Species Z e configuration S Z eff Z S Mg 12 1 s 2 2 s 2 2 p 6 3 s 2 Other e in

Species z e configuration s z eff z s mg 12 1 s 2 2 s

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Species Z e configuration S Z eff (= Z S ) Mg 12 1 s 2 2 s 2 2 p 6 3 s 2 Other e in same valence shell: 1 12 – 9.15 = 2.85 Core e in n – 1: 8 Core e in < n – 1: 2 S = 1(0.35) + 8(0.85) + 2(1.00) = 9.15 Mg + 12 1 s 2 2 s 2 2 p 6 3 s 1 Other e in same valence shell: 0 12 – 8.80 = 3.20 Core e in n – 1: 8 Core e in < n – 1: 2 S = 8(0.85) + 2(1.00) = 8.80 Mg 2+ 12 1 s 2 2 s 2 2 p 6 Other e in same valence shell: 7 12 – 4.15 = 7.85 Core e in n – 1: 2 Core e in < n – 1: 0 S = 7(0.35) + 2(0.85) = 4.15 The ionization energies of Mg, Mg + , and Mg 2+ are 731, 1450, and 7730 kJ mol –1 , respectively. The large increase in ionization energy on going from Mg + to Mg 2+ corresponds to the removal of a core electron from the n = 2 shell in Mg 2+ . A similar increase in effective nuclear charge is seen on going from Mg + to Mg 2+ , illustrating that the electrons in the n = 2 shell of Mg 2+ are more strongly attracted to the nucleus. Electron Affinities and Metallic Character 8.67 (a) Na or Rb Na has a more positive electron affinity than Rb. In column 1 electron affinity becomes less positive as you go down the column. (b) B or S S has a more positive electron affinity than B. As you trace from B to S in the periodic table you move to the right, which shows the value of the electron affinity becoming more positive. Also, as you move from period 2 to period 3 the value of the electron affinity becomes more positive. Both of these trends sum together for the value of the electron affinity to become more positive. (c) C or N C has the more positive electron affinity. As you trace from C to N across the periodic table you would normally expect N to have the more positive electron affinity. However, N has a half-filled p sublevel, which lends it extra stability, therefore it is harder to add an electron. (d) Li or F F has the more positive electron affinity. As you trace from Li to F on the periodic table you move to the right in the period. As you go to the right across a period the value of the electron affinity generally becomes more positive. 8.69 (a) Sr or Sb Sr is more metallic than Sb because as we trace the path between Sr and Sb on the peri- odic table we move to the right within the same period. Metallic character decreases as you go to the right. (b) As or Bi Bi is more metallic because as we trace a path between As and Bi on the periodic table we move down a column in the same family (metallic character increases). (c) Cl or O Based on periodic trends alone, we cannot tell which is more metallic because as we trace the path between O and Cl we go to the right across a period (metallic character decreases) and then down a column (metallic character increases). These effects tend to oppose each other, and it is not easy to tell which will predominate. 150 Chapter 8 Periodic Properties of the Elements
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(d) S or As As is more metallic than S because as we trace the path between S and As on the periodic table we move down a column (metallic character increases) and then to the left across a period (metallic character increases). These effects add together for an overall increase.
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