Atomic number 24

Chromium

Cr
+ phase
− phase

Shells Orbitals 3d 4s

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24 electrons in 4 shells: 2, 8, 13, 1.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Transition metal
Period: 4
Group: 6
Block: d
Standard atomic weight: 51.996(1) Da
Electron configuration: [Ar] 3d⁵ 4s¹
Phase at 25 °C: solid
Melting point: 1907 °C
Boiling point: 2671 °C
Density: 7.15 g/cm³
Electronegativity (Pauling): 1.66
Discovered: 1797, Louis Nicolas Vauquelin

Neighbours in the table

V Mn

By the usual filling rules chromium’s 24 electrons would end 3d⁴ 4s². In fact they end 3d⁵ 4s¹, or 2, 8, 13 and 1 across four shells. OpenStax names chromium among the exceptions to the filling order. Such exceptions arise where two subshells lie very close in energy, and small effects then reorder the filling. A half-filled subshell apparently represents a preferred stability, enough to draw one electron from 4s into 3d. No simple method, the book adds, predicts the exceptions.

In ions the 4s electron leaves first, the rule for every transition metal. Three gone leaves Cr³⁺, [Ar] 3d³, the ion stable in water. Six gone on paper is chromium(VI), the highest state OpenStax’s rule for scandium through manganese allows. Molybdenum and tungsten below it in group 6 stay mostly at +6 in solution. Their ions are larger, and an electron farther from the nucleus is easier to remove.

Heated in air the metal forms Cr₂O₃, the pigment chrome green. Chromium(VI) oxide, CrO₃, is one of two transition-metal oxides OpenStax counts as soluble in water; it dissolves as the acid H₂CrO₄. Electronegativity, an atom’s pull on the electrons it shares, rises with oxidation state. In a very high state a metal pulls almost as hard as oxygen, so its oxide is covalent, and OpenStax finds such oxides acidic.

Where it is found

PubChem’s record puts 102 milligrams of chromium in a kilogram of crust and 3 × 10⁻⁴ milligrams in a litre of seawater. The ore is chromite, FeCr₂O₄, which Los Alamos finds from Zimbabwe and Finland to Turkey and the Philippines.

The first chromium came from a different mineral. In 1797, the year Los Alamos gives, Louis Nicolas Vauquelin found a new element in Siberian red lead, the mineral crocoite, PbCrO₄. Jefferson Lab’s account in PubChem’s record has him doubting the metal could be isolated, then obtaining it in 1798 by heating the oxide in a charcoal oven. The name is from chroma, Greek for colour, after its many coloured compounds, CIAAW’s note there says.

The US Geological Survey estimates 2024 world chromite output at 47 million metric tons gross weight. South Africa’s 21 million led, then Turkey’s 8 million and Kazakhstan’s 6.5 million. The United States mined none. It bought 96 per cent of its chromite from South Africa over 2020 to 2023; in 2024 net imports met 77 per cent of consumption. Resources sit 95 per cent in Kazakhstan and southern Africa, enough, the survey judges, for centuries.

Why it matters

Stainless steel is chromium’s leading end use, and the US Geological Survey finds no substitute for it there or in superalloys. Jefferson Lab’s text puts the chromium in stainless steel at 10 per cent or more; OpenStax’s corrosion chapter calls it a small amount. Chromium also hardens steel, and chromium steels go into cutting tools, ball bearings, safes and armour plate.

Los Alamos calls the metal steel-grey, lustrous and hard; NIOSH adds brittle. Plated onto another metal it gives a hard, handsome surface that resists corrosion. That shine and that resistance, Los Alamos says, are why so much car trim is chromium.

Chromium forms many coloured compounds, Jefferson Lab notes. A chromium(III) ion has three d electrons. Ligands, the molecules or ions bound to the metal, leave its d orbitals no longer equal in energy, OpenStax explains. The gap often matches a photon of visible light, and the eye sees the complement of whatever colour the ion removes from the white.

That account stops at partly filled d subshells. Chrome yellow, lead chromate, holds chromium(VI) with no d electrons, a case the rule would leave colourless, yet it is coloured all the same. Ruby is corundum, aluminium oxide, with a small amount of chromium, OpenStax says; Los Alamos adds that chromium gives glass an emerald green.

Properties

Properties of chromium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	24
Standard atomic weight	51.996(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1907 °C	°C	Los Alamos: 1907 °C
Boiling point	2671 °C	°C	Los Alamos: 2671 °C
Density	7.15 g/cm³	g/cm³
Electronegativity (Pauling)	1.66
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵
Phase at 25 °C	solid

Safety

Chromium metal in a plated tap or a stainless pan is essentially harmless: half the reports behind PubChem’s GHS entry, 1,202 of 2,404, assign it no hazard. IARC, the International Agency for Research on Cancer, rates the metal group 3, not classifiable; the hazard statements that remain are skin and respiratory sensitisation and eye irritation. NIOSH’s card for chromium metal says fine dust burns rapidly though the bulk solid is noncombustible, and it sets a workplace exposure limit. That card and the PubChem Laboratory Chemical Safety Summary are the sheets for anyone who works with the metal.

Questions

Is chromium toxic?

It depends on the oxidation state. PubChem's safety summary puts chromium metal and chromium(III) compounds in IARC group 3, not classifiable, and reports no known ill effect from the chromium(III) in food. Chromium(VI) compounds are IARC group 1: inhaled, they are an established cause of lung cancer, and they can ulcerate the skin and the nasal septum. By one estimate the summary quotes, the gap is at least 500-fold, and the body can convert some chromium(VI) to chromium(III).

Do we need chromium in our diet?

Yes, in tiny amounts. OpenStax's anatomy text lists chromium among the trace minerals, found in cheese, lean meats, whole grains, black pepper, thyme and brewer's yeast. Its job is given as insulin function, and a shortage shows as high blood sugar, triglycerides and cholesterol. The body requires it as the chromium(III) ion, OpenStax's chemistry text says, and stable chromium isotopes have been given to patients to follow insulin function in diabetes.

Is chromium magnetic?

Only weakly. University Physics lists chromium among paramagnetic materials, with a room-temperature susceptibility of 0.00031. In such a material an applied field aligns only about a third of the atomic dipoles, and the field they add is small. Ferromagnetism needs magnetic domains, and the text says only iron, cobalt, nickel, gadolinium and dysprosium contain them. The chromium(III) ion is paramagnetic too: its three d electrons are unpaired, and an unpaired electron's spin gives an ion a magnetic moment.

Why doesn't stainless steel rust?

The chromium alloyed with the iron stops it. In OpenStax's corrosion chapter the chromium in the alloy gathers towards the surface and corrodes there, leaving an oxide layer over the iron that chemists call passivating: it protects the metal beneath. Rust on plain iron gives no such cover. It flakes off, fresh iron meets the air, and the corrosion continues. Chromium alone does this job; the US Geological Survey lists no substitute for it in stainless steel.