Ionization enthalpy comparison: O vs F. The 1st IE of O is LESS than that of N (because O has paired 2p electron easier to remove). Similarly:

**Anomaly:** 1st IE of O < N due to paired electron destabilization in O (2p⁴ vs N's stable 2p³ half-filled). **Same in 3rd row:** S (3p⁴) < P (3p³). **Similar in row 4:** Se < As, Te < Sb. **General:** half-filled subshells are especially stable.