Numford
Electronic configuration

Chromium (Cr)

[Ar] 3d⁵ 4s¹, an exception. 6 unpaired electrons, period 4, group 6, d block.

Element or ion

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H1He2Li3Be4B5C6N7O8F9Ne10Na11Mg12Al13Si14P15S16Cl17Ar18K19Ca20Sc21Ti22V23Cr24Mn25Fe26Co27Ni28Cu29Zn30Ga31Ge32As33Se34Br35Kr36Rb37Sr38Y39Zr40Nb41Mo42Tc43Ru44Rh45Pd46Ag47Cd48In49Sn50Sb51Te52I53Xe54Cs55Ba56Hf72Ta73W74Re75Os76Ir77Pt78Au79Hg80Tl81Pb82Bi83Po84At85Rn86

CrChromium, Z = 24, 51.996 u

[Ar] 3d5 4s1

In full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

[Ar]3d4s
  • Blockd block
  • Period4
  • Group6
  • Valence6
  • Unpaired6
  • MagneticPara

Last electron 3d

  • n3shell
  • l2d subshell
  • m+2orbital
  • s+½spin up
Why Cr is an exception

Plain filling would give [Ar] 3d⁴ 4s². A half-filled d subshell (d⁵) is extra stable: five electrons with parallel spins in five orbitals give the largest exchange energy and a symmetrical charge cloud. So one s electron moves into d.

Working

Step by step

  1. Chromium has 24 electrons.
  2. Fill in (n + l) order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴.
  3. That gives [Ar] 3d⁴ 4s², but the accepted configuration is [Ar] 3d⁵ 4s¹. A half-filled d subshell (d⁵) is extra stable: five electrons with parallel spins in five orbitals give the largest exchange energy and a symmetrical charge cloud. So one s electron moves into d.
  4. Write the subshells in order of n: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹.
  5. Replace the argon core with [Ar]: [Ar] 3d⁵ 4s¹.
At a glance

Chromium (Cr): the numbers

Atomic number24
Atomic mass51.996 u
Period4
Group6
Blockd block
Valence electrons6
Unpaired electrons6
Magneticparamagnetic
Last electron3d: n = 3, l = 2, m = +2, s = +1/2
Questions

Chromium (Cr): quick answers

What is the electronic configuration of chromium?

Chromium has 24 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹, or [Ar] 3d⁵ 4s¹ in short.

Why is chromium [Ar] 3d⁵ 4s¹ and not [Ar] 3d⁴ 4s²?

A half-filled d subshell (d⁵) is extra stable: five electrons with parallel spins in five orbitals give the largest exchange energy and a symmetrical charge cloud. So one s electron moves into d.

What are the quantum numbers of the last electron of chromium?

The last electron goes into 3d: n = 3, l = 2, m = +2, s = +1/2. Orbitals fill from m = −l up, spin +½ first.

Try any element or ion