Question

Why is it the valence electron that is removed from an atom upon ionization?

Answer 1

Ionization energy generally increases across a period and decreases down a group. The effective nuclear charge is the charge of the nucleus felt by the valence electron. ... Going down a group the atomic size gets larger and the electrons can be more readily removed, therefore, ionization energy decreases.

Answer 2

To remove the third electron involves a major jump. In general, successive ionization energies always increase because each subsequent electron is being pulled away from an increasingly more positive ion. Ionization energy increases from bottom to top within a group, and increases from left to right within a period.

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