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Unformatted text preview: Chapter 7: Electron Configurations and the Periodic Table 295 Chapter 7: Electron Configurations and the Periodic Table Teaching for Conceptual Understanding This chapter explains the basis for the charges on monatomic ions introduced in Chapter 3. Instead of memorizing the charges, students will be able to determine the charges from the electron configurations. Some instructors will skip over a discussion of the Bohr model of the atom and start with the quantum mechanical model. Keep in mind that the Bohr model is a concrete model that students can visualize and it provides a foundation for the abstract quantum model. Although the writing of electron configurations and the drawing of the orbital boxes are algorithmic in nature, they both can be used effectively to assess student comprehension of electron arrangement in an atom. In addition to having students write configurations and draw boxes, ask them to explain these representations. Again, it is helpful to present students with incorrect representations and ask them to explain why they are wrong. For example, some students will write 1s 2 2s 2 2p 6 3s 2 4s 2 3d 4 for Co 3+ instead of 1s 2 2s 2 2p 6 3s 2 3p 6 3d 2 . This indicates they do not understand that when electrons are added to the 3d subshell it has a lower energy than the 4s subshell. Students who are still confused by the fact that a positive ion has lost electrons and a negative ion has gained electrons will probably have trouble understanding why a positive ion is smaller than its atom and why a negative ion is larger than its atom. Transparencies of computer generated electron probability distributions for H, He, Li can be used to illustrate this concept. The H and He overlays will have similar distributions in terms of circular area because each electron is in a 1s orbital; however, the overlay for Li covers a larger area due to its electron in a 2s orbital. When Li loses an electron to form a Li + ion, it is the 2s electron that is removed. By removing the overlay for Li, it becomes clear that the remaining electrons occupy a smaller space and the Li + ion would be smaller than the Li atom. A good way to pull together all the ideas of shells, subshells, orbitals, and electron arrangement, is to develop Table 7.3 one step at a time. For each shell, ask students (1) what subshells are available, (2) how many orbitals each subshell has, (3) how many electrons can each subshell hold, and (4) how many electrons can the shell hold. Show them how the sum of electrons in the subshells equals the electrons in a given shell. Even though the concepts in this chapter are complex and abstract, the questions used to assess student understanding often take the form of simple recall or lower level applications. Combining ideas and asking integrated questions can raise the cognitive level of an item. See Questions 126-130 and 152 in the Questions for Review and Thought....
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