Consider the group 1a elements sodium (period 3), potassium (period 4), and rubidium (period 5). what would you predict about the ionization energies of these elements?

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.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.