The physics and optical properties of gold

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Chapter
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Gold: Science and Applications, 2009, pp. 13 - 30
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2009-01-01
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© 2010 by Taylor and Francis Group, LLC. The high corrosion resistance of gold is a consequence of its rst ionization potential being 9.2 eV, which is high compared to those of, for example, silver and copper, at 7.6 and 7.7 eV, respectively [6]. This results in such a large barrier to oxidation that elemental gold is ordinarily free of an oxide coating. Gold has the electronic conguration [Xe]4f145d106s1. In this conguration the 4f electrons underscreen the 5d and the 6s, p electrons from the nuclear charge, resulting in an effect analogous to lanthanide contraction. The lanthanide contraction causes the atomic radius of the lanthanides to decrease across the period as the quality of the shielding per electron decreases; in the 5d metal series this effect gives similar lattice constants to the 4d metals. For some years the stability of the Au2 dimer and the reduced lattice constant in bulk gold compared to silver were attributed to a similar contraction [7]. However, gold is in a unique spot on the periodic table where the effects of the lanthanide contraction are also superimposed on the onset of relativistic effects. The latter become increasingly important for the heavier elements because as the atomic number increases, the velocity of the 1s electrons approaches the speed of light. This causes these electrons to increase in mass and so their orbital contracts toward the nucleus. To compensate, the higher s and p orbitals, which also have signicant electron density in the vicinity of the nucleus, also contract, resulting nally in the outermost 6s and 6p orbitals being smaller than they would otherwise have been without relativistic effects (see Figure 2.1; [10-12]). The contraction of the s and p orbitals causes an increased screening of the core for the 5d and 5f electrons, which do not have much electron density in the vicinity of the nucleus and which are therefore, in the rst instance, less susceptible to relativistic effects of their own. Thus, the 5d and 5f orbitals are destabilized and expand outward, with an associated increase in energy [8,9]. The contraction of the 6s orbitals across the lanthanides and 5d metals due to relativistic effects is presented in Figure 2.2. Pyykkö and Desclaux [9] have estimated the average radial velocity, vr, of the 1s electrons as vr = Z a.u., where Z is the atomic.
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