Chromium

Chromium takes its name from the Greek for colour, and it earns it: the same ion gives ruby its red and emerald its green depending on the host lattice. Its industrial importance rests on passivation. Above about 10.5% chromium, steel grows an invisible, self-repairing chromium oxide film that stops corrosion dead — Harry Brearley in Sheffield noticed in 1913 that a rejected gun-barrel alloy in his scrap pile had failed to rust, and stainless steel followed. Chromium is also the textbook electron-configuration anomaly: instead of the expected 3d⁴ 4s², the ground state is 3d⁵ 4s¹, because a half-filled d subshell plus a half-filled s is lower in energy than the naive filling order predicts. Hexavalent chromium is a different story entirely — a potent carcinogen, and the contaminant at the centre of the Hinkley, California groundwater case.