In September, 1895, 24-year-old Ernest Rutherford said good-bye to his fiancee, Mary Newton. Rutherford was leaving the island of New Zealand, where he had grown up. His destination was Cambridge University in England, halfway around the world. He looked forward to working at the Cavendish Laboratory.
Rutherford brought with him his electromagnetic wave detector. He had found a way to detect radio signals using magnetized steel needles. In Cambridge he managed to send a message to a room three-quarters of a mile away. Impressed with Rutherford's talent, J. J. Thomson invited the young scientist to collaborate with him.
Under Thomson's guidance, Rutherford measured the electric current in gases ionized by X-rays and ultraviolet light. When the atoms in a gas acquire a positive or negative electric charge, the gas is said to be ionized. Rutherford wondered if Becquerel's uranium rays would ionize gas.
Rutherford set two parallel metal plates one above the other. He spread uranium powder on the bottom plate. After connecting the plates to a battery, he measured the electric current between them. The current told him that uranium rays were ionizing the gas.
Rutherford covered the uranium with foils of different substances and thicknesses. If the current was diminished, he knew the foils were blocking the rays.
The foils stopped some of the uranium rays. Other rays went through the foils. Rutherford concluded there were two types of rays. He called those that could not penetrate matter alpha rays, and those that penetrated matter easily beta rays.
Rutherford found that alpha rays were positively charged particles, with mass comparable to that of atoms. Beta rays turned out to be electrons. In 1900, the French physicist Paul Villard found a third type of radiation, which he called gamma. Gamma rays have no electric charge. They are electromagnetic radiation like light, X-rays, and radio waves.
While Rutherford studied uranium rays, he was looking for a new job. He wanted to start earning money so that he could marry Mary Newton. He also wanted a laboratory where he could continue to experiment with alpha rays. McGill University, in Montreal, Canada, offered him both.
In September, 1898, Rutherford left the Cavendish laboratory. He planned to get married as soon as he arrived in Montreal. But when he got there, he learned that a new professor could not immediately take a long vacation. It was another year and a half before Ernest Rutherford and Mary Newton became husband and wife.
In that year and a half, Rutherford found a new puzzle to think about. R. B. Owens, an electrical engineer, was trying to measure radiation from thorium. Every time Owens made a measurement, he got a different result. He knew that air currents were causing the problem, but he couldn't figure out how. He asked Rutherford for advice.
Rutherford realized that thorium was emitting a radioactive gas. Everything the gas touched became radioactive as well. Today we call the gas radon. In some areas of the world, radon seeps from the ground into buildings. Many people are concerned about health hazards from this naturally occurring radioactivity.
Sir William Crookes gave Rutherford yet another puzzle to solve. Crookes was studying uranium radiation. Before doing any experiments, he wanted to have a pure sample of a uranium salt. He carefully dissolved and crystallized uranium nitrate until he was sure it was pure.
Crookes placed the uranium nitrate on a photographic plate. Twenty-four hours later, he developed the plate. He expected to see a dark spot.
The plate was blank.
Crookes concluded that uranium itself could not be radioactive. A chemically distinct substance was the source of the activity. He called the unknown substance "uranium X."
Rutherford wondered if he could extract a similar substance from thorium. To find out, he needed help from a chemist. He asked 23-year-old Frederick Soddy to work with him.
Rutherford and Soddy extracted an active substance from thorium. They called it thorium X. Four days after the extraction, they found that the thorium X had lost half of its activity. The original thorium had regained exactly as much activity as the thorium X had lost.
Henri Becquerel noticed the same effect in uranium. Sir Crookes had given him some de-activated uranium nitrate and some uranium X. Many months later, when Becquerel checked it, he found the uranium X had lost its activity. The purified uranium nitrate, on the other hand, was once again radioactive.
Rutherford and Soddy figured out what was happening. Some atoms of thorium were spontaneously changing into atoms of thorium X. At the same time, some of the thorium X was losing its activity. The creation of new thorium X balanced the loss of activity.
His work on radioactivity made Rutherford famous. In 1908 he received the Nobel Prize in chemistry.
In Canada, Rutherford felt he was far away from the center of things. He longed to return to England where he would be closer to other physicists. In 1907, Arthur Schuster asked him to direct the physics laboratory at the University of Manchester. Rutherford gladly accepted the job.
At Manchester, Rutherford found a well-equipped laboratory. He also found a 25-year-old physicist named Hans Geiger. Geiger had been Professor Schuster's assistant. When Rutherford took over, Geiger agreed to stay and work with him on alpha particles.
Together, Rutherford and Geiger developed a detector that could count alpha particles. It was an early version of the Geiger-Mueller counter we use today.
Geiger used his counter to test a new way of detecting alpha particles. The new detector was a screen coated with zinc sulfide. Each time an alpha particle hit the screen, it would emit a tiny flash of light called a scintillation. In order to see the scintillations, Geiger had to peer at the screen through a microscope.
Now Geiger was ready to do an experiment. Rutherford had noticed that when a beam of alpha particles passed through a thin foil, its image on a photographic plate was blurred. The alpha particles were colliding with the atoms in the foil, and bouncing off at a small angle. When a beam of particles interacts with a target in this way, it is said to be scattered. Rutherford and Geiger hoped to learn something about atoms by counting the scattered particles.
Geiger got his alpha particles from a radioactive "source." To create a narrow beam of particles, Geiger placed his source behind a slit. Fifty-four centimenters from the slit he placed a zinc-sulfide detector. The alpha particles emerged from the slit and hit a spot in the center of the detector.
Geiger put a thin foil in front of the slit. The alpha particles went through the foil, but they did not come out in a narrow beam. They spread out like light from a flashlight.
Geiger moved his detector across the beam. At each new spot he counted the number of alpha particles hitting the screen. From his measurements he calculated the average angle of deflection of an alpha particle that had passed through the foil. It was very small, less than one degree from the center path.
At about this time, a student named Ernest Marsden joined Geiger at the lab. Geiger asked Rutherford to suggest a project for the young man. Rutherford told him to see if any of the alpha particles were being deflected by more than 90 degrees. That is, he was to see if any alpha particles were bouncing back toward the source.
A few days later, Geiger reported that Marsden had indeed observed alpha particles deflected by more than 90 degrees.
Rutherford was dumbfounded. From Geiger's measurements, he knew that the probability of small deflections adding up to more than 90 degrees was less than one in a billion. Yet when Geiger and Marsden counted alpha particles, they found that about one in 8000 was deflected by a large angle.
Rutherford could think of only one explanation for Marsden's result. The alpha particles must be colliding with something small and heavy inside the atom.
Hantaro Nagaoka had been right. The atom's positive charge was concentrated in the center, with the electrons in orbit around it. The small, heavy center of the atom is called the nucleus, from the Latin word meaning kernel, or small nut.
How did Rutherford know that the atom had a small heavy nucleus? Here are some demonstrations that will show you how physicists get information from scattering experiments.
To do the demonstrations, you will need the following:
You will need to make two cardboard pennies. Use the pencil to trace the outlines of two copper pennies on the cardboard. Carefully cut them out.
Make sure you can slide a penny easily across the table, or whatever surface you are using.
Now you are ready to do the first experiment. Place one copper penny in the middle of the table. This is your target.
Use your finger to shoot another copper penny at the target. This second penny is your projectile. With a little practice, you will be able to hit the target directly. When you do so, you will notice that the projectile slows to almost a complete stop, while the target slides away in the direction the projectile was originally moving.
Repeat this experiment using cardboard pennies for both target and projectile.
In the first experiment, you saw what happened when the projectile and the target were identical. But what if the target is much heavier than the projectile?
Use a copper penny for the target, and a cardboard penny for the projectile. When the projectile hits the target, the target will barely move. The projectile will bounce off at an angle. If it hits the target directly, the projectile will bounce straight back at you.
What if the target is much lighter than the projectile?
Use a cardboard penny as the target, and the copper penny as the projectile. After the collision, the projectile will continue to move, taking the target with it.
You have just demonstrated the law of conservation of momentum. Momentum is the mass of an object multiplied by its velocity. Velocity is speed in a specific direction. A heavy object has more momentum than a light object travelling at the same speed. An object moving quickly has more momentum than an identical object moving slowly.
Unless there is an outside force, such as gravity, acting on an object, it will move forever at a constant speed in the same direction. If it bumps into another object, it will share its momentum with the other object. Both objects may change the direction and speed of their motion, but if you add up the total momentum in any direction, it will be the same as before the collision.
Friction between your pennies and the table is an outside force. If you could eliminate the friction, then you would find that the total momentum in each of these demonstrations remains constant.
Arrange 24 copper pennies in six rows with four pennies in each rows.
Stagger the rows so that the pennies in one row line up in between the
pennies of the rows above and below it:
Leave about 30 centimeters (12 inches) between them. Without aiming at any particular target penny, shoot a cardboard penny into the arrangement. Most of the time the projectile will slide right through. Sometimes it will hit a target penny and bounce off at an angle.
Arrange the pennies as in the diagram, but this time leave only 5 centimeters (2 inches) between them. Again use a cardboard penny as your projectile. You will find that the projectile almost always hits a target penny, and that it often bounces off two or three.
Use the same arrangement of target pennies, but now use a copper penny as your projectile. When the projectile hits a target penny, the target penny will slide forward, sometimes all the way out of the array of pennies. The projectile will never bounce back.
Suppose you didn't know exactly what your target was made of. If you were to shoot it with a projectile of known mass and velocity, you could learn something about it. If the projectile always bounced back, you might conclude that the target consisted of heavy particles packed close together. If it always passed through the target, but came out at a small angle, then you would guess that the target was made of a lot of light particles.
What if the projectile usually went through, but sometimes, just once in 8000 times, it bounced back? Then you would guess that the target particles were very heavy, and very far apart. This is exactly what Rutherford guessed from Geiger and Marsden's experiment.
The demonstrations you have just done explain how particles share momentum in a scattering experiment. They do not accurately describe how the particles interact with each other. When an alpha particle from a radioactive source approaches an atomic nucleus, it does not actually touch it. The electric force between similar charges pushes it away.
You can get an idea of what is happening with the following demonstration. You will need:
Bend the coat hangar into a square. Stretch the stocking over it. Stick a toothpick in the styrofoam or clay so that is pointing straight up. Gently lower the stretched stocking over the toothpick so that the toothpick pushes the middle up. It should look like a pointy hill.
When an alpha particle approaches a nucleus, the repulsive electric force makes it feel as if it is going uphill. The faster, or more energetic, the alpha particle, the "higher," or closer to the center, it will go.
If the nucleus were infinitely small, what physicists call a "point charge," then the hill would be infinitely high. A projectile might come close, but it would never reach the top. No matter how fast they were going, some alpha particles would bounce back.
Now stick three toothpicks in the styrofoam, and lower the stocking over them. If the toothpicks are far apart, you will see three separate hills. If they are close together, you will see one hill, with three peaks on the top.
A slow projectile approaching a target never gets close to the center. The three particles close together would look like one. A fast particle might sometimes pass between the peaks, and sometimes be scattered by one of the peaks. A more energetic projectile "sees" more details of the target.
Rutherford used the idea of the repulsive electric force to develop a mathematical theory of alpha scattering. From his theory, Rutherford could predict how many alpha particles would be scattered in any direction.
To test Rutherford's theory, Geiger and Marsden made more measurements. By the time they were finished, they had counted more than 100,000 scintillations. Their experiments proved that Rutherford's theory was correct.
Rutherford used his theory to estimate the size of the nucleus. He found it was less than 6 X 10^-14, or 0.00000000000006 meters across. Today we know it is about a fifth that size. The atom is about 10^-10 meters across, nearly 10,000 times the size of the nucleus. The nucleus fills less than a billionth of the atom's volume.
Here was a picture of the atom nobody had imagined--a vast, nearly empty space with a tiny charged sphere in the center. If the nucleus were the size of a marble, then the atom would be nearly one kilometer (0.6 mile), across. Somewhere in that space were even tinier electrons. In a few strokes of the pen, Rutherford had transformed the solid world around us into empty space.
Rutherford's picture of the atom had the same problem as Nagaoka's Saturnian atom. It could not be stable. The electrons should have radiated energy and collapsed into the nucleus. Yet atoms remain stable for millions of years. How could this be?
A young Danish physicist named Neils Bohr thought of an explanation. He said that the electrons in an atom travelled around the nucleus in fixed orbits, like planets around the sun. Unlike a planet, an electron could not exist between orbits. To go from one orbit to another, it had to make a "quantum leap." When an electron was in its orbit, it did not radiate. It radiated only when it jumped from one orbit to another.
Bohr's theory explained how electrons were arranged around the nucleus, but the nucleus itself remained a mystery. What exactly was it made of? It would be nearly two decades before physicists had a satisfactory answer to that question.