If there are cracks or fractures in the crystal, the radon gas escapes from the crystal and as a result the ratio would yield too young of a date. If the radon gas is still held within the crystal, it would decay back to a solid, eventually as lead. Lead is not found initially within zircon crystals, and lead would only be found within zircons from the decay of uranium isotopes, allowing radiometric dating.

During the 1940s and 1950s a young scientist named Clair Cameron Patterson was trying to determine the age of the Earth by dating zircons and meteorites. Rather than look at zircons, he was trying to date meteorites, which contain the stable isotope lead-204, and used a type of uranium-lead dating simply called lead-lead dating. Clair Patterson used an isochron, which graphically compares the ratios of lead produced through the decay of uranium and thorium, lead-206 and lead-207 with stable lead-204 (an isotope not produced by radioactive decay of uranium and thorium), by plotting these ratios on a graph the resulting slope would indicate the age of the sample, the line is called an isochron, meaning the same age. Using lead isotopes recovered from the Canyon Diablo meteorite from Arizona, Patterson calculated that the Earth was between 4.5 and 4.6 billion years old in 1956.

To acquire these ratios of lead, Clair Patterson developed the first chemical clean room, as he quickly discovered abundant lead contamination in the environment around him traced to the widespread use of lead in gasoline, paints, and water pipes in the 1940s and 1950s. Patterson dedicated much of his later life fighting corporate lobbying groups and politicians to enact laws prohibiting the use of lead in household products, such as fuel and paints. The year 1956 was also the year that the solution to the uranium-lead problem was solved by the brilliant scientist George Wetherill who published a solution to the problem, something called a Concordia diagram, or sometimes called the Wetherill diagram, which allowed direct dating of zircons. There are two types of Uranium isotopes in these zircons: Uranium-238 (the most common) which decays to Lead-206, and Uranium-235 (the next most common) which decays to Lead-207 (with different half-lives).

If you could measure these two ratios, in a series of zircon crystals and compare the ratios graphically, you could calculate the true ratios of the zircons as if they had not lost any daughter products. Using this set of ratios, you can determine where the two ratios would cross with a given age, and hence where they would be in accordance with each other. It was a brilliant solution that solved the issue of daughter products escaping from zircon crystals. Today geologists can analyze particular points on individual zircon crystals, and hence select the best spot on the crystal that has the minimum amount of leakage of daughter products. Using the Concordia diagram allows a correction to these resulting ratios.

Uranium-Lead dating requires you to determine two ratios, Uranium-238 to Lead-206, and Uranium-235 to Lead-207. The Uranium-238 to Lead-206 has a half-life of 4.46 billion years, while Uranium-235 to Lead-207 has a half-life of 704 million years, making them great for both million-year and billion-year scales of time. To do Uranium-Lead dating on zircons, rock samples are grounded, and zircons are extracted using heavy liquid separation. These zircons are analyzed under a microscope. Zircons found in sedimentary rock will yield the age when the zircon initially formed from magma, not when it was re-deposited in a sedimentary layer or bed. Zircons found in sedimentary rocks are called detrital zircons, and will yield maximum ages; for example, a detrital zircon that is 80 million years old, could be found in sedimentary rock deposited 50 million years ago, and the 30-million-year difference is the time when the zircon was exhumed and eroded from igneous rocks and transported into the sedimentary rock. A 50-million-year old zircon will not be found in sedimentary rocks that are in fact 80 million years old, so detrital zircons will tell you only that the rock is younger than the zircon age.

Zircons deposited or forming within igneous rock or volcanic ash layers that contain fresh zircons can yield very reliable dates, particularly when the time between crystallization and deposition is minimal.

The first step of Uranium-Lead dating is finding and isolating zircon crystals from the rock, usually by grinding the rock up, and using heavy liquid to separate out the zircon crystals. The zircons are then studied under a microscope to determine how fresh they are. If zircons were found in a sedimentary rock, they are likely detrital, and the damage observed in the crystal will tell you how fresh they are. Detrital zircons are dated in studies to determine the source of sedimentary grains by sedimentary geologists, however, they often lack the resolution for precise dates, unless the zircon crystals were deposited in a volcanic ash, and have not be eroded and transported. Once zircons are selected, they are analyzed using laser ablation inductively coupled plasma mass spectrometry, abbreviated as LA-ICP-MS, which zaps the crystals with a laser, the ablated material is sucked up and ionized in the mass spectrometer under extremely hot temperatures, and a plasma is created which passes the atoms along a tube at high speed measuring the atomic mass of the resulting atoms scattered along the length of the plasma tube. LA-ICP-MS measures larger atomic mass atoms, such as lead and uranium. LA-ICP-MS does not require much lab preparation, and zircons can be analyzed quickly resulting in large sample sizes for distributions of zircons, giving very precise dates. Zircon Uranium-Lead dating is the most common type of dating seen today in the geological literature, exceeding even the widely used Argon-Argon dating technique. It is also one of the more affordable methods of dating, requiring less lab preparation of samples.

Dating using electron energy states
One of the things you will note about these dating methods is that they are used either to date organic matter that is less than 100,000 years old, or volcanic or igneous minerals that are much older between 1-million to 5-billion years old.

That leaves us with a lot of materials that we cannot date using those methods, including fossils directly that over 100,000 years old, and sedimentary rocks, since detrital zircons will only give you the date when they turned into a solid crystal, rather than the age the sedimentary rocks they are found in. Also using these methods, we cannot determine the age of stone or clay pottery artifacts, the age of glacial features on the landscape, or fossilized bone directly.

One place that is notoriously difficult to date are cave deposits that contain early species of humans, which are often older than the limits of radiocarbon dating. This problem is exemplified by the controversial ages surrounding the Homo floresiensis discovery, a remarkably small species of early humans found in 2003 in a cave located on the island of Flores in Indonesia. Physical anthropologists have argued that the species shares morphological similarities with Homo erectus, which lived in Indonesia from 1.49 million years ago to about 500,000 years ago. Homo erectus was the first early human to migrate out of Africa, and fossils discovered in Indonesia were some of the oldest, as determined from potassium-argon and zircon fission track dating. However, radiocarbon dating from the cave where the tiny species Homo floresiensis was found were much younger than expected, yielding radiocarbon dates of 18,700 years and 17,400 years old, which is old, but not as old as the anthropologists had suggested if the species was closely related to Homo erectus. Researchers decided to conduct a second analysis, and they turned to luminescence dating.

Luminescence (optically and thermally stimulated)
There are two types of luminescence dating, optically stimulated and thermally stimulated. They measure the time since the sediment or material was last exposed to sunlight (optical) or heat (thermal). Luminescence dating was developed in the 1950s and 1960s initially as a method to date when a piece of pottery was made. The idea was that during the firing of clay in a pottery kiln to harden the pottery, the quartz crystals within the pottery would be subjected to intense heat and energy, the residuals of this energy would dim slowly long after the pottery was cooled down. Early experiments in the 1940s on heating crystals and observing the light emitted after subjecting the crystals to heat or light, showed that materials could fluorescence (spontaneously glow) and phosphorescence as a delay of light given off by the material for a longer period of time, long after the material was subjected to the initial light or heat.

If you have ever played with glow in the dark objects, you can see this when you expose the object to light, then turn off the light, there is a glow to the object for a long while until it dims to the point you cannot see it anymore. This effect is called phosphorescence. It was also known that material near radioactive-materials would also give off either spontaneous fluorescence and phosphorescence which would last, so it does not have to be heated or in light, radioactive particles can also excite material to glow as well.

What causes this glow is that by exciting electrons in the atom with intense heat or exposure to sunlight (photons) or even radioactivity, the electrons move up in energy levels, however these electrons quickly drop down in energy levels, and in doing so emit photons as observable light as the object cools or was removed from the light. In some materials these electrons become trapped at these higher energy levels, and slowly and spontaneously pop back down to the lower energy levels over a more extended period of time. When electrons drop down from their excited states they emit photons, prolonging the glow of the material over a longer time period and perhaps thousands of years.

Scientists wanted to measure the remaining trapped electrons in ancient pottery. The dimmer the glow observed the older the pottery would be. Earlier experiments were successful, and later this tool was expanded to materials exposed to sunlight, rather than heat. The way it works is to determine two things, first is the radiation dose rate, as this will tell you how much radiation the crystal is absorbing over time. This is usually is done by measuring the amount of radioactive elements in the sample and surrounding it. The second thing to measure is the total amount of absorbed radiation, which is measured by exposing the material to light or heat, and measuring the number of photons emitted by the material. Using these two measurements you can calculate the age since the material was subjected to the initial heat or light. There are three types of Luminescence dating.

The first is TL (or thermal luminescence dating) using heat to measure the amount of photons given off of the material. The second is infrared stimulated or IRSL, and the third is optical stimulated or OSL, both of these methods refer to how the photons are measured in the lab by stimulating them with either infrared light or visible optical light. The technique works well, but there is a limit to how dim the material can be to give you useful information, so it works well for materials that are 100 to 350,000 years old, similar to ranges found with radiocarbon dating, but can be carried out on different material, such as pottery, stone artifacts, and the surfaces of buried buildings and stone work.

Researchers in addition to determining the radiocarbon age of Homo floresiensis, used luminescence dating and found a TL maximum date of 38,000 years old, and an IRSL minimum date of 14,000 years old, suggesting that the 18,000 years old date was correct for the skeletons found in the cave. These ages are when these sediments were last exposed to sunlight, and not when they were actually deposited in their current place in the cave, so there is likely a lot of mixing going on inside the cave.

Uranium series dating
As a large atom, uranium decays over a very long half-life to lead, and that there are two uranium decay chains, one for uranium-235 which decays to the stable isotope lead-207 and one for uranium-238 which decays to stable isotope lead-206. Scientists look at it just as a segment of that long decay chain, the decay of uranium-238 to uranium-234, which is the first part of uranium-238 decay.

And just measure the amount of uranium-234 decaying to thorium-230. Uranium-238 decays to thorium-234 with half-life of 4.27 billion years, thorium-234 decays to protactinium-234 with a half-life of 27 days, then protactinium-234 decays to uranium-234 also with a half-life of 27 days and finally Uranium-234 with a half-life of 245,500 years decays to thorium-230.

The decay between Uranium-234 and thorium-230 can be used to measure things within a few hundred thousand years. There is a problem with this method, since scientists do not know the initial amount of uranium within the bone or sediment that we are measuring. There is an unknown amount of uranium-234 starting out in the bone or sediment. Uranium-oxide is often carried by groundwaters moving into and out of the fossil and pores between sediment grains.

So unlike other dating methods, were the initial amount of daughter product was assumed to be zero, or a way to determine it experimentally, such as in carbon-14 dating, we cannot make that case. So we have to build a diffusion model, often this is called modeled ages.

The way this is done is that the bone is sectioned, cleaned and laser ablated at various points across its depth measuring the ratios between uranium-234 and thorium-230. Because the bone absorbed more uranium-234 over time, the outer layers of the bone will be enriched in uranium-234 compared to the internal part of the bone, using a gradient of uranium-238, uranium-234 and thorium-230 a diffusion model can be made to determine the amount of uranium-234 likely in the bone when the fossil organism died, and the amount of thorium-230 resulting from the decay of this uranium-234 as additional uranium-234 was added during the fossilization process. Because of this addition of uranium-234, and the fact that uranium-234 is very rare, as it is produced only by the decay of uranium-238, this method is reserved for difficult cases, such as dating fossils deposited in hard-to-date cave deposits, especially in the upper limits of radiocarbon dating, between 100,000 to 500,000 year-old fossils.

Uranium series dating was used to reexamine the age of Homo floresiensis by looking at the actual fossil bone itself. Uranium series dating of the bone of Homo floresiensis resulted in ages between 66,000 and 87,000 years old (link to revised age), older than the radiocarbon dates from the nearby charcoal (17,400-18,700 years old) and luminescence dating of sediment in the cave (14,000-38,000 years old), but modeled on the actual bones themselves. These are modeled ages, since you have to determine the diffusion of uranium into the pores of the bone as it was fossilized in the cave, which can yield somewhat subjective dates.

Uranium series dating was also done for another problematic early human cave discovery, the age of Homo naledi from the Rising Star cave in South Africa. Fossil teeth were directly dated using uranium series dating, yielding a minimum age of 200,000 years old (link to paper on the age of the fossil), which had been predicted to be about 1,000,000 years old.

Although, uranium series dating tends to have large error bars, due to the modeling of the diffusion of uranium into the fossils and rocks. Depending on how quickly and how much uranium-238 and uranium-234 was added to the fossil over time in the cave. Uranium series dating is used really only in special cases where traditional dating such as radiocarbon dating and uranium-lead dating cannot be done.

Electron Spin Resonance (ESR)
In April of 1986 the Chernobyl Nuclear Power Plant suffered a critical meltdown resulting in an explosion and fire that released large amounts of radioactive materials into the nearby environment. The accident led to the death of 31 people directly from radiation, and 237 suffered acute radiation sickness. Worry spread across Europe as to how to measure the exposure to radiation from the accident, and electron spin resonance was developed by Soviet scientists to measure the exposure to radiation by looking at teeth, particularly baby teeth of children living in the area.

Electron spin resonance is the measurement of the number of unpaired electrons within atoms. When exposed to radiation, electrons will unpair from their typical covalent bonds, and become unpaired within the orbitals resulting in a slight difference in the magnetism of the atom. This radiation damage, results in the breaking of molecular bonds, and the reason radiation causes cancers and damage to living cells. At the atomic level radiation can break molecules, resulting in abnormally high errors in DNA and proteins within living cells. Electron spin resonance measures the amount of free radical electrons with a material.

Using this measurement, scientist measured the amount of electron spin resonance in teeth from children who lived near the accident to determine the amount of exposure they had to radiation fall-out from the Chernobyl accident. The study worked, which led to the idea of using the same technology in fossilized teeth, exposed to naturally occurring radiation in the ground.

Dating using electron spin resonance requires that we know the amount of uranium and radioactivity that is in the surrounding material through its history, and calculate the length of exposure time to this radiation. The issue however is that you have to model the amount of uranium uptake within the fossil over time, similar to the model you develop with uranium series dating. This is because in both methods of dating you cannot assume that uranium (and amount of radioactivity) in the material remained the same, as the uptake of fresh uranium over time likely occurred. Often scientists will focus on the dense crystal lattice structure of enamel, a mineral called hydroxyapatite, as it is less susceptible to the uptake of uranium.

Electron spin resonance is often paired with uranium series dating, since it has a similar range of ages that it can be used for from a 100 up to 2,000,000 years. Unpaired electrons within atoms is a more permanent state than electrons at higher energy levels seen in Luminescence dating, so older fossils can be dated, up to 2 million years old. This dating method cannot be used for the vast majority of fossils older than 2 million years old, but can be used to date the length of time a fossil or rock was buried up to that limit. Note that electron spin resonance dating is determining the length of time a fossil was buried in sediment that has a background radiation that can be measured.

Surface Exposure Dating or Beryllium-10 dating
This dating method has revolutionized the study of past Ice Ages over the last 2.5 million years, and study of the glacial and interglacial cycles of Earth’s recent climate. Surface exposure dating can determine the length of time a rock has been exposed to the sun. Ascertaining the length of time, the rock has been exposed to the sunlight allows geologists to discover the age of when that rock or boulder was deposited by a melting glacier, and the timing of the extent of those glaciers on a local level throughout past ice age events.

The way it works is that when rocks are exposed to sunlight, they are bombarded by cosmic rays from the sun, which contain neutrons. These rays result in something called spallation of the atoms in mineral crystals, resulting in the build-up of cosmogenic nuclides.

There are a number of different types of cosmogenic nuclides. For example, we had previously talked about potassium-40 being hit with neutrons in a lab setting and producing Argon-39, in argon-argon dating. The same thing happens in nature, when rocks are left in the sun for a long time, and you could measure the amount of argon-39. Most geologists instead look for atoms which form solids as they are easier to extract from the rock, including Beryllium-10, one of the most widely used cosmogenic nuclides to measure.

Beryllium-10 is not found in quartz minerals common in rocks and boulders when they form, but will accumulate when the oxygen atoms within the crystal lattice structure are exposed to cosmic rays containing short-lived free neutrons. The beryllium-10 will build up within the crystals, as long as the rock is exposed to the sun. Beryllium-10 is an unstable, radioactive isotope, with a half-life of 1.39 million years, making it ideal for most applications of dating during the Pleistocene Epoch. Most rocks studied so far have exposure ages of less than 500,000 years, indicating that most rocks get re-buried within half a million years.

Surface Exposure Dating is different because we are looking at the amount of beryllium-10 building up within the surface of the rock over time, so the more beryllium-10 is within the rock the longer it has been exposed to sunlight. If the rock becomes obscured from the sun through burial, or a tree grows next to it, then the build-up of the beryllium-10, will be slowed or turned off, and over time will decay to boron-10, emptying the beryllium-10 out of the rock, and resetting the clock for the next time it is exposed to the sunlight. Geologists have to be sure that the rock has been well exposed to sunlight and not shaded by any natural feature in the recent past, like trees.

Geologists will select a boulder or rock, and carefully record its location, as well as the horizon line surrounding the rock, to account for the length of sunlight exposure of any given day at that location.

A small explosive charge is drilled into the rock, and rock fragments are collected of the surface edge of the rock. The sample is grounded into a powder back in the lab, and digested with hydrofluoric acid, to isolate quartz crystals, which is turned into a liquid solution within a very strong acid. This solution is reacted to various chemicals to isolate the beryllium into a white powder, which is then passed through a mass spectrometer to measure the amount of Beryllium-10 in the rock. This amount is then compared to a model of how much sunlight the rock was exposed to at that location, with the topography of the surrounding features, and determine the length of time that rock has sat there on the surface of the Earth. It’s a pretty cool method which, has become highly important in understanding the glacial history of the Earth through time.

Magnetostratigraphy
Magnetostratigraphy is the study of the magnetic orientations of iron minerals within sedimentary rocks. These orientations record the direction of the magnetic pole when the sedimentary rocks were deposited. Just like a magnetic compass, iron minerals when transported in lava, magma, or in sediment will orient to the current Earth’s magnetic field. The Earth magnetic field is not stationary, but moves around. In fact, the orientations of the poles switch randomly every few hundred thousand years or so, such that a compass would point toward the south pole rather than the north pole. This change in the orientation of the iron minerals is recorded in the rock layers formed at that time. Measuring these orientations between normal polarity, when the iron minerals point northward, and reversal polarity, with the iron minerals point southward, gives you events that can be correlated between different rock layers.

The thickness of these bands of changing polarity can be compared to igneous volcanic rock as well, which record both the absolute age using potassium-argon dating for example, as well as the polarity of the rocks at that time, allowing correlation between sedimentary and igneous rocks. Magnetostratigraphy is really important because it allows for the dating of sedimentary rocks, that contain fossils, even when there is no volcanic ash layers present.

There are a couple problems with magnetostratigraphy. One is that rocks can become demagnetized in sedimentary rocks. For example, the rock being struck by lightning will scramble the orientations of the iron grains. It can be also difficult to correlate layers of rock if the sedimentation rates vary greatly or you have unconformities you are unaware of. However, it really works well for many rock layers, and is a great tool to determine the age of rocks by documenting these reversals in the rock record. It was also one of the key technologies to demonstrate the motion of Earth’s tectonic plates.

Rock samples are collected in the field, by recording their exact orientation and carefully extracting the rock not to break it. The rock is then taken to a lab, where the rock is placed in an iron cage to remove the interference of magnetic fields from the surrounding environment. The rock sample is cryogenically cooled down to extremely cold temperatures just above absolute zero, where the residual magnetism of the rock will be easier to measure, because sedimentary rocks are not very magnetic. More magnetic rocks, like igneous rocks do not have to be cooled. The rock is slowly demagnetized and the orientations vectors are recorded and spatially plotted.

These data points will fall either more toward the north or south, depending on the polarity of the Earth at the time of deposition. The time span between polar reversal events is short, a few hundred years, with the majority of time the polarity is either in normal or reverse state. Sometimes the polarity changes rapidly, while other times the polarity does not change for millions of years, such long intervals with lack of change occurred during the Cretaceous Period, during the age of dinosaurs for 40 million years, which is called the Cretaceous Superchron where the polarity stayed normal for a very long time, and geologists do not know why this happened.


text: Category:Book:End-user Computer Security/Book part/Main content/Chapters/«Mind-reading attacks», chapter 6

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text: Category:Book:End-user Computer Security/Hash-tagged/＃Mind-reading


text: Category:Book:End-user Computer Security/Book part/Main content/Chapters/«Simple security measures», chapter 7


text: Category:Book:End-user Computer Security/Hash-tagged/＃Simple security measures


text: Category:Book:End-user Computer Security/Hash-tagged/＃Security induced by specific computer power state


text:
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text:
Planet Earth/3e. The Periodic Table and Electron Orbitals

Electrons: how atoms interact with each other
If it was not for electrons inside atoms, atoms would never bond or interact with each other to form molecules, crystals and other complex materials. Electrons are extremely important in chemistry because they determine how atoms interact with each other. It is no wonder that the Periodic Table of Elements, found in most science classrooms is displayed rather than the more cumbersome Chart of the Nuclides, since the Periodic Table of Elements organizes elements by the number of protons and electrons, rather than the number of protons and neutrons.

As discussed previously, electrons are wayward subatomic particles that can increase their energy states and even leave atoms altogether to form plasma, which is also called electricity. Electricity is the flow of free electrons which can move near the speed of light across conducting material, like metal wires. In this next section we will look in detail at how electrons are arranged within atoms in orbitals. However, remember that highly excited atoms bombarded with high levels of electromagnetic radiation, such as increasing temperatures and high pressures, electrons can leave atoms, while at very cold temperatures, near absolute zero Kelvin electrons will be very close to the nucleus of atoms forming Bose–Einstein condensate. When we think of temperature (heat), what is really indicated is the energy states of electrons within the atoms of a substance, whether a gas, liquid or solid. The hotter a substance becomes the more vibrational energy electrons will have.

Electrons orbit around the nucleus at very fast speeds and under no discrete orbital path, but as an electromagnetic field called an orbital shell. The Heisenberg principle describes the impossible nature of measuring these electron orbital shells, because anytime a photon is used by a scientist to measure the position of an electron, it will move and change its energy level. There is always an uncertainty as to the exact location of an electron within the orbit around the atom’s nucleus. As such electron orbital shells are probability fields where an electron is likely to exist at any moment in time.

Negatively charged electrons are attracted to positively charged protons, such that equal numbers of electrons and protons are observed in most atoms.

Early chemists working in the middle 1800s knew of only a handful of elements, which were placed into three major groups based on how reactive they were with each other, the Halogens, Alkali Metals and Alkali Earths. By 1860, the atomic mass of many of these elements were reported, allowing the Russian scientist Dmitri Mendeleev to arrange elements based on their reactive properties and atomic mass. While working on a chemistry textbook, Mendeleev stumbled upon the idea of each set of elements having increasing atomic mass, such that a set of Halogens would have elements of differing mass. Without knowing the underlying reason, Mendeleev organized the elements by their atomic number (number of protons), which is related to atomic mass and the number of electron orbitals, which is related to how reactive an element is to bonding with other elements. While these early Periodic Tables of Elements look nothing like our modern Periodic Table of Elements, they excited chemists to discover more elements. The next major breakthrough came with the discovery and wide acceptance of Noble gasses, which include Helium and Argon, which are the least reactive elements known.

The Periodic Table of Elements
So how does an atom’s reactivity relate to its atomic mass? Electrons are attracted to the atomic nucleus in equal number to the number of protons, which is half the atomic mass. The more atomic mass, the more protons, and the more electrons will be attracted. However, electrons prefer to fill electron orbital shells in sets, such that an incomplete electron orbital shell will attract other electrons, despite there being an equal number of electrons to protons. If an atom has a complete set of electrons which matches the number of protons, it will be non-reactive, while elements that need 1 less or gain 1 more electron to fill an orbital set are the most reactive types of elements.

The First Row of the Periodic Table of Elements (Hydrogen & Helium)
The first row of the Periodic Table of Elements contains two elements, Hydrogen and Helium.

Hydrogen has 1 proton, and hence it attracts 1 electron. However, the orbital shell would prefer to contain 2 electrons, so hydrogen is very reactive with other elements, for example in the presence of oxygen, it will explode! Hydrogen would prefer to have 2 electrons within its electron orbital shell, but can’t because it has only 1 proton, so it will “steal” or “borrow” other electrons from nearby atoms if possible.

Helium has 2 protons, and hence attracts 2 electrons. Since 2 electrons are the preferred number for the first orbital shell, helium will not react with other elements, in fact it is very difficult (nearly impossible) to bond helium to other elements. Helium is a Noble Gas, which means that it contains the full set of electrons in its orbital shell.

The columns of the Period Table of Elements are arranged by the number of electrons within each orbital shell, and the atomic number (number of protons), is represented by rows.

The Other Rows of the Periodic Table of Elements
The first row of the Periodic Table of Elements is the when the first 2 electrons fill the first orbital shell, called the 1s orbital shell. The second row is when the next 2 electrons fill in the 2s orbital shell, and 6 fill the 2p orbital shell. The third row is when the next 6 orbitals fill in the 3p orbital shell, and 2 fill the 4s orbital shell. The fourth row is when the next 10 orbitals fill in the 3d orbital shell, and 6 fill in the 4p orbital, and 2 fill in the 5s orbital.

Valence Electrons
A valence electron is an outer shell electron that is associated with an atom, but not completely filling the outer orbital shell, and as such is involved in bonding between atoms. The valence shell is the outermost shell of an atom. Elements with complete valence shells (noble gases) are the least chemically reactive, while those with only one electron in their valence shells (alkali metals) or just missing one electron from having a complete shell (halogens) are the most reactive. Hydrogen, has one electron in its valence shell but also is just missing one electron from having a complete shell has unique and very reactive properties.

The number of valence electrons of an element can be determined by the periodic table group, the vertical column in the Periodic Table of Elements. With the exception of groups 3–12 (the transition metals and rare earths), the columns identify by how many valence electrons are associated with a neutral atom of the element. Each s sub-shell holds at most 2 electrons, while p sub-shell holds 6, d sub-shells hold 10 electrons, followed by f which holds 14 and finally g which holds 18 electrons. Observe the first few rows of the Periodic Table of Elements to see how this works to determine how many valance electrons ( < ) are in each atom of a specific element.

Notice that Helium and Neon have 0 valance electrons, which means that they are not reactive, and will not bond to other atoms. However, Lithium has 1 valance electron, if this 1 electron was removed, it would have 0 valence electrons, this makes Lithium highly reactive. Also notice that Fluorine just needs 1 more valence electron to complete its set of 2s and 2p orbitals, making Fluorine highly reactive as well. Carbon has the highest number of valence electrons in this set of elements, which will attract or give up 4 electrons to complete the set of 2s and 2p orbitals.

Understanding the number of valence electrons is extremely important in understanding how atoms form bonds with each other to form molecules. For example, the first column of elements containing Lithium on the periodic table all have 1 valence electron, and likely will bond to elements that need 1 valence electron to fill the orbital shell, such as elements in the fluorine column on the Periodic Table of Elements.

Some columns of the periodic table are given specific historical names. The first column of elements containing Lithium collectively are called the Alkali Metals (hydrogen a gas is unique and often not considered within Alkali Metals), the last column of the elements containing Helium all have 0 valence electrons, and are collectively called the Noble Gases. Elements under the Fluorine column require 1 valence electron to fill the orbital shell and are called the Halogens, while elements under Beryllium are called the Alkaline Earth Metals, and have 2 valence electrons. Most other columns are not given specific names (sometimes collectively called Transitional Metals), but can be used to determine the number of valence electrons, for example Carbon and elements listed below will have 4 valence electrons, while all elements listed under Oxygen will have 2 valence electrons. Notice that after the element Barium, there is an insert of two rows of elements, these are the Lanthanoids and Actinoids, which contain electrons in the 4s, 4p, 4d, 4f orbitals, for a possible total of 32 electrons, a little too long to include in a nice table, and hence often these elements are shown at the bottom of the Periodic Table of Elements.

A typical college class in chemistry will go into more detail on electron orbital shells, but it is important to understand how electron orbitals work, because the configuration of electrons determines how atoms of each element form bonds in molecules. In the next section, we will examine how atoms come together for form bonds, and group together in different ways to form the matter that you observe on Earth.


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Planet Earth/3f. Chemical Bonds (Ionic, Covalent, and Others)

There are three major types of bonds that form between atoms, linking them together into a molecule, Covalent, Ionic, and Metallic. There are also other ways to weakly link atoms together, because of the attractive properties related to the configuration of the molecules themselves, which includes Hydrogen bonding.

Covalent Bonds
Covalent bonds are the strongest bonds between atoms found in chemistry. Covalent bonding is where two or more atoms share valence electrons to complete their orbital shells. The most-simple example of a covalent bond is found when two hydrogen atoms bond. Remember that each hydrogen atom has 1 proton, and 1 electron, however to fill the s1 orbital requires 2 electrons. Hydrogen atoms will group into pairs, each contributing an electron to the s1 orbital shell. Chemical hydrogen will be paired, which is depicted by the chemical formula H2. Another common covalent bond can be illustrated, by introducing oxygen with hydrogen. Remember that oxygen needs two valence electrons to fill its set of electron orbitals, hence it bonds to 2 hydrogen atoms, each having 1 valence electron to share between the atoms. H2O the chemical formula for ice or water is where 2 hydrogen atoms, each with an electron, bond with an oxygen atom that needs 2 electrons to fill its s2 p2 orbitals. In covalent bonds, atoms share the electron to complete the orbital shells, and because the electrons are shared between atoms covalent bonds are the strongest bonds in chemistry. Oxygen for example, will pair up to share the 2 electrons (called a double bond), forming O2. Nitrogen does the same, pairing up to form N2, by sharing 3 electrons (called a triple bond). However, in the presence of nitrogen and hydrogen, the hydrogen will bond with nitrogen forming NH3 (ammonia) because it would require 3 electrons each from a hydrogen atom to fill all the orbitals. Carbon which has 4 valence electrons most often bonds with hydrogen to form CH4 (methane or natural gas), because it requires 4 electrons each from a hydrogen atom. Bonds that form by two atoms sharing 4 or more electrons are very rare. The electrons shared equally between the atoms makes these bonds very strong. Covalent bonds can form crystal lattice structures, when valence electrons are used to link atoms together. For example, diamonds are composed of linked carbon atoms. Each carbon atom is linked to 4 other carbon atoms, which each share an electron between them. If the linked carbon forms a ring, rather than a lattice structure, the carbon is in the form of graphite (used at the end of pencils). If the linked carbon forms a lattice structure the crystal form is much harder, a diamond. Hence the only difference between graphite pencil lead and a valuable diamond is how the bonds between the carbon atoms are linked together in covalent bonds.

Ionic Bonds
Ionic bonds are a weaker type of bond between atoms found in chemistry. Ionic bonding is where one atom gives a valence electron to complete another atom’s orbital shell. For example, lithium has a single valence electron, and would like to get rid of it, so it will give or contribute the electron to an atom of fluorine, which needs an extra valence electron. In this case the electron is not shared by the two atoms, however, when lithium gives away its valence electron, it becomes positively charged because it has fewer electrons than it has of protons. While fluorine, will have more electrons than protons, and will be negatively charged. Because of this charge, the atoms will be attracted together. Atoms that have different numbers of protons and electrons are called ions. Ions can be positively charged like Lithium, which are called cations or negatively charged like Fluorine which are called anions.

An excellent example of ionic bonding you have encountered is table salt, which is composed of Sodium (Na) and Chloride (Cl). Sodium has one extra valence electron that it would like to give away, and Chloride is looking to pick up an extra electron to fill its orbital, this results in Sodium (Na) and Chloride (Cl) ionically bonding to form table salt. However, the bonds in salt are easy to break, since they are held not by sharing electrons, but by their different charges. When salt is dropped in water, the pull of the water molecules can break apart the sodium and chloride, resulting in Sodium and Chloride ions (the salt is dissolved within the water). Often chemical formulas of ions are expressed as Na+ and Cl− to depict the charge, where + sign indicates a cation and − sign indicates an anion. Sometimes an atom will give away or receive two or more electrons, for example Calcium will often give up two electrons, resulting in the cation Ca2+.

The difference between covalent bonds and ionic bonds is that in covalent bonds the electrons are shared between atoms, while in ionic bonds the electrons are given or received between atoms. A good analogy to think of is friendship between two kids. If the friends are sharing a ball, by passing it between each other, they are covalently bonded to each other since the ball is shared equally between them. However, if one of the friends has an extra ice cream cone, and gives it to their friend, they are ionically bonded to each other.

Some molecules can have both ionic and covalent bonds. A good example of this is with a common molecule Calcium Carbonate CaCO3. The carbon atom is covalently bonded to three oxygen atoms, which means that it shares electrons between the carbon and oxygen atoms. Typically carbon only covalently bonds to two oxygen atoms (forming carbon dioxide CO2), each sharing two electrons, for a total of 4. However, in the case of carbonate, three oxygen atoms are bonded to the carbon, with 2 sharing 1 electron, and one sharing 2 electrons, this results in 2 extra electrons. Hence CO3-2 has two extra electrons that it would like to give away, and is negatively charged. Calcium atoms have 2 electrons more than a complete shell, and will lose these electrons resulting in a cation with a positive charge of +2, Ca+2. Hence the ions CO3-2 and Ca+2 have opposite charges that will bond together and form CaCO3, calcium carbonate, a common molecule found in limestones and shelled organisms that live in the ocean. Unlike salt, CaCO3 does not readily dissolve in pure water, as the ionic bonds are fairly strong, however if the water is slightly acidic, CaCO3, calcium carbonate will dissolve.

A solution is called an acid when it has an abundance of ions of hydrogen within the solution. Hydrogen ions, lose 1 electron, forming a cation H+. When there is an excess of hydrogen ions in a solution, these will break ionic bonds by bonding to anions. For example, in CaCO3, the hydrogen ions can form bonds with the CO3-2, forming HCO3- ions, called bicarbonate, dissolving the CaCO3 molecule. Acids break ionic bonds, by introducing ions of hydrogen, which can dissolve molecules that form these ionic bonds. Note that a solution with an abundance of anions, such as OH-, can also break ionic bonds, and these are called bases. So a basic solution, is one with an excess of anions. In this case the calcium will form a bond with the OH- anion, forming Ca(OH)2, calcium hydroxide, which in a solution of water is known as limewater.

The ratio of ions of H+ and OH- is measured in pH, such that a solution with a pH of 7 has equal numbers of H+ and OH- ions, while acidic solutions have pH less than 7, with more H+ cations, while basic solutions have pH more than 7, with more OH- anions.

Metallic bonding
Metallic bonding is a unique feature of metals, and can be described as a special case of ionic bonding involving the sharing of free electrons among a structure of positively charged ions (cations). Materials composed of metallic bonded atoms exhibit high conductivity of electricity, as electrons are free to pass between atoms across its surface. This is why electrical wires are composed of metals like copper, gold, and iron, since they can conduct electricity across their surface, since electrons are shared evenly between many atomic bonds. Material composed of metallic bonding also have a metallic luster or shine, and can be more ductile (bend) easily because of the flexibility of these bonds. Metallic bonds are susceptible to oxidation. Oxidation is a type of chemical reaction in which metallic bonded atoms lose electrons to an oxidizing agent (most often atoms of oxygen), resulting in metallic atoms becoming bonded covalently with oxygen. For example, iron (Fe) which can be either cations of Fe2+ (Iron-II) or Fe3+ (Iron-III) lose electrons and can receive these missing electrons with oxygen (O-2), which has two extra electrons in its orbitals, resulting in a series of molecules called iron oxides such as Fe2O3. This is why metals, such as iron, rust or corrode and silver tarnishes over time. These metallic bonds react with the surrounding oxygen by gaining extra electrons from them. Oxygen is common in the air, within water and within acidic solutions (corrosive solutions), and the only way to prevent oxidation in metals is to limit the exposure to oxygen (and other atoms with an excess of electrons like fluorine).

When electrons are gained the reactive is called a reducing reaction, and is the opposite of oxidation. Collectively these types of chemical reactions are called "Redox" reactions and they form an important aspect in chemistry. Furthermore, the transfer of elections in oxidation-reduction reactions are a useful way to store excessive electrons (electricity) in batteries.

Hydrogen bonding
Covalent, Ionic and Metallic bonding all require the exchange of electrons between atoms, and hence are fairly strong bonds, with covalent bonds being the strongest type of bond. However, molecules themselves can become polarized because of the arrangement of the atoms, such that a molecule can have a more positive and more negative side. This frequently happens with molecules containing hydrogen atoms bonded to larger atoms. These types of bonds are very weak and easily broken, but produce very important aspects in the chemistry of water and organic molecules essential for life. Hydrogen bonds form within water and is the reason for the expansion in volume between liquid water and solid ice. Water is composed of oxygen bonded to two hydrogen atoms covalently (H2O). The distribution of these two hydrogen atoms contribute an electron to the p2 orbitals, which require 6 electrons. Hence the two hydrogen atoms are pushed toward each other slightly because of the pair of electrons in the first p2 orbital, forming a “mouse-ear” like molecule. These two hydrogen atoms are more positively charged and give the molecule a slight positive charge at the hydrogen atom side compared to the other side of the oxygen atom which lacks a hydrogen atom. Hence water molecules oriented themselves with weak bonds between the positively charged hydrogen atoms and the open space negative charge side of the atoms. Hydrogen bonds are best considered an electrostatic force of attraction between hydrogen (H) atoms which are covalently bound to more electronegative atoms such as oxygen (O) and nitrogen (N). Hydrogen bonds are very weak, but provide important bonds in living organism, such as the bonds within the helix in the double helix structure of DNA (Deoxyribonucleic acid), and hydrogen bonds are important in capillary forces with water transport in plant tissue and blood vessels, as well as hydrophobic (water repelling) and hydrophilic (water attracting) organic molecules in cellular membranes.

Hydrogen bonding is often considered as a special type of the weak Van der Waals molecular forces which cause the attraction or repulsion of electrostatic interacts between electronically charged or polarized molecules. These forces are weak, but play a role in making some molecules more “sticky” than other molecules. As you will learn later on, water is a particularly “sticky” molecule because of these hydrogen bonds.


text:
User:Masking1969/Collections/health

 * GNU Health
 * GNU Health/Preface
 * GNU Health/Introduction
 * GNU Health/Resources
 * GNU Health/First Steps

title: User:Masking1969/Collections/health


text:
Icelandic/Grammar/Adverbs

Atviksorð
 * Adverbs

Adverbs are words that add description to a verb or adjective.

Forming adverbs
-lega is the most common suffix added to nouns, verbs, or adjectives to form the adverb. If the adjective ends in -legur (the most common suffix used to form adjectives from nouns), then that is simply changed to -lega. Examples:
 * lýðræðis lega  ("democratically")
 * from lýðræði ("democracy")
 * fal lega  ("beautifully")
 * from fallegur ("beautiful")

Inflection
Adverbs are not inflected in any way other than degree (positive, comparative, and superlative). The comparative of -lega adverbs is -legar, while the superlative is -legast.

title: Icelandic/Grammar/Adverbs


text:
Planet Earth/3g. Common Inorganic Chemical Molecules of Earth

Goldschmidt Classification
With 118 elements on the periodic table of elements, there can be a nearly infinite number of molecules with various combinations of these 118 elements. However, on Earth some elements are very rare, while others are much more common. The distribution of matter, and of the various types of elements across Earth’s surface, oceans, atmosphere, and within its inner rocky core is a fascinating topic. If you were to grind up the entire Earth, what percentage would be made of gold? What percentage made of oxygen? How could one calculate the abundances of the various elements of Earth? Insights into the distribution of elements on Earth came about during World War II, as scientists developed new tools to determine the chemical makeup of materials. One of the great scientists to lead this investigation was Victor Goldschmidt.

On November 26, 1942, Victor Goldschmidt stood among the fearful crowd of people assembled on the pier in Oslo, Norway, waiting for the German ship Donau to transport them to Auschwitz. Goldschmidt had a charmed childhood in his home in Switzerland, and when his family immigrated to Norway, Goldschmidt was quickly recognized for his early scientific interests in geology. In 1914 he began teaching at the local university after successfully defending his thesis on the contact metamorphism in the Kristiania Region of Norway. In 1929 he was invited to Germany to become the chair of mineralogy in Göttingen, and had access to scientific instruments that allowed him to detect trace amounts of elements in rocks and meteorites. He also worked with a large team of fellow scientists in the laboratories whose goal it was to determine the elemental make-up of the wide variety of rocks and minerals. However, in the summer of 1935 a large sign was erected on the campus by the German government that read, “Jews not desired.” Goldschmidt protested, as he was Jewish and felt that the sign was discriminatory and racist. The sign was removed, but only to reappear later in the Summer, and despite his further protest against the sign, the sign remained as ordered by the new Nazi party. Victor Goldschmidt resigned his job in Germany and returned to Norway to continue his research, feeling that any place where people were injured and persecuted only for the sake of their race or religion, was not a welcome place to conduct science. Goldschmidt had with him vast amounts of data regarding the chemical make-up of natural materials found on Earth, particularly rocks and minerals. This data allowed Goldschmidt to classify the elements based on their frequency found on Earth.

The Atmophile Elements
The first group Goldschmidt called the Atmophile elements, as these elements were gases and tended to be found in the atmosphere of Earth. These included both Hydrogen and Helium (the most abundant elements of the solar system), but also Nitrogen, as well as the heavier noble gasses: Neon, Argon, Krypton and Xenon. Goldschmidt believed that Hydrogen and Helium as very light gasses were mostly stripped from the Earth’s early atmosphere, with naturally occurring Helium on Earth found from the decay of radioactive materials deep inside Earth, and trapped, often along with natural gas underground. Nitrogen forms the most common element in the atmosphere, as a paired molecule of N2. It might be surprising that Goldschmidt did not classify oxygen within this group, and that was because oxygen was found to be more abundant within the rocks and minerals he studied, in a group he called Lithophile elements.

The Lithophile Elements
Lithophile elements or rock-loving elements are elements common in crustal rocks found on the surface of continents. They include oxygen and silicon (the most common elements found in silicate minerals, like quartz), but also a wide group of alkali elements belong to this group including lithium, sodium, potassium, beryllium, magnesium, calcium, strontium, as well as the reactive halogens: fluorine, chloride, bromine and iodine, and with some odd-ball middle of the chart elements, aluminum, boron, phosphorous, and of course oxygen and silicon. Lithophile elements also include the Rare Earth elements found within the Lanthanides, and making a rare appearance in many of the minerals and rocks understudy.

The Chalcophile Elements
The next group are the Chalcophile elements or copper-loving elements. These elements are found in many metal ores, and include sulfur, selenium, copper, zinc, tin, bismuth, silver, mercury, lead, cadmium and arsenic. These elements are often associated in ore veins and concentrated with sulfur molecules.

The Siderophile Elements
The next group Goldschmidt described where the Siderophile elements or iron-loving elements, which include iron, as well as cobalt, nickel, manganese, molybdenum, ruthenium, rhodium, palladium, tungsten, rhenium, osmium, iridium, platinum, and gold. These elements were found by Goldschmidt to be more common in meteorites (most especially in iron-meteorites) when compared to rocks found on the surface of the Earth. Furthermore, these elements are common in iron-ore and associated with iron-rich rocks when they are found on Earth’s surface.

The last group of elements are simply the Synthetic elements, or elements that are rarely found in nature, which include the radioactive elements found on the bottom row of the Periodic Table of Elements and produced only in labs.

Meteorites, the Ingredients to Making Earth
A deeper understanding of the Goldschmidt classification of the elements was likely being discussed at the local police station in Oslo on that chilly late November day in 1942. Goldschmidt’s Jewish faith resulted in his imprisonment seven years later, when Nazi Germany invaded Norway, and despite his exodus from Germany, the specter of fascism had caught up with him. Jews were to be imprisoned, and most would face death in the concentration camps scattered over Nazi-occupied Europe. Scientific colleagues argued with the authorities that Goldschmidt’s knowledge of the distribution of valuable elements was much needed. The plea worked, because Victor Goldschmidt was released, and of the 532 passengers that boarded the Donau, only 9 would live to see the end of the war. With help, Goldschmidt fled Norway instead of boarding the ship and would spend the last few years of his life in England writing a textbook, the first of its kind on the geochemistry of the Earth.

As a pioneer in understanding the chemical make-up of the Earth, Goldschmidt inspired the next generation of scientists to study not only the chemical make-up of the atmosphere, ocean, and rocks found on Earth, but to compare those values to extra-terrestrial meteorites that have fallen to Earth from space.

Meteorites can be thought of as the raw ingredients of Earth. Mash enough meteorites together, and you have a planet. However, not all meteorites are the same, some are composed mostly of metal iron, called iron meteorites, other meteorites have equal amounts of iron and silicate crystals, called stony-iron meteorites, while the third major group, the stony meteorites are mostly composed of silicate crystals (SiO2). If the Earth formed from the accretion of thousands of meteorites, then the percentage of chemical elements and molecules found in meteorites would give scientists a starting point for the average abundance of elements found on Earth. Through its history Earth’s composition has likely changed as elements became enriched or depleted in various places, and within various depths inside Earth. Here are the abundances of molecules in meteorites: (From Jarosewich, 1990: Meteoritics)

If Earth was a homogenous planet (one composed of a uniform mix of these elements) the average make-up of Earthly material would have a similar composition to a mix of stony and iron meteorites. We see some indications of this fact, for example SiO2 (silica dioxide) is the most common molecule in stony meteorites at 38.2%, with silica bonded to two oxygen molecules. Silicon and oxygen are the most common molecules found in rocks, forming a group of minerals called silicates, which include quartz, a common mineral found on the surface of Earth. The next three molecules, MgO, FeO, and CaO are also commonly found in rocks on Earth, however, iron (Fe) which is very common in iron meteorites, and also makes up a significant portion of stony meteorites with various molecules containing FeO, FeS, and Fe in native metal form. Yet typical rocks found on the surface of Earth contain very little iron. Where did all this iron go?

Goldschmidt suggested that iron (Fe) is a Siderophile element, as well as nickel (Ni), manganese (Mn) and cobalt (Co), which sank into the core of the Earth during its molten stage. Hence over time the surface of the Earth became depleted in these elements. A further line of evidence for an iron rich core is Earth’s magnetic field observed with a compass. This magnetic field supports the theory of an iron rich core at the center of Earth. Hence siderophile elements can be thought of as elements that are more common in the center of the Earth, than on Earth’s near surface. This is why other rare siderophile elements like gold, platinum and pallidum are considered precious metals at the surface of Earth.

Goldschmidt also looked at elements common in the atmosphere, in the air that we breath and that readily form gasses with Earth’s temperatures and pressures. These atmophile elements include hydrogen and helium, which are only observed in meteorites as H2O and very little isolated helium gas. This is despite the fact that the sun is mostly composed of hydrogen and helium. If you have ever lost a helium balloon, you likely know the reason why there is so little hydrogen and helium on Earth. Both hydrogen and helium are very light elements and can escape into the high atmosphere, and even into space. Much of the solar system’s hydrogen and helium is found in the sun, which has a greater gravitational force, as well as the larger gas giant planets in the outer solar system, like Jupiter which has an atmosphere composed of hydrogen and helium. Like the sun, larger planets can hold onto these light elements with their higher gravitational forces. Earth has lost much of its hydrogen and helium, and almost all of Earth’s hydrogen is bonded to other elements preventing its escape.

Nitrogen is only found in trace amounts in meteorites, as the mineral carlsbergite, which is likely the source of nitrogen in Earth’s atmosphere. Another heavier gas is carbon dioxide (CO2), which accounts for about 0.1% of stony meteorites. However, in the current atmosphere it accounts for less than a 0.04%, and as a total percentage of the entire Earth much less than that. In comparing Earth to Venus and Mars, carbon dioxide is the most abundant molecule in the atmosphere of Venus and Mars, accounting for 95 to 97% of the atmosphere on these planets, while on Earth it is a rare component of the atmosphere. As a heavier molecule than hydrogen and helium, carbon dioxide can stick to planets in Venus and Earth’s size range. It is likely that Earth early in its history had a similar high percentage of carbon dioxide as found on Mars and Venus, however over time it was pulled out of the atmosphere. This process was because of Earth’s unusual high percentage of water (H2O). Notice that water is found in stony meteorites, and this water was released as a gas during Earth’s warmer molten history, and as the Earth cooled, it resulted in rain that formed the vast oceans of water on its surface today. There has been a great debate in science as to why Earth has these vast oceans of water and great ice sheets, while Mars and Venus lack oceans or significantly large amounts of ice. Some scientists suggest that Earth was enriched in water (H2O) from impacts with comets early in its history, but others suggest that enough water (H2O) can be found simply from the molten gasses that are found in rocks and meteorites that formed the early Earth.

So how did this unusual large amount of water result in a decrease of carbon dioxide in Earth’s atmosphere? Looking at a simple set of chemical reactions between carbon dioxide and water, you can understand why.

Note that g stands for gas, l for liquid, and aq as an aqueous solution (dissolved in water), and also notice that this reaction goes in both directions with the double arrows. Each carbon atom takes on an additional oxygen atom, which results in two extra electrons, this results in the ion CO3-2. This ion forms ionic bonds to two hydrogen ions (H+), forming H2CO3. Because these hydrogen ions can break apart from the carbon and oxygen, this molecule in a solution forms a weak acid called carbonic acid. Carbonic acid is what gives soda drinks their fizz. If water falls from the sky as rain, the amount of carbonic acid would cause a further reaction to solid rocks composed of calcium. Remember that calcium forms ions of Ca+2, making these ions ideal for reacting with the ions to form Calcium Carbonate (CaCO3) a solid.

Note that there is a 2 before the ion so that the amount of each element in the chemical reaction is balanced on each side of the chemical reaction.

Over long periods of time the amount of carbon dioxide will decrease from the atmosphere, however, if the Earth is volcanically active and still molten with lava, this carbon dioxide would be re-released into the atmosphere as the solid rock composed of calcium carbonate is heated and melted (a supply 178 kJ of energy will convert 1 mole CaCO3 to CaO and CO2).

This dynamic chemical reaction between carbon dioxide, water and calcium causes parts of the Earth to become enriched or depleted in carbon, but eventually the amount of carbon dioxide in the atmosphere will reach an equilibrium over time, and during the early history of Earth water scrubbed significantly amounts of carbon dioxide out of the atmosphere of Earth.

Returning to the bulk composition of meteorites, oxygen is found in numerous molecules, including some of the most abundant (SiO2, MgO, FeO, CaO). One of the reasons, Goldschmidt did not include oxygen in the atmophile group of elements was because it is more common in rocks, especially bonded covalently with silicon in silica dioxide (SiO2). Pure silica dioxide is the mineral quartz, a very common mineral found on the surface of the Earth. Hence oxygen, along with magnesium, aluminum, and calcium, is a lithophile element. Later we will explore how Earth’s atmosphere became enriched in oxygen, an element much more commonly found within solid crystals and rocks on Earth’s surface.

Isolated carbon (C) is fairly common (0.5%) in meteorites, but carbon bonded to hydrogen CH4 (methane) or in chains of carbon and hydrogen (for example C2H6) are extremely rare in meteorites. A few isolated meteorites contain slightly more carbon (1.82%) including the famous Murchison and Banten stony meteorites which exhibit carbon molecules bonded to hydrogen. Referred to as hydrocarbons, these molecules are important in life, and will play an important role in the origin of life on Earth. But why are these hydrocarbons so rare in meteorites?

This likely has to do with an important concept in chemistry called Enthalpy. Enthalpy is the amount of energy gained or lost in a chemical reaction at a known temperature and pressure. This change in enthalpy is expressed as (ΔH) and expressed in Joules of energy per Mole. A Mole is a unit of measurement that relates the number of atoms per gram of a molecule’s atomic mass. A positive change in enthalpy indicates an endothermic reaction (requiring heat), and while negative change in enthalpy releases heat resulting in an exothermic reaction (producing heat). In the case of hydrocarbon (like CH4) and the presence of oxygen, there is an exothermic reaction, that releases 890.32 kilojoules of energy as heat per mole.

The release of energy via this chemical reaction makes hydrocarbons such a great source for fuels, since they easily react with oxygen to produce heat. In fact, methane or natural gas (CH4) is used to generate electricity, heat homes and used to cook food on a gas stove. This is also why hydrocarbons are rarely found when closely associated with oxygen. Hydrocarbons are however of great importance, not only because of their ability to combust with oxygen in these exothermic reactions, but because they are also the major elements found in living organisms. Other elements that are important for living organisms are phosphorous (P), nitrogen (N), oxygen (O), sulfur (S), sodium (Na), magnesium (Mg), calcium (Ca) and iron (Fe). All of these lithophile elements are found in complex molecules within life forms near the surface of Earth which are collectively called organic molecules, which bond with carbon and hydrogen in complex molecules found within living organisms. The field of chemistry that study these complex chains of hydrocarbon molecules is called organic chemistry.

Goldschmidt’s classification of the elements is a useful way to simplify the numerous elements found on Earth, and way to think about where they are likely to be found, whether in the atmosphere, in the oceans, on the rocky surface, or deep inside Earth’s core.