Atomic number and electron count
Chromium, \(\mathrm{Cr}\), has atomic number \(Z=24\). A neutral chromium atom therefore contains \(24\) electrons. Electron configuration notation distributes these \(24\) electrons among orbitals subject to the Pauli exclusion principle and Hund’s rule.
Aufbau prediction and the chromium exception
The usual Aufbau order places the \(4s\) subshell slightly below the \(3d\) subshell when building configurations for neutral atoms, so a straightforward prediction after argon is
\[ \text{predicted: }\mathrm{Cr} \;=\; \mathrm{[Ar]\,3d^4\,4s^2} \]
Chromium is one of the classic exceptions because the energy difference between \(4s\) and \(3d\) is small, and electron–electron effects make a half-filled \(3d\) subshell unusually stable.
A half-filled \(d\) subshell, \(3d^5\), maximizes parallel-spin occupancy across the five \(d\) orbitals and reduces electron pairing. This increases stabilization associated with exchange energy and lowers repulsion relative to a more paired arrangement.
Ground-state electron configuration of Cr
The experimentally supported ground-state electron configuration of chromium is
\[ \mathrm{Cr} \;=\; \mathrm{[Ar]\,3d^5\,4s^1} \]
The configuration corresponds to one electron occupying \(4s\) and five electrons occupying \(3d\) with parallel spins distributed one per \(d\) orbital (Hund’s rule). The total electron count matches \(24\):
\[ \mathrm{[Ar]} = 18,\quad (3d^5)=5,\quad (4s^1)=1,\quad 18+5+1=24 \]
Full configuration notation
The condensed \(\mathrm{[Ar]}\) core expands to the full configuration:
\[ \mathrm{Cr} \;=\; 1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^5\,4s^1 \]
Orbital occupancy diagram
The orbital-box view below shows the valence-region occupancy for chromium: one electron in \(4s\) and one electron in each of the five \(3d\) orbitals. The diagram inherits the site’s link/accent color for emphasis without hard-coded colors.
Comparison of predicted vs observed configuration
| Model description | Condensed configuration | Key feature |
|---|---|---|
| simple Aufbau prediction | \(\mathrm{[Ar]\,3d^4\,4s^2}\) | paired electrons appear sooner in \(3d\) |
| chromium ground state | \(\mathrm{[Ar]\,3d^5\,4s^1}\) | half-filled \(3d^5\) with five unpaired electrons |
Electron configuration of common chromium ions
Transition-metal cations lose electrons from the \(4s\) orbital before the \(3d\) orbital in typical ionic formation. Chromium therefore follows the pattern “\(4s\) first” for ionization.
| Species | Configuration | Electron-removal note |
|---|---|---|
| \(\mathrm{Cr}\) | \(\mathrm{[Ar]\,3d^5\,4s^1}\) | ground state |
| \(\mathrm{Cr^+}\) | \(\mathrm{[Ar]\,3d^5}\) | \(4s\) electron removed |
| \(\mathrm{Cr^{2+}}\) | \(\mathrm{[Ar]\,3d^4}\) | next electron removed from \(3d\) |
| \(\mathrm{Cr^{3+}}\) | \(\mathrm{[Ar]\,3d^3}\) | additional \(3d\) removal |
Common pitfalls
The chromium exception is often memorized as \(\mathrm{[Ar]\,3d^5\,4s^1}\), with the physical origin tied to the small \(3d/4s\) energy gap and the special stability of a half-filled \(d\) subshell.
Chromium ions commonly create confusion because electron removal typically occurs from \(4s\) before \(3d\), even though \(4s\) fills before \(3d\) in neutral-atom Aufbau writing.