1. Show students the Quantum Mechanics video and lead a class discussion of the contemporary atomic model. Ask students:

• What is the basic structure of the atom? (a small nucleus surrounded by tiny orbiting electrons and much space)
• What keeps electrons in orbit around the nucleus? (Electrons orbit in specific energy levels within the space surrounding the nucleus.)
• What can we know about individual electrons, and what is it impossible to know? (The exact position and momentum of an electron cannot be determined, but the probability of it occupying a particular location can be determined using wave equations.)

2. Pass out copies of the Quantum Mechanical Atom (PDF) handout and the Periodic Table of the Elements chart (PDF) handout and make sure that all students have scrap paper and pencils. Work through the Quantum document slowly with the entire class. Before moving on to the "Electron Configuration" section of the document, check to make sure that students have a clear understanding of the two tables in the "Levels and Sublevels" section.

3. Demonstrate how to write the electron configuration for lithium, the example given in the "Electron Configuration" section. Write the electron configurations for at least two more elements, such as beryllium and boron, so that students can begin to see a pattern.

4. Individually or in pairs, have students work out the electron configurations for the rest of the elements in row 2 of the periodic table (ending with neon). Then have students work out the electron configuration for sodium, in row 3, before reconvening as a class to discuss any difficulties they might have had moving up to a higher energy level.

5. Read the "Repeating Electron Patterns" section of the document with the class and work out the electron configurations for a few sample elements with atomic numbers of 21 or higher.

6. Now read the "Abbreviated Configurations" section of the document. Explain to the class that there are different ways to write electron configurations. Chemists usually prefer to use the most abbreviated notation possible. The electron configuration for krypton, for example, is usually written as [Ar] 4s² 3d¹⁰ 4p⁶. Write this configuration on the board.

7. Then write the following:

• 1s¹ 1s² 2s¹ 2s² 2p¹ 2p² 2p³ 2p⁴ 2p⁵ 2p⁶ 3s¹ 3s² 3p¹ 3p² 3p³ 3p⁴ 3p⁵ 3p⁶ 4s¹ 4s² 3d¹ 3d² 3d³ 3d⁴ 3d⁵ 3d⁶ 3d⁷ 3d⁸ 3d⁹ 3d¹⁰ 4p¹ 4p² 4p³ 4p⁴ 4p⁵ 4p⁶

Explain that this is the unabbreviated quantum number string for krypton, in which each individual piece of notation -- the 3p⁵, for example -- represents a single electron. It's obviously much more cumbersome than the abbreviated form, but this type of notation can help us make the connection between electron configuration and the shape of the periodic table.

8. Start by circling the s sublevel electrons in the string, one at a time. After circling each one, ask students to find on the Periodic Table of the Elements chart (PDF) handout the element for which that electron was the last to be added. For example, when you circle 3s², students should look for the element whose highest energy electron is in the 3s² position. This is magnesium. When you circle 4s¹, students should look for the element whose highest energy electron is in the 4s¹ position: potassium. Continue through all of the s sublevel electrons in the above string.

9. Repeat this exercise for all of the p sublevel electrons in the string.

10. By this time, most students will have recognized that those elements whose highest energy electrons are in s sublevels occupy the first two columns of their chart, and those whose highest energy electrons are in p sublevels occupy the last six columns. Discuss with students why they think this is. Remind students that there are two more sublevels, d and f. They hold a maximum of 10 and 14 electrons respectively. Each of these sublevels should also have its own block. Ask students how many columns they think each of these blocks should have.