Answer:
Mg
Step-by-step explanation:
Mg requires more ionization energy to ionize because it has more protons than Na. If an atom, located within the same period, has more protons, the pull on the electrons will be greater. This makes it more difficult to remove these electrons during ionization.
As a general rule, ionization energy decreases as you go down a column and increases as you move to the right on the periodic table. Therefore, the elements with the highest ionization energies are located on the top right of the periodic table. Ionization energies increase across a group due to the quantity of protons increasing. Ionization energies decrease as you go down a column because the outermost electrons are further from the nucleus (where the protons are), making it easier to remove them.