# Why does ionization energy change on the periodic table?

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.