Hi there! When we look at ionization energy it is easiest to have a periodic table as a reference. Ionization energy is defined as the minimum amount of energy to remove an electron. There is a trend that we can see on the periodic table that will show us how to compare elements ionization energy. The farther up and the farthest to the right of the periodic table will have the higher ionization energy. Ex: He having the highest Fr having the lowest.
When I looked at these models selenium germanium and gallium were in the same row but since some were further to the right than others we were able to determine the highest to lowest ionization energies.