Lesson 2 of 7
Objective: SWBAT explain why some isotopes are radioactive and how nuclear decay by particle emission results in a more stable nucleus..
I was absent the day between the Nuclear Fission lesson and today's lesson, so students completed the Intro to Isotopes and Stability PhET Lab in my absence. When I got to school that morning, I was able to quickly scan all the student work and found that while not complete, they had gotten the main ideas. This helped me alter my lesson plan a little, and take time to review and wrap up Fission before moving on.
Today is spent on defining the term "radioactive", and introducing the students to spontaneous decay. I utilize both PowerPoint and ExploreLearning Gizmos in today's lesson to keep students active. In past years, I included all the details on alpha, beta and gamma decay in the PowerPoint, but I am working very hard this year on having multiple activities in each lesson.
This lesson is aligned to the NGSS via HS-PS1-8: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay. We will also use computer models to understand how alpha decay occurs, in alignment with Science and Engineering Practice 2
Fission Review -- Opener
When students enter the classroom, I pass back their Fission Inquiry sheets from the day before. I ask students what is required to make the Uranium atom unstable to start fission. A student volunteer replies "It had to add a neutron from somewhere." I then ask, "Does fission happen all on its own then?" and call on a different student volunteer who responds "No, but once it starts, it can keep going."
I ask what it's called when the reaction keeps going on its own. Students flip their paper over and find the term "chain reaction". I then show them this YouTube clip, with the sound off due to the overbearing music.
I ask, "How that is an accurate model of a nuclear chain reaction?" A student says that the "mousetraps are like atoms, and the ping pong balls are like the neutrons-- when you add one, it starts a reaction and then another and another."
I then state: "Since fission needs something to start it, lets look at when atoms break down on their own." and pass out the student version of the notes.
On the opening slide of the PowerPoint, I refer back to the students doing the Isotopes and Stability PhET simulation. While they struggled with the details, they got that some atoms are naturally stable, and some are not. I ask if they remember why certain isotopes of elements were unstable. Regardless of answer, I use their response to transition into the next slide.
When we talk about stability, it is about the isotopes of an element having the right ratio of neutrons to protons. If they have too many of either particle, it makes them unstable. We then look at the graph and discuss how the correct ratio changes as elements get larger. I ask the students why they think that is. In one of my four classes a student responded "If they have lots of protons, won't they try to repel each other and break up the nucleus." In the other classes, I had to probe more and ask which particle would push on itself or the other particle, and coach out that the positive protons would repel if they don't have the right number of neutrons.
On the next screen I show students who the super stable elements are, and which elements tend to be unstable or radioactive. I do not differentiate between isotopes and elements here as we did not go into the math of the weighted average atomic masses. I explain the theory that I first read in The Disappearing Spoon by Sam Kean that the super stable nuclei are the most perfectly spherical, and that the really radioactive nuclei are oblong, shaped like footballs. I compare the super stable elements to the noble gases: noble gases had the right number of electrons, the super stable elements have the perfect ratio of neutrons to protons. This analogy led a student to ask "Then is Helium the most perfect element?" I responded that I didn't know, but that based on our discussion, it would certainly seem to be.
On the next slide, we define radioactivity, and bring in the root words to help students understand it. I remind students that the discovery of radioactivity is less than 150 years old, and that we continue to learn more about it every year.
We finish with outlining the three main types of radiation: alpha, beta, and gamma decay. When I've taught honors chemistry in the past, I've included more information such as positron emission and the like, but our general chemistry curriculum is limited to these three types of decay. I explain to students that these are the first three letters of Greek alphabet, and that they got their name from the order of their discovery.
As the students move to the computers and log in, I pass out the Nuclear Decay exploration sheet. This is not modified very much from the original document available on the ExploreLearning site. The warm-up orients students to the emission of alpha particles and gamma rays. Activity A goes further with investigation of how alpha particles change the identity and mass of the unstable isotope, and students perform their first decay equation.
I encourage students to complete the prior knowledge questions and read the Gizmo Warm-up sections while the computer is loading. I explain that they are only expected to get through the first two pages today, since we have less than half the period.
Students struggle initially on the review of mass and atomic number. They often think that everything is a completely new topic. When I, or their partner, remind them that we have been doing this since September, they relax and think and do just fine on the Prior Knowledge section.
In the Warm-up, students struggle with #2, because the gamma ray exits the viewing screen very quickly. I remind students that if they aren't seeing things properly, they should slow down the Gizmo or pause it and advance slowly.
When students move into Activity A, most of their struggles come from not going step by step in the directions. My students like to jump to the question marks without reading the instructions. Most of my time today is referring them back to the directions, making sure they follow them.
Even when following directions students still rush and make errors. In this student example, he correctly predicts what will happen to the mass and atomic numbers of an element undergoing alpha decay. All of his work in the "Calculate" section is correct, and then on part D, he flips them when creating his rule. Confusion atomic number and mass number is the most common error, but students generally did this mini-investigation well without much assistance.
Most students do not have time to finish part A, and get only through the first two pages. Students who complete all of part A make far fewer errors on confusing the atomic and mass numbers, and benefit from the additional practice with the alpha decay equations.
When students finish, or as we reach the final two minutes of the period, I encourage students to log out and turn in their papers.
Students who only complete the first two pages are still a little confused on what is happening, and how the decay equation is representing the changes to the nucleus. It is apparent that many students will need some check-ins during the following day's instruction and practice on alpha decay.