copper chemistry
Copper was one of the first metals humans ever used, and it is still everywhere: the reddish wires that carry our electricity, the green patina on old roofs and statues (the Statue of Liberty is copper), and the blue of copper sulfate in the garden and the lab. Sitting near the end of the transition row, copper has a slightly different feel from its neighbors — its full-ish d shell makes it less typical — but it still shows the colors, complexes and variable states that mark the block.
Copper lives mainly in two states, copper(II) and copper(I). Copper(II), the d9 'cupric' ion, is the common one in water: the familiar blue [Cu(H2O)6]2+, which deepens to royal blue [Cu(NH3)4]2+ when ammonia replaces the water. As a d9 ion, copper(II) shows a textbook Jahn-Teller distortion — its octahedral complexes are not perfect octahedra but are stretched along one axis, because an unevenly filled eg level makes the symmetric shape unstable, so two bonds lengthen and four shorten. Copper(I), the d10 'cuprous' ion, is colorless (a full d shell means no d-d color) and, crucially, unstable in water: it disproportionates, 2 Cu+ giving Cu2+ + Cu metal, so copper(I) survives in water only when locked up in insoluble salts (like CuCl, CuI) or stable complexes. Copper metal itself is unusually unreactive for a first-row metal — it does not displace hydrogen from acids — reflecting its position near the noble end of the block.
Copper's uses lean on its physical and electrical excellence: it is second only to silver as an electrical conductor, so it wires the world; it alloys into bronze (with tin) and brass (with zinc); and copper(II) compounds serve as fungicides and pigments. Biologically copper is essential, sitting in the electron-carrying and oxygen-handling enzymes (like cytochrome c oxidase, the final step of respiration, and the blue copper proteins). Copper's combination of a nearly full d shell, easy Cu+/Cu2+ switching, and Jahn-Teller-distorted complexes makes it a fitting capstone to the descriptive chemistry of the first transition row.
Copper(I) cannot survive freely in water: 2 Cu+ gives Cu2+ + Cu, an example of disproportionation. So a copper(I) salt like CuI stays as copper(I) only because it is insoluble; dissolve copper(I) in water as a free ion and it immediately splits into blue copper(II) and a sludge of copper metal.
Free copper(I) disproportionates in water into copper(II) and copper metal; it survives only when insoluble or complexed.
Copper(II) is the stable aqueous state, not copper(I) — the reverse of what the simple 'noble-gas core' guess might suggest. The d9 copper(II) ion also reliably shows Jahn-Teller distortion, so its 'octahedral' complexes are really stretched, with four short and two long bonds.