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## Repeating Patterns: The Shape of the Periodic Table

Students explore element periodicity, the shape of the periodic table, and how we are able to predict the characteristics of elements yet to be discovered or created.

### Overview

This lesson--the third in a series of three lesson plans about the Periodic Table of Elements--explains why the elements exhibit periodicity, why the periodic table of elements is shaped the way it is, and how we are able to predict the characteristics of elements yet to be discovered or created. Students create electron configuration diagrams that describe the arrangement of electrons around the nucleus. This lesson is the third of three lessons and is intended as an enhancement activity following completion of the first two lessons. The first lesson, explored the origin of the periodic table. The second lesson, The Strange World of the Electron, described the structure of the atom.

### Objectives

• Discover that electron sublevel structure is responsible for the periodicity of elements
• Understand that the periodic table arranges elements in groups based on the energy sublevel of an element's highest energy electron

### Suggested Time

• Two class periods

### Part I: The Modern Atomic Model

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.)

### Part II: Electron Configuration

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 ins sublevels occupy the first two columns of their chart, and those whose highest energy electrons are inp 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.

### Check for Understanding

Ask students to find information on the physical properties of at least three elements on the periodic table, each from a different group, or column. Have students write a short paragraph about each element, explaining, in terms of electron configuration, why their chosen elements might have the properties they do.

## Educational Standards

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