Practice!

Sample Problem

Which of the following explains why ionization energy decreases down a group?

Solution

Electrons are added down a group, but the addition of new shells that accompany it overwhelms the positive force of the nucleus (the number of valence electrons remain the same). Ionization energy refers to the energy required to remove an electron. Because the atomic radius is larger down a group, the energy needed to fully remove an electron is smaller, so ionization energy decreases.