Q.Write the ground-state electronic configuration of molybdenum ($Z = 42$) and explain the anomaly, if any, in its filling.

Molybdenum has $Z = 42$ electrons. The simple Aufbau order predicts $[\text{Kr}]\,4d^4\,5s^2$ (36 core electrons in $[\text{Kr}]$, plus 4 in $4d$ and 2 in $5s$). Experimentally, molybdenum's actual ground-state configuration is

Exactly as with chromium directly above it in the same group, one electron that the simple rule would place in $5s$ instead moves into $4d$, so that the $4d$ subshell becomes exactly half-filled (one electron in each of its five orbitals, parallel spin) while the $5s$ subshell is left with a single electron. …

Write the Aufbau-predicted configuration first, then compare with molybdenum's known experimental exception, and explain it by ana …

Assuming only chromium and copper show such anomalies and writing $4d^4\,5s^2$ for molybdenum as if no exception applied; confusing molybdenum's half-fil …