Electronic Structures and General Characteristics

As was shown in Chapter 6, a high resistance to oxidation for a metal, as expressed in its oxidation potential, may result from a high heat of sublimation to the atoms, from a high ionization potential, from a low energy of hydration for the positive ion produced, or from combinations of these three factors. Both the heat of sublimation and the ionization potential of a metal depend upon how efficiently the nuclear charge is shielded from the valence electrons. Now, in an atom of silver, the valence electron is a 5s electron. A good portion of the density of this outer elec- 

Iron cloud lies closer to the nucleus than the periphery of the completed 4d subshell. The valence electron is shielded from the positive nucleus only incompletely, thus being held more firmly (ionization potential 7.57 ev), than is the valence electron in the rubidium atom (ionization potential 4.19 ev), for which the shielding is more nearly complete. Likewise, there is attraction between the incompletely shielded nuclear charge of one atom in silver metal and the peripheries of the electron clouds of adjacent atoms and breakup of the metal structure to the individual atoms is far more difficult for silver (heat of sublimation 67 kcal per gram-atom) than for any of the alkali metals (heats of sublimation ranging from 20 to 36 kcal). If any one factor may be said to explain the nobility of the coinage metals, it would thus be the incomplete shielding of the valence electron by the inner d orbitals. 

The stabilities of the oxidation states within this series are disturbingly free from trends. For copper, the +2 state predominates for silver, the + 1 state is most important finally, of the known gold compounds for which a valence state of the metal may be easily assigned, most contain trivalent gold. At higher temperatures, the cuprous state of copper becomes more important than the cupric. 

Aside from the CuF 3 complex, the maximum coordination number for ions of the coinage metals is 4 the complexes of Cu(II) and Au(III) nearly always are tetracoordinated. Coordination numbers of 4 and 3 are known for the univalent ions, but coordination number 2 is much the more usual. 

The univalent ions of this group (and most of the compounds derived therefrom) are colorless they are characterized by complete d subshells. The polyvalent ions have incomplete d subshells their compounds would be expected to be (and generally are) colored (Chap. 7). 

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