Chromium at a glance
- Atomic number
- 24
- Symbol
- Cr
- Atomic mass
- 51.996 u
- Category
- Transition metal
- Group / period
- 6 / 4
- Electron configuration
- [Ar] 3d 5 4s 1
- State at 25 °C
- Solid
- Melting point
- 1907 °C
- Boiling point
- 2671 °C
- Density
- 7.15 g/cm³
- Electronegativity
- 1.66
- 1st ionization energy
- 653 kJ/mol
- Oxidation states
- +3, +6, +2
- Discovered
- 1797
Atomic structure
A neutral chromium atom has 24 protons and 24 electrons, and its most common isotope carries about 28 neutrons. One atom weighs about 8.63 × 10⁻²⁶ kg, so a mole of chromium (6.022 × 10²³ atoms) weighs 51.996 g. That nucleus holds 1.17 neutrons for every proton. The electrons fill 4 shells (K 2, L 8, M 13, N 1), which gives the configuration [Ar] 3d 5 4s 1. As a d-block element, both its outer s electrons and its inner d electrons can take part in bonding, which is why it shows more than one oxidation state. Its configuration breaks the simple filling order: one electron moves so that a subshell becomes half-full or full, which is more stable. It has 4 stable isotopes: Cr-50, Cr-52, Cr-53 and Cr-54. No other element has a stable isotope with mass number 52, so Cr-52 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) chromium is a solid. It melts at 1907 °C and boils at 2671 °C, so it stays liquid across a 764-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. Its density is 7.15 g/cm³, 7.15 times as dense as water. Its atoms have a radius of about 140 pm (Vanadium next door is 135 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1.66 on the Pauling scale. Pulling off its outermost electron takes 653 kJ/mol. A bond between chromium and chlorine would be polar covalent, with an electronegativity difference of 1.5. It releases 65 kJ/mol when it gains an extra electron. Its common oxidation states are +3, +6, +2, so in compounds you will usually meet it as Cr³⁺, Cr⁶⁺ and Cr²⁺. Group 6 also holds Molybdenum, Tungsten and Seaborgium, which share its outer-shell pattern.
Where it comes from
It makes up 102 mg/kg of Earth's crust, the 21st most abundant element there. In the human body it accounts for about 0.000003 % of body mass. It was first identified in 1797, making it the 32nd element to be discovered (counting the 11 known since antiquity together).