coinage metals
Copper, silver and gold are the three metals of group 11, sitting at the right-hand edge of the transition block. Humans have prized them for thousands of years and stamped them into coins — hence 'coinage metals' — because they are attractive, workable, and (silver and especially gold) resist tarnishing and corrosion. They are also the best electrical conductors of all the elements, with silver first, copper second and gold third. As a trio they show how a nearly full d shell tames the wilder transition-metal behavior.
Each has a single s electron over a full d10 shell (configurations like 3d10 4s1 for copper), so a +1 oxidation state, leaving the closed d10 core, looks natural — and silver(I) and gold(I) and copper(I) all exist. But the d10 core is not as inert as in the alkali metals, so higher states appear too, and there is a curious split: copper's stable aqueous state is copper(II) (d9), silver's is silver(I) (d10), and gold's most stable common state is gold(III) (d8, square-planar). Gold also shows striking relativistic effects — the fast-moving inner electrons of such a heavy atom are pulled in by relativity, contracting its 6s orbital, which is part of why gold is golden-colored (it absorbs blue light) rather than silvery, and why gold is so unreactive. All three readily form complexes, especially with soft ligands like cyanide and ammonia, which is exploited to extract gold and silver from their ores.
Their uses follow directly from their character. Copper wires the world (cheap and conductive); silver leads in conductivity and in photography, antimicrobial coatings and electronics; gold, almost perfectly unreactive, is used for jewellery, currency reserves, dentistry, and corrosion-proof electrical contacts. Silver and gold are 'noble' metals like the platinum group — they sit near the bottom of the reactivity series and below hydrogen, so they do not dissolve in ordinary acids (gold needs aqua regia). The coinage metals close the transition block by showing how, with the d shell full, these elements become beautiful, conductive and chemically calm.
Gold is so unreactive that no single common acid dissolves it; you need aqua regia, a 3:1 mix of concentrated hydrochloric and nitric acids. The nitric acid oxidizes a trace of gold to gold(III), and the chloride immediately ties it up as the stable complex [AuCl4]-, which pulls the equilibrium along and lets more gold dissolve. Neither acid alone would do it — they work only as a team.
Aqua regia dissolves gold because nitric acid oxidizes it and chloride locks it up as [AuCl4]- — neither acid works alone.
The three coinage metals do not share one favorite oxidation state: copper prefers +2, silver +1, gold +3. And gold's color and inertness are genuinely relativistic effects of its heavy nucleus, not just a quirk — a reminder that for the heaviest elements relativity becomes part of everyday chemistry.