The 3d transition series (Sc to Zn) has general configuration [Ar] 3d^(1-10) 4s^(1-2). Two anomalies: Cr = [Ar]3d^5 4s^1 (half-filled d gives extra exchange energy stabilisation) and Cu = [Ar]3d^10 4s^1 (fully filled d is stable). When forming ions, 4s electrons are removed first (despite being filled first) because 3d contracts below 4s in energy upon ionisation. Thus Fe^2+ = [Ar]3d^6 (NOT 3d^4 4s^2), Cr^3+ = [Ar]3d^3, Cu^2+ = [Ar]3d^9. Common error: writing Cu^+ as 3d^9 4s^1 instead of correct 3d^10.