Question

Explain the exceptional electronic configuration of Cr.

Answer 1

1. Chromium (Cr) has an exceptional electronic configuration due to the extra stability of half-filled subshells.
2. The expected configuration is 3d⁴ 4s², but this is unstable because 3d is neither half-filled nor fully filled.
3. One electron from 4s enters 3d to make 3d⁵ 4s¹, giving both 3d and 4s half-filled subshells.
4. Half-filled subshells (like 3d⁵) provide extra stability because of symmetrical distribution and exchange energy.
5. This configuration lowers repulsion and stabilizes the atom more than the expected filling order.

Thus, Cr’s configuration is [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s² for extra stability from a half-filled d-subshell.