Across period 2 the first ionization energy generally rises, yet oxygen falls below nitrogen. Which element has the higher first ionization energy?

Core Concept

Nitrogen's 2p subshell is half-filled (2p³), a particularly stable arrangement with its three 2p electrons in separate orbitals. Oxygen's fourth 2p electron is the first forced into a pairing, and that added repulsion makes it easier to remove. Measured values confirm it: about 1402 kJ/mol for nitrogen versus 1314 for oxygen. Small subshell stability effects can override the broad periodic trend.