Subatomic Particles and Isotopes

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Students will be able to determine the number of protons, neutrons, and electrons in an atom, explain what an isotopes is, and calculate average mass.

Big Idea

Atoms are composed of subatomic particles; the number of these particles determines an atom's identity and mass.


In this lesson students learn how to determine the number of different types of subatomic particles in atoms, what an isotope is, and how to calculate a weighted average. 

  • This lesson aligns with NGSS Physical Science Performance Expectation (HS-PS1-1): "Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms".
  • This lesson highlights NGSS Science and Engineering Practice 2: Developing and using models because it leads students in refining an earlier model of an atom, has them draw atoms, construct atoms with manipulatives, and finally to build atoms on the computer using a computer animation.
  • In addition, students are constantly asked to look for patterns in understanding how to determine the number of subatomic particles for an atom and to help lead them to the definition of an isotope.  Therefore this lesson deals with NGSS Crosscutting Concept 1: "Patterns".


There are several resources that I utilize in this activity.

  1. When students construct isotopes using manipulatives I give them a bag of "stuff" to use.  I include bingo chips, string, paperclips, pipecleaners, and little plastic pieces.
  2. When students work on the PhET activity they need to have access to computers or iPADs.


15 minutes

In order to engage students in the lesson I begin by having students revisit the model of the atom that they stared in the previous lesson.

  • Students are instructed to get out their atom models paper and to draw a final model in the "Final Model" box.  I remind students that, "A model should be a physical representation of a mental idea and used to explain a concept."  Additionally I tell students, "Your models should be well labeled so that others can understand what you are drawing."
  • As students complete their final models I instruct them to reflect on their final vs. initial models through performing two analysis questions:
  1. What is the same in your two model?
  2. What is different in your two models?
  • As students complete the questions I have them share with their partners at their table.
  • As all students are done and talking with their partners I choose several students to share their models with the class.  As they speak about their models I will take their papers and place under the document camera so other students can see their models.
  • This is an example of one student's model paper.  Notice that although his model has improved quite a bit that it still shows some gaps in terms of where neutrons are located, what exactly the nucleus is, and size/location of electrons.
  • Here is another example of a student's model paper.  In this model the student has gained the knowledge that there is an nucleus with protons and neutrons and electrons outside the nucleus in orbitals, but definitely shows the electrons as a Bohr model in exact orbits around the nucleus.
  • This is a Video that gives you a glimpse of how I have students perform these models in my classroom.


15 minutes

In this section I introduce students to the idea of how we can determine the number of subatomic particles in an atom through using atomic number and atomic mass.

I use the Bohr model of the atom to show students how to draw models reflecting the number of protons, neutrons, and electrons.  At this point I do not have students worry about where they put the electrons.  I tell them, "to put two electrons in the first energy level (circle), eight in the next, and up to eighteen in the third energy level, and that we will discuss more about where to put electrons in the next class."

I present information using the first six slides of the attached PowerPoint and students fill in information on the first page of their Graphic Organizer.   Here is a copy of one student's filled in notes.

Some students get confused when doing examples when they see the atomic mass.  I make sure to stress to students to round to the nearest whole number before they subtract atomic number from atomic mass.


10 minutes

In this next section of the lesson students are given time to explore three isotopes of Hydrogen to help them determine the definition of an isotope.  Students do this using the top half of the second page of their notes graphic organizer.

My goal is that students are able to determine that all isotopes have the same number of protons, same number of electrons, and same atomic number (questions number 2 and 5).  Additionally, they are to determine that isotopes have a differing number of neutrons and atomic mass (questions 3 and 4).  Through this analysis their definition of an isotope should be that they are atoms that have the same atomic number (or number of protons/electrons), but different atomic mass (or number of neutrons).

Over the years I found that students struggle with the concept of isotopes so this way of helping them figure out the definition on their own helps students to better remember the concept.

As students complete the analysis I review answers on a blank copy of the graphic organizer and call on groups to help with the answers to the questions.  As students answer I fill in the answers on the top part of the graphic organizer as can be seen in this example.

Here is a Video showing how this works in my classroom in terms of leading students in the process of discovering the definition of isotopes.


30 minutes

In this section students have time to synthesize what they have learned about atoms and isotopes through completing two group activities:

  • For the first group activity I have students use manipulatives found in a bag (I include string, pipe cleaners, paperclips, bingo chips, and plastic pieces) to make 3 models of the isotopes of Carbon (Carbon-12, Carbon-13, and Carbon-14).  
    • The instructions I give students are found on the 10th slide of the Notes PowerPoint. 
    • Here is a Video which shows how I introduce this activity to students and shows students working on the activity.
  • As students are working I walk around and ask what each manipulative represents.  I make sure to ask all group members for answers. 
    • These are some examples of students models (ex 1, ex2, ex3.)
    • Here is a Video of my checking for understanding why students are performing their activity to ensure that they are all understanding what the manipulatives represent in terms of isotopes and atoms.
  • When students are done I have them start the second activity.  For the second activity students go up to their lab stations and complete the PhET computer simulation "Build an Atom".  This activity allows students to build atoms.  I have questions that they answer on a worksheet to ensure that they understand what happens to atoms when you add protons, neutrons, and electrons.  Although we have not yet discussed ions, this allows students to begin to see what would happen so it will make more sense when we refer back to it later in the unit.  
    • This is an example of student answers to the activity.
    • Here is a Video which shows students working on the activity and my checking to make sure that they are learning what happens to an atom when you change protons, neutrons, and/or electrons.
  • If students are done with the computer activity before others I have them continue to play with the simulation on Build Atom screen or have them play the Atom Game.



30 minutes

In this section of the lesson I help students deepen their understanding of isotopes through understanding the idea of weighted averages. I lead students using slides 12 - 17 on the Power Point and they fill in the second half and third page of the notes graphic organizer.

I scaffold this lesson by having students first think about normal straight averages through using examples of test results.  I then go through examples of how to calculate weighted averages using the same test results, but with weights applied.

This Video shows how I am able to scaffold new chemistry content using the idea of weighted averages.

After this, I lead students through examples dealing with Chemistry.  I help students a lot with the first problem, and then give them time to work through the other problems on their own and walk around to help out.  As students complete each problem I go over answers on the board.

Here is an example of student work with answers to the questions.


20 minutes

For the last part of class students have time to begin their homework.  Any work they do not complete in class they will finish at home and bring in the next day when I will stamp and review.  This is the answer key to the homework.

On the homework most students feel comfortable with calculating protons, neutrons, and electrons. 

The major area where students struggle is with calculating average atomic mass.  I make sure to review the answers by going through examples on the board and stress that they just need to follow the steps that we went over in class.

Although weighted averages is an important concept, it is not one that I end up testing students on.  I find that it is a "nice to know" concept, but not one that I stress that all students understand.