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IonSizes - 606 CHAPTER I3 Bonding GeneraL Concepts FIGURE...

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Unformatted text preview: 606 CHAPTER I3 Bonding: GeneraL Concepts FIGURE I13 Sizes of ions related to positions of elements in the periodic table. Note that size generally increases down a group. Also note that in a series of isoelectronic ions. size decreases with increasing atomic number. The ionic radii are given in units of picometers. For isoelectronic ions. size generally decreases as Z increases. Li+ Be2+ - . . ' J 9 60 31 140 136 Mg” Als+ _ 184 181 C1334- 8114+ Sb5+ :0 00.) 113 198 221 216 from predominantly metals on the left-hand side of the periodic table to non- metals on the right-hand side. A given period thus contains both elements that give up electrons to form cations and ones that accept electrons to form anions. One trend worth noting involves the relative sizes of a set of isoelectronic ions—ions containing the some number of electrons. Consider the ions 02—, F‘, Na+, Mg“, and A13”. Each of these ions has the neon electron configu- ration. How do the sizes of these ions vary? In general, there are two impor- tant facts to consider in predicting the relative sizes of ions: the number of electrons and the number of protons. Since these ions are isoelectronic, the number of electrons is 10 in each case. Electron repulsions should therefore be about the same in all cases. However, the number of protons increases from 8 to 13 as we go from the 02— ion to the Al3+ ion. Thus, in going from 02— to A13 +, the 10 electrons experience a greater attraction as the positive charge on the nucleus increases. This causes the ions to become smaller. You can con- firm this by looking at Fig. 13.8. In general, for a series of isoelectronic ions, the size decreases as the nuclear charge (Z) increases. Example 13.3 Arrange the ions Se2_, Br‘, Rb+, and Sr2+ in order of decreasing size. Solution This is an isoelectronic series of ions with the krypton electron con- figuration. Since these ions all have the same number of electrons, their sizes will depend on nuclear charge. The Z values are 34 for Se2_, 35 for Br', 37 for Rb+, and 38 for Sr“. Since the nuclear charge is greatest for Sr“, it is the smallest of these ions. The Se2_ ion is largest. Se2' > Br" > Rb” > Sr2+ T T Largest Smallest ...
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