Are All Isotope Radioactive?

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SWBAT compare and contrast nuclear processes (fission, fusion and radioactive decay) in terms of number of protons, neutrons, electrons, type of energy.

Big Idea

The ratio of protons to neutrons is what determines if an isotope is stable or unstable. Isotope notation is a convenient way to represent key information about isotpes


In this lesson students explore the NGSS Performance Expectation 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 In order for students to understand radioactive decay they must first learn that there are different versions of atoms based on atomic mass called isotopes.  They will also learn that isotopes are dependent on the number neutron and that an isotope can be unstable and radioactive due to the difference in neutrons.  Finally they will learn how to write the correct isotope notation for elements and how they will change based on the number of neutrons on the atom.  

Since nuclear chemistry is difficult for students to explore in a traditional lab setting, they must use the NGSS SEP, Developing and using models to illustrate the nuclear decay process. In this lesson they use a PhET simulation that models how changing the number neutrons in an atom can affect the stability of an atom.  This simulation, just like many of PhET’s simulations provides students with a good visual that helps them utilize NGSS Science and Engineering Practice 2, developing and using models.

As a result of being able to model various isotopes, students will begin to see the Cross Cutting Concept, Energy and Matter, is an underlying theme in this unit.  This is first seen as students view an atom become unstable as they change the number of neutrons in the simulation.  This will be developmental in the process of understanding that in nuclear processes, atoms are not conserved, but the total number of protons and neutrons are conserved.


10 minutes

At the beginning of class as students walk in, I instruct them to grab a computer and the assignment for the day and then to begin the pre-lab vocabulary assignment of the Intro to Isotopes and Stability PhET Lab.

This vocabulary activity is a good introduction, as one of the main objectives of this lesson involves calculating different masses based on the number of neutrons. Most of the words that will be defined (proton, neutron, atomic # and mass number) are review and serve as a reminder about the atomic structure and how to calculate the mass of an atom.  The other words, isotope and radioactive, help students develop an understand that are a variety of individual atoms may contain a different number of neutrons than what is seen on the periodic table. This will lead into the idea that atoms become unstable as the number of neutrons change.

I give about 10 minutes to log on and define the definitions.  On the board, as they are logging in, I have and as suggested sites to use to find definitions.  As an alternative they can use their textbook to define the words; I allow this because some students are more comfortable using the glossary of the text, instead of a computer.

Once completed I instruct them to go to the PhET website and follow the procedures just below the definitions.  Most students have been to this site and are familiar with how to run the simulations.  I personally did not develop this activity, I believe it originated from the PhET website where several different activities are posted.  However, I found it on the internet and have slightly modified it by changing the analysis questions about heavy water to focus more on a comparison of heavy water to regular water.


25 minutes


The first part of this section has students play around with the simulation.  I do this so they can become familiar with it by making observations and have a little fun before getting into the assessment questions.  As they are playing I walk around to ensure that they have turned on the symbol   and abundance tabs.  After about 3-5 minutes I have them start answering questions 1-6 on the first page of the lab handout:

  1. How does the number of protons change as atomic number increase by one? By one
  2. How does the mass of the atoms change as atomic number increases by one? By one
  3. What effect does adding a neutron have on the atom’s identity? It can make the atom unstable
  4. What effect does adding a neutron have on the atom’s mass? Increase it by one
  5. Draw the nucleus of the most abundant isotope of each of the following atoms in the boxes below.  Be sure to count and label the protons and neutrons.  Also show the full atomic symbol.  Hydrogen has been done for you.  This can be seen on the answer key

I give them about 10 minutes to complete this before we go over the answers.  Most students are aware of the answers to 1-4 because they have calculated atomic mass in the beginning of the year, so I do not much time on this part.  Question 5-6 are the questions that students sometimes struggle with because it’s a new way of representing an atom.  I provide them with hydrogen and expect them to do carbon, oxygen and neon.  The answers to 5 and 6 can be obtained using the simulation by observing the symbol and abundance tabs (see key). 


As they are working on questions 1-6, I find it important to walk around to ensure they are getting the answers correct.  I let them know that the element notation, showing the symbol, atomic number and mass number, will be used throughout the unit to represent isotopes and it is key they understand how to perform this task.  

After I've ensured that all students have completed questions 1-6, I project the answers on the board to make sure everyone has the drawing of the atoms nucleus and element notation correct 


After showing the correct answers, they get extra practice on writing element notation by complete the chart on the second page of the lab.  To get them started I go over the first hydrogen (1st row) which can be seen on the answer key.  I then ask them if they can tell me what is different about the second form of hydrogen (row 2).  I am looking for students to respond with a different atomic mass, and specifically a different number of neutrons.

Next I guide the conversation towards stability and ask, if Hydrogen-2 and hydrogen-3 are stable or unstable?  I wait for students to respond with H-2 is stable, H-3 is unstable based on what they discovered using the simulation to answer questions 1-6.

I conclude the questioning by asking, what does unstable mean?   Even though most students do not understand that unstable elements are radioactive, it does begin to plant the idea in their minds that atoms can be unstable and radioactive in the process.  In the event that no one answers the question correctly, I do tell them the answer. This will prepare them for later in the unit when it comes to predicting nuclear decay.

As students are working on the table, I walk around and help students in need.  The students that tend to need the most help are the ones that struggled in the first unit on atomic structure.


10 minutes

As students are finishing the chart I instruct them to answer analysis questions 1-7 by doing an internet search (see Phet isotope):

  1. Water is H2O.  How many isotopes of hydrogen exist in nature?  (even unstable ones)
  2. Use the internet to search for “heavy water.”  What is this stuff?
  3. How does it behave, compared to ordinary water?
  4. Is Heavy water radioactive?  Why?
  5. Observe the atoms you determined to be unstable.  What can you conclude about the ratio of neutrons to protons and a nucleus’ stability?
  6. What makes Carbon-14 so useful in “carbon dating” or “radio dating”?
  7. Could a stable isotope of carbon be used in the same way?  Why or why not?

Even though most students have not heard of heavy water, they will begin to see that there are other forms of water based on the molecular weight (isotopes).  It also serves to inform students that not all isotopes are radioactive.  They should come across the fact that heavy water (which is not very abundant, 0.051%) is not radioactive, it is just slightly heavier due to two extra neutrons (one extra per hydrogen). 

Question 6 has them briefly research something that most have hear of before, carbon dating.  The goal of researching this is to give them an idea that there are practical applications for radioactive materials, such as dating ancient artifacts.

I end their internet search with 5 minutes left in class, so that they can complete an exit slip.

A couple of websites that I will suggest to get them started on heavy water are: and  These sites are a good platform to get them started, but I encourage them to check some other sites for homework.  After showing these sites on the board, I let them know that they will be responsible for searching 6 (carbon dating) on their own.  This helps build research skills that they will use throughout the course, a important part of the CCSS and NGSS.  If your students do not have access to computers, I highly recommend you show the movie on heavy water; it is extremely informative. 


5 minutes

As they are logging off the computers I hand out an exit slip.  I don’t collect the lab for a few reasons and use an exit slip: (1) I've helped most students throughout the process of completing the lab, (2) the exit slip serves an assessment that tells me who grasps the concept of isotopes and who does not, (3) I will go over it the following day at the beginning of class so they can have all the correct answers and (4) it generates conversation about the analysis questions the following day. 

Once completed I have them put them at the edge of their lab table face down till I pick them up.