Electronic configuration of chromium and copper

Table 8.2 indicates that the expected electronic configuration of chromium (Z = 24) differs from the observed configuration. This can be explained on the basis of the concept of additional stability associated with the completely filled and half filled subshells.

The general electronic configuration of the elements of the 3d series is $\mathrm{3d^{1-10}\ 4s^2}$, with the exceptions of Cr and Cu. The 3d and 4s orbitals are close in energy, and in order to gain extra stability the last electron, instead of occupying the 4s orbital, occupies the 3d orbital. That assigns Cr the $\mathrm{3d^5,\ 4s^1}$ and Cu the $\mathrm{3d^{10},\ 4s^1}$ configuration:

• chromium — a half filled 3d subshell: $\mathrm{[Ar]\ 3d^5\ 4s^1}$, rather than the expected $\mathrm{[Ar]\ 3d^4\ 4s^2}$;
• copper — a completely filled 3d subshell: $\mathrm{[Ar]\ 3d^{10}\ 4s^1}$, rather than the expected $\mathrm{[Ar]\ 3d^9\ 4s^2}$. …