It takes energy to ionize any atom. It takes progressively more and more energy for each successive electron that is removed and the increase is somewhat smooth. For the removal of which electron does boron exhibit a sudden marked increase in its ionization energy?

Removal of Third Electron

Explanation:

a major jump is required to remove the third electron. 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.