Summary

In this activity, students will compare two models of the atom using cognitive scaffolding to move from the more simplistic Bohr model to the more abstract and accurate quantum mechanical model. They will examine experimental data and use it to explain periodic trends that cannot be accounted for with the Bohr model.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• 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.
• Scientific and Engineering Practices:
  • Developing and Using Models
  • Analyzing and Interpreting Data

Objectives

By the end of this lesson, students should be able to:

• Compare/contrast the Bohr model and the quantum mechanical model.
• Use the quantum mechanical model to explain exceptions in periodic trends that cannot be explained by previous atomic models.

Chemistry Topics

This lesson supports students’ understanding of

• Atomic structure
• Bohr model
• Quantum mechanical model
• Electron configurations and orbital diagrams
• Periodic trends

Time

Teacher Preparation: 10 minutes
Lesson: 45 minutes

Materials

• Student handout

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• Prior to completing this activity, students should understand the basic structure of the atom (location, charges, relative masses of subatomic particles) and periodic trends. This activity is structured based on the assumption that students are already familiar with both the Bohr model and the quantum mechanical model. They will be asked to calculate the effective nuclear charge and write electron configurations and orbital notations. Many students have seen the Bohr model in middle school and this activity is a good way to help them become more comfortable with the more abstract (and more accurate) quantum mechanical model.
  • For the Bohr model diagrams, students will write the number of protons and neutrons in the center of the diagram. Students should recall that the number of neutrons is determined from the mass number of an atom, not the atomic mass. However, for this activity, they will use the most common isotope, and the most common isotope for the atoms selected have mass numbers equivalent to the atomic number rounded to the nearest whole number.
• This activity includes questions about periodic trends in ionization energy, so they should be familiar with this concept as well. The idea that half-filled sublevels are relatively stable (though still less than completely filled sublevels) should also be a familiar idea.
• I highly recommend that students complete the “Electron Configuration” POGIL activity before completing this activity. The POGIL activity uses an ingenious analogy of a hotel (the atom) with various types of “rooms” (orbitals) that are filled with “guests” (electrons) in a very strict pattern. The analogy really helps the students understand the more abstract orbital diagrams.
• If students are not yet very comfortable with electron configurations and orbital diagrams, it may be helpful to walk them through the first example or two as a class, and students may benefit from completing the activity in pairs or small groups.
• Comparing and contrasting the two models will foster the growth of scientific reasoning skills. Students will use the quantum mechanical model to explain what might appear to be abnormalities in periodic trends based on the Bohr model but are in fact predictable when viewed through a quantum mechanics lens. For instance, they will use the relative stability of half-filled sublevels to explain the lower ionization energy of oxygen vs. nitrogen. The Bohr model cannot explain this since it does not account for electrons pairing up within orbitals or slightly different energies in different sublevels of the same energy level.
• My preference is to use one color to represent the shielding electrons and another color for the valence electrons.
• The effective nuclear charge (Z_eff) in this activity is calculated as Z_eff = Z – S, where Z is the number of protons and S is the number of core electrons. (Note: This is a simple way of approximating the effective nuclear charge. A slightly more advanced way to approximate Z_eff would be to use Slater’s rules to account for shielding effects of same-level electrons, which could be added if the class is more advanced. It is still an approximation, but it is a little more nuanced than just subtracting the number of core electrons.)
• If students would like evidence that the sublevels exist, then photoelectron spectroscopy (PES) can be discussed. This is primarily an AP Chemistry topic, and there is an AP Chemistry POGIL “Photoelectron Spectroscopy” activity that helps students grapple with this concept.