Why does ionization energy change on the periodic table?
Because the nuclear charge incrementally changes, and also shielding by other electrons periodically changes.
Two factors determine ionization energy: (i) nuclear charge; and (ii) shielding by other electrons. As nuclear charges increase with
Once a shell is filled, the nuclear charge is shielded somewhat, and electrons join a new valence shell, further removed from the nucleus. You should look at the data, at a table of ionization energies. You will be able to account for them considering periodicity.