Ionization energy is the amount of energy required to remove an electron from an atom.
Across the period the number of energy levels remains the same, however, more protons are added to the nucleus, and the more protons an atom has, the higher its ionization energy will be, therefore, as you move across a period on the periodic table, the ionization energy increases.
As one progresses down a group on the periodic table, the ionization energy will likely decrease since the valence electrons are farther away from the nucleus and experience greater shielding.
