First ionization enthalpy generally rises across a period, but exceptions occur where electronic configuration confers special stability. Nitrogen has the configuration 1s^2 2s^2 2p^3, with an exactly half-filled 2p subshell that is unusually stable due to symmetrical electron distribution and favourable exchange energy. Removing an electron from this stable arrangement requires extra energy. Oxygen has the configuration 1s^2 2s^2 2p^4, where the fourth 2p electron must pair up in an already occupied orbital, introducing electron-electron repulsion that makes it easier to remove. Consequently oxygen's first ionization enthalpy is slightly lower than nitrogen's. The option of smaller nuclear charge for oxygen is false, since oxygen has more protons. The option of more shells is wrong, as both are in period two. The option that nitrogen is a metal is incorrect. This half-filled stability anomaly is a key JEE Advanced periodicity point. Understanding ionization enthalpy anomaly in this way ties directly into the wider study of classification of elements and periodicity in properties, where the same reasoning recurs across many problems. Plausibility check: the analogous dip occurs between phosphorus and sulphur, confirming that half-filled stability causes the reversal.