Chromium

Chromium breaks the expected filling pattern: its configuration is [Ar]3d5 4s1 rather than 3d4 4s2, because a half-filled d subshell, with five unpaired electrons, is especially stable. Its most important oxidation states are +3, found in green chromium(III) oxide and in very stable complexes, and +6, found in yellow chromate and orange dichromate, which are powerful oxidizing agents. The same Cr3+ ion makes rubies red and emeralds green; the difference comes from how strongly the surrounding crystal splits the energies of its d orbitals.

Chromium is a hard, lustrous, bluish-white metal that melts at 1907 °C. Like titanium and aluminium it protects itself with a thin oxide layer, and it passes that ability to steel: alloys with at least about 10.5 percent chromium form a self-healing Cr2O3 film and become stainless. Most of the chromium mined, as the ore chromite, ends up in stainless and alloy steels. The rest goes into plating, pigments, refractory bricks and leather tanning, though toxic hexavalent compounds are being phased out of many uses.

Discovery

In 1797 the French chemist Louis Nicolas Vauquelin analyzed crocoite, a brilliant orange-red lead mineral from the Ural Mountains known as Siberian red lead. He discovered a new oxide in it and in 1798 isolated the metal by heating the oxide with charcoal. He named it from the Greek chrōma, meaning color, because its compounds come in so many vivid hues.