Successive Ionization Enthalpies

Successive ionization enthalpies increase steadily as electrons are removed, but a sudden very large jump occurs when removal begins to break into a stable noble-gas inner core. The position of this jump reveals the number of valence electrons. A large jump between the second and third ionization enthalpies means that two electrons are removed relatively easily but the third faces the stable inner core, indicating the element has exactly two valence electrons. This corresponds to Group 2, the alkaline earth metals with an outer ns^2 configuration. Group 1 would show the jump after the first electron. Group 13 would show it after the third electron. Group 14 would show it after the fourth electron. Using the jump in successive ionization enthalpies to deduce valence-electron count and group is a classic JEE Advanced reasoning task from NCERT. It is worth emphasising that this is not a special case but a representative example of how successive ionization enthalpies operates throughout classification of elements and periodicity in properties. Plausibility check: removing the two outer electrons of a Group 2 metal leaves a noble-gas core, so the third removal must require dramatically more energy, matching the observed jump.