Which has larger ionization energy and why: Ca or Mg

Consult a periodic table of elements online and check where Magnesium and Calcium are in relation to each other. There are two trends to consider when understanding how ionization energy increases or decreases. Ionization is the ability for an atom to become "ionized" or stripped of a electron. This ionization increases from left to right on the periodic table since it is harder for an atom to be ionized with more electrons filling the outer shell. Ionization decreases from top to bottom due to electron shielding which is the ability of inner electrons to shield valence electrons from the protons in the nucleus.