Chromium (Cr)

Chromium is the reason stainless steel does not rust: above about 11% it forms a self-repairing chromium oxide film a few atoms thick, and scratching it simply makes a new one. The name comes from the Greek for colour, because its compounds run through the whole spectrum — the red of ruby and the green of emerald are both trace chromium in otherwise colourless crystals. Its configuration is one of the classic textbook exceptions: [Ar] 4s¹ 3d⁵, with an electron promoted out of the 4s shell to give a half-filled d subshell, not the 4s² 3d⁴ that filling order predicts.

Properties

Atomic number

24

Symbol

Cr

Standard atomic weight

51.996 u

Category

Transition metal

Group

6

Period

4

Block

d-block

Electron configuration

[Ar] 4s¹ 3d⁵

Electrons per shell

2, 8, 13, 1

Melting point

2,180 K — 1906.8 °C — 3464.3 °F

Boiling point

2,944 K — 2670.8 °C — 4839.5 °F

Density

7.15 g/cm³

Electronegativity

1.66 (Pauling)

Oxidation states

+6, +3, +2

Discovered

1797 by Louis-Nicolas Vauquelin

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵

Noble gas shorthand

[Ar] 4s¹ 3d⁵

Shells

K: 2 · L: 8 · M: 13 · N: 1

What it is used for

• Stainless steel — everything above about 11% chromium is stainless
• Chrome plating on taps, tools and older car trim
• Leather tanning, which uses chromium(III) salts
• Refractory brick lining furnaces and kilns
• Green and yellow pigments, and the red of a ruby

Hazards

• Chromium(VI) compounds are carcinogenic by inhalation and readily absorbed through skin
• Welding or grinding stainless steel can generate hexavalent chromium fume
• Chromium(III), the form in food and supplements, is far less hazardous than chromium(VI)

Anomalous electron configuration: [Ar] 4s¹ 3d⁵ rather than the predicted 4s² 3d⁴. A half-filled d subshell plus a half-filled s is lower in energy than the filling-order arrangement.

Neighbours in the table

Group 6

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Chromium sits in group 6 with molybdenum, tungsten, seaborgium.