Question

Why does ionization energy change on the periodic table?

Answer 1

Because the nuclear charge incrementally changes, and also shielding by other electrons periodically changes.

Explanation:

Two factors determine ionization energy: (i) nuclear charge; and (ii) shielding by other electrons. As nuclear charges increase with `Z`, the atomic number, ionization energies INCREASE, and atomic radii decrease. Clearly, this reflects the effect of electrostatic attraction. Incomplete electronic shells shield nuclear charge very imperfectly, and this imperfect shielding accounts for the periodicity of the elements.

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.