Assertion A: The first ionization enthalpy of O is lower than that of N and F. Reason R: The loss of an electron from O leads to a stable half-filled p orbital. Choose the most appropriate answer:

Solution & Explanation

First ionisation enthalpy generally rises across a period, but half-filled and fully-filled subshells are extra stable, creating exceptions. Configurations: N is 2p³ (stable half-filled), O is 2p⁴, F is 2p⁵. Nitrogen's half-filled 2p³ is unusually stable, so its IE₁ is higher than oxygen's. In oxygen the 4th 2p electron is paired, suffering extra electron–electron repulsion, which makes it easier to remove ⇒ O has a lower IE₁ than N (and lower than F too). So the Assertion is correct. Reason: removing one electron from O (2p⁴) gives O⁺ (2p³), the stable half-filled configuration, which favours ionisation. This is precisely why O has the low IE₁, so the Reason correctly explains the Assertion. Answer: option A.