Gold, electron affinity

Bromine has a lower electron affinity and electrode potential than chlorine but is still a very reactive element. It combines violently with alkali metals and reacts spontaneously with phosphorus, arsenic and antimony. When heated it reacts with many other elements, including gold, but it does not attack platinum, and silver forms a protective film of silver bromide. Because of the strong oxidising properties, bromine, like fluorine and chlorine, tends to form compounds with the electropositive element in a high oxidation state. 

Bashong, W., Lucocq, J. M., and Roth, J. (1985) Thiocyanate-Gold small (2-3 nm) colloidal gold for affinity cytochemical labeling in electron microscopy. Histochem. 83,409 11. 

Bendayan, M. (1984) Enzyme-gold electron microscopic cytochemistry a new affinity approach for the ultrastructural localization of macromolecules. J. Electron Micros. Tech. 1, 349-372. 

The energy with which electrons are bound in conducting materials is known as the electron affinity of the material. Materials with a high electron affinity bind electrons strongly and exhibit noble behavior (i.e., are relatively inert and do not oxidize spontaneously in air). Gold is an example. On the other hand, metals such as aluminum or copper are less noble and their surfaces, once exposed to air, are readily oxidized. When two dissimilar electronic conductors are placed in contact with each other, electrons flow from the material that is less noble (e.g., copper) to the more noble material (e.g., palladium) until an equilibrium is reached and the contact potential is formed at their junction. Because of the multitude of possible combinations of conductors in the real world, contact potential is the most ubiquitous of all junction potentials. 

Gold is more difficult to ionize and, in consequence, to obtain cations. This situation also explains the high electron affinity and the possibility to accept electrons to give anions. 

On the other hand, Gold shows large relativistic effects (the Gold maximum — see eg. [21]). In fact, it has been explicitly demonstrated that for Au relativistic and arc-effects are nonadditive [22]. This is most obvious for its electron affinity While a nonrelativistic Cl-calculation [23] gives a value of 1.02 eV and a fully relativistic Coupled-Cluster calculation [22] yields 2.28 eV, the corresponding nonrelativistic and relativistic Hartree-Fock values are 0.10 eV [22] and 0.67 eV, respectively. Thus immediately the question arises to which extent the GGA s failure for metallic Au is due to the neglect of relativistic arc-contributions in Exc[n. ... 

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... 

The electron affinities of clusters behave in a similar manner. This fact, undoubtedly, has a role to play in the chemistry exhibited by nanometals that has been reported in the literature recently. For example, it has been shown that Au atoms (Gold is a noble metal in the bulk state) supported on a TiOa substrate shows a marked size effect in their ability to oxidize the diatomic gas CO to CO2 via a mechanism involving O2 dissociative chemisorption and CO adsorption (Valden et al., 1998). Small Ni particles have also been found to dissociate CO (Doering et al., 1982). Smaller nanoparticles of Ag can dissociate molecular oxygen to atomic oxygen at low temperatures, whereas in the bulk state, the species adsorbed on the Ag surface is O2 (Rao et al., 1992). 

The small affinities of lithium and sodium are of little importance, but copper, silver and gold, with completed d shells, possess marked electron affinity and whereas the alkaline earth metals, with completed s levels, have negative electron affinities, mercury, with a completed d and s level, has a high positive electron affinity. 

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