Answer: The order predicted would be: ionization energy of sodium > ionization energy of potassium > ionization energy of rubbidium.
Explanation:
1) The ionization energy is the energy that has to be added to a gaseous atom to remove an electron and form a positive ion.
2) The ionization energy decreases when you move down a group.
3) The reason for this decrease of the ionization energy down a group is that, as you descend inside a group, the atom size increases, which implies that the valence electrons (the electrons in the outermost shell) are further away from the nucleus, and so less energy is needed to remove those electrons.
