Question

Why does ionization energy increase on the periodic table?

Answer 1

Because of the increase in nuclear charge across the Period from left to right, as we face it.........

Explanation:

See this old answer. Ionization energy depends on (i) nuclear charge (i.e. `"Z, the atomic number"`), (ii) shielding by other electrons. As explained in the link, shielding of the nuclear charge by electrons in partly-filled valence shells, is VERY ineffective.