JEE Main · 2006 · 30 Apr 2006 · Offline · Classification of Elements and Periodicity

First ionisation enthalpy generally rises across a period and falls down a group, but half-filled stability shapes the trend here. Boron, losing a single $2p$ electron, has the lowest value; sulphur, $3p^4$, is lowered by electron-pair repulsion and sits below phosphorus; phosphorus, with a stable half-filled $3p^3$ shell, is higher than sulphur; and fluorine, small and highly charged with a $2p^5$ shell, has the highest value. The increasing order is therefore $\mathrm{B}<\mathrm{S}<\mathrm{P}<\mathrm{F}$.