Chromium (Cr)
Chromium (Cr) is a transition metal with atomic number 24 and a standard atomic weight of 51.996. At room temperature it is a solid that melts at 1,907 °C and boils at 2,671 °C.
Chromium key facts
Chromium at a glance
Identity
| Symbol | Cr |
|---|---|
| Atomic number (Z) | 24 |
| Atomic weight | 51.996 ± 0.001 (CIAAW 2024) |
| Category | Transition metal |
| Group | 6 |
| Period | 4 |
| Block | d-block |
| CAS number | 7440-47-3 |
Atomic properties
| Electron configuration | [Ar] 3d5 4s1 Full: 1s2 2s2 2p6 3s2 3p6 3d5 4s1 |
|---|---|
| Electrons per shell | 2, 8, 13, 1 |
| Oxidation states | +3, +6 Less common: -4, -2, -1, 0, +1, +2, +4, +5 |
| Electronegativity (Pauling) | 1.66 |
| First ionisation energy | 6.766 eV (652.9 kJ/mol) 2nd: 16.486 eV · 3rd: 30.959 eV |
| Electron affinity | 0.666 eV |
| Atomic radius (empirical) | 140 pm |
| Covalent radius | 122 pm |
| Van der Waals radius | 206 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 1,907 °C · 3,464.6 °F · 2,180.15 K |
| Boiling point | 2,671 °C · 4,839.8 °F · 2,944.15 K |
| Density | 7.15 g/cm³ |
| Specific heat capacity | 0.449 J/(g·K) |
| Thermal conductivity | 93.9 W/(m·K) |
| Crystal structure | Body-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 52Cr 83.789%, 53Cr 9.501%, 50Cr 4.345%, 54Cr 2.365% |
| Stable isotopes | 5 |
| Known isotopes | 30 |
| Abundance in Earth's crust | 102 mg/kg |
| Abundance in seawater | 0.0003 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Chromium electron configuration
Chromium has 24 electrons. Written in short form, its ground-state configuration is [Ar] 3d5 4s1 — the electrons of the noble gas argon, plus 3d5 4s1.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d5 4s1.
An exception to the filling rule. The simple aufbau (Madelung) order predicts 4s2, 3d4, but chromium actually has 4s1, 3d5. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.
Discovery and name
1797 — Louis-Nicolas Vauquelin · France.
Greek chrōma, colour — its compounds are vividly coloured.
Known for
Gives stainless steel its corrosion resistance and chrome plating its shine. Rubies are red and emeralds green because of chromium ions.
Isotopes of chromium
Naturally occurring chromium is a mix of 4 isotopes. Each has 24 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 52Cr | 24 | 28 | 83.789% | stable |
| 53Cr | 24 | 29 | 9.501% | stable |
| 50Cr | 24 | 26 | 4.345% | stable |
| 54Cr | 24 | 30 | 2.365% | stable |
30 isotopes of chromium are known in our data set (half-lives as published in nuclear data tables).
Chromium compared with group 6
The other elements in group 6, from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Chromium | 24 | 51.996 | 1,907 °C | 2,671 °C | 7.15 g/cm³ | 1.66 |
| Molybdenum | 42 | 95.95 | 2,622 °C | 4,639 °C | 10.2 g/cm³ | 2.16 |
| Tungsten | 74 | 183.84 | 3,422 °C | 5,930 °C | 19.3 g/cm³ | 2.36 |
| Seaborgium | 106 | [269] | — | — | 35 g/cm³ (pred.) | — |