IE1 = energy to remove the most loosely bound electron from a gaseous atom in ground state. General trend: IE increases across a period (increasing Zeff), decreases down a group (valence electron farther from nucleus). Critical exceptions in Period 2: (1) IE(B) < IE(Be): Be = 2$s^2$ (stable full subshell), B = 2$s^2$ 2$p^1$ (removing the 2p electron is easier — higher energy orbital, less penetration). (2) IE(O) < IE(N): N = 2$p^3$ (half-filled, extra exchange energy stability), O = 2$p^4$ (paired electron in one 2p orbital experiences inter-electronic repulsion, easier to remove). Same pattern repeats in Period 3: IE(Al) < IE(Mg) and IE(S) < IE(P). These exceptions are among the most tested concepts in JEE.