$word_{count}$: 260

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 $\frac{2372 kJ}{mol}$. Lowest among stable elements: Cs $\frac{376 kJ}{mol}$.

Critical exceptions (most tested in JEE):

1. IE(B) < IE(Be): B removes a 2p electron (higher energy, less penetration) vs Be's 2$s^2$ (fully filled, stable). Same pattern: Al < Mg in Period 3.
2. IE(O) < IE(N): N's half-filled 2$p^3$ has extra exchange energy stability. O's 2$p^4$ 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.