The 3d transition series (Sc to Zn) has general configuration [Ar] 3d^(1-10) 4s^(1-2). Two anomalies: Cr = [Ar]3$d^5$ 4$s^1$ (half-filled d gives extra exchange energy stabilisation) and Cu = [Ar]3$d^{10}$ 4$s^1$ (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]3$d^6$ (NOT 3$d^4$ 4$s^2$), $Cr^{3+}$ = [Ar]3$d^3$, $Cu^{2+}$ = [Ar]3$d^9$. Common error: writing $Cu^+$ as 3$d^9$ 4$s^1$ instead of correct 3$d^{10}$.