Chromium

Chromium (Cr) is a transition metal with atomic number 24 and an atomic mass of 51.996. Its electron configuration is [Ar] 3d5 4s1.

Chromium key data

Symbol	Cr
Atomic number	24
Atomic mass (standard atomic weight)	51.996
Molar mass	51.996 g/mol
Protons	24
Neutrons (most abundant isotope, chromium-52)	28
Electrons (neutral atom)	24
Category	Transition metal
Period	4
Group	6
Block	d-block
Noble gas notation	[Ar] 3d5 4s1
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d5 4s1
Electrons per shell	2, 8, 13, 1

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of chromium

From PubChem, the US National Institutes of Health. Public domain.

Melting point	2180 K (1906.9 °C), 15th of 103 by melting point
Boiling point	2944 K (2670.9 °C), 43rd of 93 by boiling point
Density	7.15 g/cm³, 47th of 96 by density
State at room temperature	Solid
Electronegativity	1.66 (Pauling), 45th of 95 by electronegativity
Van der Waals radius	189 pm, 72nd of 99 by atomic radius
First ionisation energy	6.767 eV (652.9 kJ/mol), 54th of 102 by first ionisation energy
Electron affinity	0.666 eV (64.3 kJ/mol), 32nd of 57 by electron affinity
Common oxidation states	+6, +3, +2
Discovered	1797

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Chromium electron configuration

The full electron configuration of chromium is 1s2 2s2 2p6 3s2 3p6 3d5 4s1. With the noble-gas core collapsed it is [Ar] 3d5 4s1, and the outermost subshell is 4s1.

Chromium does not follow the simple filling order. Its measured configuration differs from the one the aufbau rule predicts, because a half-filled d subshell sits lower in energy than the arrangement the rule would give. This is a measured fact about the atom rather than an exception to be derived.

Bohr model of chromium (Cr): a nucleus and four shells holding 2, 8, 13 and 1 electrons from the inside out. — Bohr model of chromium (Cr). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of chromium (Cr), 1s2 2s2 2p6 3s2 3p6 3d5 4s1: one box per orbital, filled by Hund’s rule, showing six unpaired electrons, in 3d and 4s. — Orbital diagram of chromium (Cr). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one chromium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 51.996 / (6.02214076 × 10²³)
= 8.6341 × 10⁻²³ g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 51.996
= 1.1582 × 10²² atoms

Chromium on the periodic table

Chromium is in period 4, group 6, in the d-block.

The other elements of group 6 are molybdenum, tungsten and seaborgium.