As an atom’s radius increases, 1. its ionization energy increases. 2. its ionizations energy increases or decreases depending on whether you are going up a column or across a row. 3. its ionization energy does not change. 4. its ionization energy decreases

The correct option is: 4. its ionization energy decreases

Explanation:

Ionization energy refers to the energy required to pull a valence electron completely from the valence shell of a gaseous atom.

Now, in the periodic table of chemical elements, as we down a group, the atomic radius increases, so the effective nuclear charge experienced by the valence electron decreases. Therefore, the ionization energy decreases, down the group.

As we across a period, the atomic radius usually decreases, so the effective nuclear charge experienced by the valence electron increases. Therefore, usually the ionization energy increases, across the period.

Therefore, as the atomic radius increases, the effective nuclear charge experienced by the valence electron decreases and thus the ionization energy also decreases.