Wednesday, November 9, 2011

Copper's Special Nature

GOLD is for the mistress--silver for the maid --
Copper for the craftsman cunning at his trade.

from Cold Iron by Rudyard Kipling (1910)
Copper, silver, and gold--we call them coinage metals for obvious reasons. They antedate recorded history because all three were found essentially pure in their uncombined "native" state. Later came smelting and the secrets for winning even more of them from their ores. I once read that around 85 % of all the copper ever mined is still in use.

We had a Copper Age followed by a Bronze Age, during which the metal played a paramount role. But ever since electrification, copper's biggest use has been for wire to conduct electricity. The secrets to copper's utility are its ductility, its conductivity, and its longevity; all three are related to electronics.

Here's a recipe for a metal having high ductility, conductivity, and nobility: Give it a full set of d-orbitals--a resilient layer of 10 perfectly paired electrons surrounding an inner core of 18 perfectly paired electrons (28 in all)--to fend off rapacious oxygen and the harsh world of oxidation. The perfection of copper's 3d subshell adds a sort of resonance like that in the noble gases.

But copper needs one more electron--29 in all. The 29th electron, the outer valence electron, is spherically symmetric. This 29th electron is responsible for copper binding to copper. In theory, just two copper atoms could couple to make a dicopper molecule and it's been studied. But Cu2 is unstable because the bonding is too weak. Instead, the atoms associate into metal.  But there's no directionality to their bonding and the bonds are very weak, lacking covalent character. In a sense, copper is like frozen mercury. This means that copper should easily deform--and so it does. What's more, because each copper atom has a single valence electron, there's room for easily moving electrons; this property translates from metal atoms to bulk metal and copper has an awesome conduction band--thus the conductivity exceeded only by silver for a pure metal.

What about copper's pretty red color? That comes from bathing in visible light. But copper only gives back the reddish portion of the spectrum.

The chemistry of copper is dominated by the chemistry of copper (I), "cuprous ion," from the loss of the 29th electron. But copper can also lose a second electron to make copper (II), "cupric" ion. I once got into an argument over on Althouse over whether copper has one or two valence electrons. Clearly it has two. The word "valence" derives from an atom's combining power and indicates how many electrons an atom can give or take.

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