Final answer:
Ionization energy decreases as you move down a group in the periodic table because atomic size increases and valence electrons are farther from the nucleus, making them easier to remove.
Step-by-step explanation:
The ionization energy of elements decreases as you go down a group/family on the periodic table. This is because as the atomic number increases within a group, the valence electrons are further from the nucleus, thus experiencing less electrostatic pull. As a result, it becomes easier to remove an electron since the atomic size is larger and the effective nuclear charge on the valence electron is reduced.
It's worth noting that ionization energy is the energy required to remove an electron from an atom in its gaseous state. The periodic trend demonstrates that ionization energy typically increases across a period from left to right. This increase is due to a decrease in atomic size coupled with an increase in electrostatic interactions between the nucleus and valence electrons, creating a stronger attraction that makes removing electrons more challenging and thus requires more energy.