Chromium
Element 24 · Transition Metal
Chromium is a hard, lustrous, steel-gray transition metal best known for two things: the dazzling finish of chrome plating and its role in stainless steel, where it forms a protective oxide film that stops rust. It famously breaks the expected electron-configuration pattern — a half-filled 3d subshell ([Ar] 3d5 4s1) is more stable than the predicted 3d4 4s2.
Chromium chemistry spans colorful, relatively benign +3 compounds and toxic +6 compounds such as chromates and dichromates, which are strong oxidizers and known carcinogens handled only with strict safety controls.
Key facts
| Atomic number | 24 |
|---|---|
| Atomic weight | 51.996 |
| Category | Transition Metal |
| Group / Period | 6 / 4 |
| Block | d |
| Electron configuration | [Ar] 3d5 4s1 |
| Common oxidation states | +6, +3, +2 |
| State at STP | Solid |
| Electronegativity | 1.66 |
| Atomic radius | 166 pm |
| Density | 7.19 g/cm³ |
| Melting point | 1907 °C |
| Boiling point | 2671 °C |
Occurrence
Chromium is obtained from the ore chromite (iron chromium oxide), mined mainly in South Africa, Kazakhstan, and India. It never occurs as a free metal in nature, and its name comes from the Greek chroma, meaning color, for the brilliant hues of its compounds.
Applications
Stainless steel requires at least 10.5% chromium for corrosion resistance
Chrome plating gives a hard, mirror-like finish to car parts and fixtures
Chrome yellow and chrome green pigments color paints and inks
Leather tanning uses chromium(III) salts for most modern leather
Refractory bricks for furnaces use chromite minerals
Common compounds
- chromium(III) oxide
- potassium dichromate
- sodium chromate
- chromium trioxide
The red of a ruby and the green of an emerald both come from traces of chromium.
Data source: IUPAC standard atomic weights 2021