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CHAPTER 8 ELECTRON CONFIGURATION AND CHEMICAL PERIODICITY 8.1 Elements are listed in the periodic table in an ordered, systematic way that correlates with a periodicity of their chemical and physical properties. The theoretical basis for the table in terms of atomic number and electron configuration does not allow for an “unknown element” between Sn and Sb. 8.2 Today, the elements are listed in order of increasing atomic number. This makes a difference in the sequence of elements in only a few cases, as the larger atomic number usually has the larger atomic mass. One of these exceptions is iodine, Z = 53 which is after tellurium, Z = 52, even though tellurium has a higher atomic mass. 8.3 Plan: The value should be the average of the elements above and below the one of interest. Solution: a) Predicted atomic mass (K) = Na Rb 2 + = 22.99 85.47 2 + = 54.23 amu (actual value = 39.10 amu) b) Predicted melting point (Br 2 ) = 2 2 Cl I 2 + = 101.0 113.6 2 + = 6.3°C (actual value = –7.2 ° C) c) Predicted boiling point (HBr) = HCl HI 2 + = 84.9 ( 35.4) 2 + = –60.15 = –60.2°C (actual value = –67.0 ° C) 8.4 The allowed vales of n : 1, 2, 3, 4... The allowed values of l : 0, 1, 2, ... n –1 The allowed values of m l : – l , (– l + 1), ... 0, ... ( l – 1), + l The allowed values of m s : ±1/2 8.5 The quantum number m s relates to just the electron; all the others describe the orbital. 8.6 Within an atom, no two electrons may have the same four quantum numbers. Within a particular orbital, there can be only two electrons and they must have paired spins. 8.7 In a one-electron system, all sublevels of a particular level have the same energy. In many electron systems, the principal energy levels are split into sublevels of differing energies. This splitting is due to electron-electron repulsions. Be 3+ would be more like H since both have only one 1 s electron. 8.8 Shielding occurs when inner electrons protect or shield outer electrons from the full nuclear attractive force. The effective nuclear charge is the nuclear charge an electron actually experiences. As the number of inner electrons increases, the effective nuclear charge decreases. 8.9 Penetration occurs when the probability distribution of an orbital is large near the nucleus, which results in an increase of the overall attraction of the nucleus for the electron, lowering its energy. Shielding results in lessening this effective nuclear charge on outer shell electrons, since they spend most of their time at distances farther from the nucleus and are shielded from the nuclear charge by the inner electrons. The lower the l quantum number of an orbital, the more time the electron spends penetrating near the nucleus. This results in a lower energy for a 3 p electron than for a 3 d electron in the same atom. 8-1
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