Question

Why is there a large increase between th first and second ionization energies of the alkali metals?

Answer 1

Because the valence shell of the alkali metal contains ONE electron only........

Explanation:

Two factors are important here. `"A priori"` we would expect that it would be harder to remove an electron from a cation than from a neutral species. This is from simple considerations of electrostatics.

Neverthless, the alkali metals metals contain only the one valence electron. Ionization of an `"M"^+` ion requires that the electron be removed from an inner shell, that is closer to the metal nucleus, and therefore much more strongly held:

`M(g) + Delta_1rarrM^(+)(g) + e^(-)`

`M^(+)(g) + Delta_2rarrM^(2+)(g) + e^(-)`

`Delta_2">>"Delta_1`

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