Shiny, hard, and practically immune to corrosion — chromium is the element that makes steel "stainless." Add just 10% of this metal to ordinary iron, and it stops rusting. An invisible oxide film only 5 nanometers thick forms on the surface, shielding the metal from destruction.
But chromium is far more than gleaming faucets and bumpers. It paints rubies red and emeralds green. Without chromium there would be no heat-resistant jet turbines, no tool steels, and no lasers. It is the element that unites beauty and strength.
In nature, chromium occurs mainly as the mineral chromite (FeCr₂O₄). It is the fourth most abundant transition metal in Earth's crust. Its electron configuration [Ar]3d⁵4s¹ is unusual: instead of the expected 3d⁴4s², one electron "jumps" to the d-orbital for the extra stability of a half-filled shell.
Chromium has the highest hardness of any metal — 8.5 on the Mohs scale. This makes it indispensable for protective coatings and wear-resistant alloys.
Metallic chromium and trivalent Cr(III) compounds have low toxicity. In fact, Cr(III) is an essential trace element for humans, involved in glucose metabolism. By contrast, hexavalent chromium Cr(VI) is a potent carcinogen recognized by IARC. It causes lung cancer when inhaled and severe skin burns. Welding stainless steel produces toxic Cr(VI) aerosols. A respirator, gloves, and exhaust ventilation are mandatory when handling chromium compounds.
The name "chromium" comes from the Greek "chroma" — color. Its compounds produce vivid yellow, green, orange, and red shades. No other metal offers such a palette.
The same Cr³⁺ ion makes rubies red and emeralds green. The only difference is the crystal lattice: in corundum it creates red, in beryl — green.
Chromium is the hardest metal on Earth. Its Mohs hardness is 8.5. For comparison: iron is only 4, and diamond is 10.
Chromium is the only solid element that is antiferromagnetic at room temperature. Its magnetic moments line up antiparallel and cancel each other out.
A "chrome" bumper is coated with a chromium layer only 0.25 micrometers thick. Yet even that thin layer gives a mirror shine and scratch protection.
The chromium oxide film on stainless steel is only 2–5 nanometers thick. If scratched, it heals itself within seconds.
| Isotope | Mass (u) | Abundance | Half-life | Decay |
|---|---|---|---|---|
50Cr | 49.946044 | 4.34% | stable | — |
52Cr | 51.940508 | 83.79% | stable | — |
53Cr | 52.940649 | 9.50% | stable | — |
54Cr | 53.938880 | 2.37% | stable | — |
Analysis of crocoite