Modeling Atoms and Ions

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Students will be able to use modeling software to ascertain rules about what makes a stable and neutral atom.

Big Idea

Atoms need to balance protons and neutrons in order to be stable. They are considered neutral when the number of protons and electrons are equal, but charge changes when electrons are gained or lost.


In this lesson students use modeling software available for free from PhET Interactive Simulations
University of Colorado found at this link. The software allows students to build an atom by dragging protons, neutrons, and electrons into a space, and they can monitor the effect that these additions have on the stability, charge, and mass of the atom they build. They then use this information to try to develop rules for how the subatomic particles affect the atom.

This lesson aligns with the NGSS Practice of the Scientist 2 Developing and using models by allowing students the opportunity to build something, to make predictions, and then see how their prediction plays out. It corresponds to the NGSS Disciplinary Core Idea of Structure and Properties of Matter by working with the concept "Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons."

Students do this lesson in my class after they have already been introduced to the periodic table, elements, and atoms as prescribed in this lesson and this lesson. However, this lesson could also serve as an introductory lesson for a unit on atoms and the periodic table.

Materials list:

  • Computer access for students with internet access
  • A projector for the teacher computer


Do Now/Activator

10 minutes

Do Now: Students begin class by signing out a computer and getting it up and running. I have them do this so that this will not slow us down later. I then administer the Element Quiz, which they have been studying for throughout the week. They have to correctly spell and locate the first twenty elements on using this template. As first articulated in this lesson, my rationale for memorizing makes the case for memorizing as one important type of student activity.

Activator: After ten minutes I have students exchange papers and use a periodic table from their chemistry book to correct another student quiz. I caution them that they will lose points if they give someone credit for an incorrect answer, and that they should pay special attention to the spelling of the elements. 

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: I begin the lesson by discussing the importance of modeling, as shown in this modeling video. I start the atom modeling portion of class by demonstrating how to use the model. You can drag protons, neutrons, and electrons into the nucleus by clicking on the subatomic particle and dragging it. You have to drag it to the right spot in order for it to stay. You can open up the boxes on the right by clicking on the plus signs, and this will reveal the symbol, mass number and net charge of the atom you are building. I ask students to play with this until they are comfortable with how the model works.

Guided Practice: I walk around and make sure students are comfortable with how to use the basics of this software. One reason that I like to use the software is that is provides a chance for students to get modeling experience. This NGSS practice is an important tool when manipulating an object is impractical. The software further assists with this goal by providing instant feedback to students about how things like charge and nuclear stability are affected by choices they make in designing their atom, which is feedback they won't get from the plastic models we use to model the atom.


25 minutes

Student Activity: During this time students work on a self-guided Atom Building exercise like the one found at this website. I use a slightly modified Phet atom builder lab that you could also use.

The point of either activity is that students learn about atoms by experimenting, coming up with a possible rule, and then testing to see if the rule works. students have the opportunity to grapple with coming up with a rule for what makes an atom neutrally, positively, or negatively charged, as well as what makes an atom stable, and what subatomic particle is responsible for creating an atom for a specific element.

I do not believe that this activity alone is going to be enough for students to learn the material that the model exposes them to. For example, in this student work the student does not understand the difference between the mass number and the atomic number.

However, I do believe that the model will give students another way to concretize subatomic particles so that they are less abstract. I can then refer back to the model when reteaching these concepts. I chose this method of instruction rather than embedding each of the activities into its own lesson because I think it is more efficient to concentrate on this one activity. Students were engaged in the modeling, and most reported that they had a better conceptual understanding of the atom after they got to "play" with one.

Catch and Release Opportunities: After most students had a chance to finish a section, I go back to that section and ask students to share what they learned. This debriefing video is an example of what this looked like.


10 minutes

I debrief this lab by demonstrating some of the rules that the class discovered using the model by using the model and a projector so that all students can see my work in real time.

First, atoms are stable when there are a similar number of protons and neutrons.

Second, the number of protons determines the element.

Third, a neutral atom is an atom with no charge; it has equal amouts of protons and electrons. A negative atom has more electrons than protons, while a positive atom has less electrons than protons.

Finally, the mass of the atom is determined by the number of protons and neutrons.

I tell students that we will return in more depth to each of these subjects, but my hope for today’s class was that they begin to understand the ideas, and to have a mental picture of the ideas we discussed today. 

Subsequent lessons on electrons, electron configuration, and ions all build on the subjects introduced by student use of the modeling software.