It can be predicted that the ionization energies of these elements increase as you go up a period. This means that sodium has the greatest ionization energy, while rubidium has the least. This is due to the placement or distance of the valence electrons from the nucleus of these elements. When you go up a period, this distance decreases, making it harder to overcome the attraction between the nucleus and the electrons. The harder it is to overcome this attraction, the higher the ionization energy needed.
