Cue Column

• Why is Cr anomalous?
• Why is Cu anomalous?
• What is the driving force?
• How many electrons differ from expected?
• Which 4s orbital is affected?

Notes Column

Chromium (Cr, Z = 24)

Expected: $[\text{Ar}]\, 3d^4\, 4s^2$

Actual: $[\text{Ar}]\, 3d^5\, 4s^1$

One electron migrates from 4s to 3d. This gives a half-filled d-subshell (3d^5) which is exceptionally stable due to maximum exchange energy — every pair of parallel-spin electrons contributes stabilisation via the exchange interaction. With five unpaired electrons in 3d, the number of possible exchanges is maximised (C(5,2) = 10 pairs).

Copper (Cu, Z = 29)

Expected: $[\text{Ar}]\, 3d^9\, 4s^2$

Actual: $[\text{Ar}]\, 3d^{10}\, 4s^1$

One electron migrates from 4s to 3d. A completely filled d-subshell (3d^{10}) is maximally symmetrical and gives the highest total exchange energy. Both 3d^5 and 3d^{10} are pseudo-noble-gas-like in symmetry.

Key Rule: When the choice is between a half-filled or fully-filled d-subshell and a doubly occupied 4s, nature always chooses the former.

Summary Box

Two anomalies: Cr = half-filled $3d^5$, Cu = fully-filled $3d^{10}$. Both sacrifice one 4s electron. Reason: exchange energy stability.