How does ionization energy change as you move across a period?
First off, ionization energy is defined as the amount of energy needed to remove the outermost electron from an atom in the gaseous state.
In this example, one valence electron is being removed from gaseous hydrogen in order to produce a cation. The removal of the electron requires 1312.0 kJ/mol of energy.
The ionization energy increases because, as you move across the periodic table, the atomic radius decreases. Since the atomic radius decreases, the outermost electrons (valence electrons) are held more tightly to the increasing number of protons in the nucleus. Because the electrons are held more tightly, more energy is needed to overcome those attractive forces in order to remove one or more valence electrons.