The general trend for ionization energy as the atomic number increases within a group is a decrease. In other words, ionization energy tends to decrease as you move down a group in the periodic table.
Ionization energy refers to the energy required to remove an electron from a neutral atom in its gaseous state. When you move down a group, the principal energy level (shell) in which the valence electrons reside increases. The valence electrons are located farther away from the nucleus and are shielded by inner electrons, which reduces the attractive force between the nucleus and the valence electrons.
As a result, it becomes easier to remove an electron from an atom as you move down a group because the electrons are less tightly held by the nucleus. Therefore, ionization energy decreases as you go down a group in the periodic table.