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IE = minimum energy to remove the outermost electron from a gaseous atom in ground state. Always positive (endothermic).
General trends: IE increases across a period (higher Zeff), decreases down a group (electron farther from nucleus, more shielding). Highest IE: He (2372 kJ/mol). Lowest among stable elements: Cs (376 kJ/mol).
Critical exceptions (most tested in JEE):
- IE(B) < IE(Be): B removes a 2p electron (higher energy, less penetration) vs Be's 2s^2 (fully filled, stable). Same pattern: Al < Mg in Period 3.
- IE(O) < IE(N): N's half-filled 2p^3 has extra exchange energy stability. O's 2p^4 has a paired electron with repulsion. Same pattern: S < P in Period 3.
Successive IE: IE1 < IE2 < IE3... A large jump indicates core electron removal. Used to identify group: count "easy" ionisations before the jump = number of valence electrons. Example: jump after IE3 → Group 13.
Special cases in d-block: IE varies irregularly. Half-filled d^5 (Mn) and fully filled d^10 (Zn) show slightly higher IE due to extra stability. Cr (d^5 s^1) has low IE because the lone 4s electron is easy to remove.