JEE Main · 2020 · 08 Jan 2020 · Shift 1 · Classification of Elements and Periodicity

First ionization energy rises across a period, but aluminium breaks the trend: its outermost electron is a $3p$ electron, better shielded and more easily removed than the $3s$ electron of magnesium, so aluminium's value dips below magnesium's. The resulting order is $\mathrm{Na}(496)<\mathrm{Al}(577)<\mathrm{Mg}(737)<\mathrm{Si}(786)$ kJ/mol, which for Na, Mg, Al, Si reads $496,737,577,786$.