Electron Affinities and Metallic Character

Electron affinity and metallic character also exhibit periodic trends. Electron affinity is a measure of how easily an atom will accept an additional electron and is crucial to chemical bonding because bonding involves the transfer or sharing of electrons. Metallic character is important because of the high proportion of metals in the periodic table and the large role they play in our lives. Of the roughly 110 elements, 87 are metals. We examine each of these periodic properties individually in this section. 

The alkali halogenide gas molecules MX present a still more extreme case, the bonds being essentially ionic with only a small amount of covalent character. For cesium chloride, involving the most electropositive of the metals and one of the most electronegative of the nonmetals, the electron affinity of the nonmetal (86 kcal/mole) is about as great as the ionization potential of the metal (89 kcal/mole), so that at large intemuclear distances the ionic structure Cs+Cl is about as... 

Palladium ions were reduced by hydrogen at room temperature. The zeolite thus formed has hydroxyl groups identical to those found in de-cationated Y zeolites and probably has a Bronsted acid character. Furthermore, hydrogen reduction produces metallic palladium almost atomically, dispersed within the zeolite framework as demonstrated by our IR, volumetric, and x-ray (23) results. Palladium atoms are located near Lewis acid sites which have a strong electron affinity. Electron transfer between palladium atoms and Lewis acid sites occurs, leaving some palladium atoms as Pd(I). Reduction by hydrogen at higher temperatures leads to a solid in which metal palladium particles are present. The behavior of these particles for CO adsorption seems to be identical to that of palladium on other supports. 

The degree of ionicity in the bond between a metal atom and a polymer, or molecule, is related to the ionization potential and electron affinities of the substituents. The metals we have studied are of interest as electron injecting contacts in electronic devices. These metals must have a low ionization potential (or work function), of the same order as the electron affinity of the polymer, in order for the charge transfer process to occur. If the ionization potential of the metal is lower than the polymery-electron affinity, spontaneous charge transfer occurs which is the signature of an ionic bond. Thus, the character of the charge distribution in the metal-polymer complexes we are studying is related to the situation in the electronic device. 

Another clear example is the effect of methyl groups on B-subgroup metals. Table 11 compares stability data for mercury, thallium, and lead systems. There is now an effective reduction of (b)-class character due to the extra ligands CH3- This must be attributed to the lowered electron affinity of the metal orbitals if our discussion is correct. 

The chemistry of gold is more diversified than that of silver. Six oxidation states, from -I to III and V, occur in its chemistry. Gold(-I) and Auv have no counterparts in the chemistry of silver. Solvated electrons in liquid ammonia can reduce gold to give the Au" ion which is stable in liquid ammonia (E° = -2.15 V). In the series of binary compounds MAu (M = Na, K, Rb, Cs), the metallic character decreases from Na to Cs. CsAu is a semiconductor with the CsCl structure and is best described as an ionic compound, Cs+Au . The electron affinity of gold (—222.7 kJ mol"1) is comparable to that of iodine (-295.3 kJ mol-1). Gold in the oxidation state -I is also found in the oxides (M+ Au O2 (M = Rb, Cs) these, too, have semiconducting properties.1... 

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