Page 218 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

mercury; the gold and silver content dissolved, forming liquid alloys. The metals
were then obtained through simple distillation of mercury. However, the method
of choice for gold production is cyanidation that usually follows a froth flotation
process. The crushed ore is treated with an aqueous NaCN solution, along
with enough CaO to neutralize any acid present in the rock that would generate
highly toxic HCN. This results in the formation of a cyanoaurate complex (Eq. 22);
any silver that is present also forms the analogous cyanoargentate complex. Finely
divided zinc metal is then added to reduce the metal ions (Eq. 23). The addition of
base regenerates cyanide through formation of zinc hydroxide, which is more stable
than Zn(CN) 4 (Eq. 24).

ð22Þ 4Au þ O 2 þ 2H 2O þ 8CN ! 4 Au(CNÞ þ 4OH
2
2
ð23Þ 2 Au(CNÞ þ Zn ! Zn(CNÞ þ 2Au
2 4
2 2
ð24Þ Zn(CNÞ þ 4OH ! Zn(OHÞ þ 4CN
4 4
1
10
The electronic configuration of the coinage metals is nd (n +1)s . Hence, one may
suspect that Cu, Ag, and Au would share similar properties to the isoelectronic alkali
metals. However, it should be noted that a filled d shell is far less effective at shielding
an outer s electron from nuclear attraction than the less diffuse p shell. As a result, the
first ionization energy of the coinage metals is much higher than the alkali metals, and
their bonding is significantly more covalent in nature. This explains their relatively
higher melting points, hardness, density, and inertness relative to the alkali metals.
The unreactivity of the coinage metals increases dramatically from Cu to Au;
whereas copper and silver readily react with sulfur and halogens, gold is completely

unreactive to all reagents except very strong oxidizing acids such as aqua regia
(3:1 HCl/HNO 3 ). Acid rain produced from gaseous sulfur compounds (SO x ) react
with copper surfaces, eventually resulting in the formation of a basic copper sulfate
film, as evidenced by a green-blue-patina color. Another useful property of copper is
its antifungal behavior; fine granules of ceramic-coated copper oxide are now placed
within specialized asphalt shingles to prevent discoloration by algae. The activity
can last as long as the shingles, 25–30 years, until all of the Cu 2+ ions are leached
from the porous ceramic granules.
Although all metals possess metallic luster, the only metals that exhibit colors in
their bulk state are copper and gold. The familiar reddish and golden colors of these
elements arise from the filled d shell near the top of the conduction band of the solid.
By definition, the highest-occupied energy level at 0K is referred to as the Fermi
level. For copper, the gap from the top of the 3d-band to the Fermi surface is ca.
544 nm (Figure 3.30). Hence, energy in the green/blue region of the spectrum may
be absorbed, resulting in an observed red/orange color. For gold, the gap is ca.
400 nm, corresponding to absorption in the blue region of the spectrum and an
observed golden color. By contrast, the analogous energy gap for silver is 311 nm,
resulting in UV absorption and an observed white/silver color (i.e., equal reflection
of all visible wavelengths).