Chromium (Cr)
“Its very name means colour — its compounds are green, yellow and orange, and a few percent of it turn rust-prone iron into stainless steel.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
Characteristics
- Configuration [Ar] 3d⁵ 4s¹ — a famous rebel against the aufbau rule: instead of the expected 3d⁴ 4s², the atom empties one 4s electron to half-fill its d-subshell, buying extra stability.
- Among the hardest of the pure metals — a chromium nugget scratches glass — with a melting point of 1907 °C, well above iron’s.
- Passivation champion: the self-healing Cr₂O₃ film keeps chromium and its stainless alloys indifferent to water, air and most acids.
- Its compounds keep chemistry colourful: Cr³⁺ is green, chromate CrO₄²⁻ yellow, dichromate Cr₂O₇²⁻ orange, and the orange crystals of potassium dichromate are a school-lab icon.
A steely-grey, lustrous metal, hard enough for a pure sample to scratch glass, taking a mirror-bright polish that the years cannot dull. The lasting shine is a chemical trick: air instantly cloaks the surface in a film of Cr₂O₃ only a few atoms thick, and that shield lets nothing through.
Where you meet it
✦ Uses
- Stainless steel — cutlery, kitchen sinks, surgical instruments and water tanks all use iron alloyed with about 18% chromium, the shield that refuses rust.
- Chrome plating — the mirror-bright handlebars, rims and bumpers of bicycles, motorbikes and buses are electroplated chromium laid over nickel.
- Potassium dichromate — the orange oxidising agent of school and college titrations, turning green as it accepts electrons from the analyte.
- Pigments and tanning — chrome yellow once painted taxis and school buses; chromium sulfate still “tans” most of the world’s leather.
◍ Everyday examples
- The stainless steel spoon and tiffin-carrier of a Bangladeshi kitchen carry their shine only because chromium hides inside the alloy.
- The mirror-flash of a bicycle’s chrome handlebars in morning traffic is electroplated chromium quietly doing its second-favourite job.
- A goldsmith’s ruby and the green of an emerald both borrow their fire from chromium atoms trapped inside the crystal.
History & name
In 1797 the Parisian chemist Louis-Nicolas Vauquelin, handed the dazzling orange-red crystals of crocoite — “Siberian red lead” — from a Russian gold mine, found the colour came from a lead salt of an unknown acid, and hiding inside it a brand-new metal. Early in 1798 he roasted the new oxide with charcoal and won the first tiny grey beads of chromium. Because the element’s compounds flared in colour after colour — chrome yellow, chrome green, the ruby’s red — he named it chromium, from the Greek chroma, “colour”. Nature, it turned out, had been employing it as her painter all along: traces of chromium make rubies red and emeralds green.