explaining trends

Question

how would you Explain the opposing trends of ionization energies and atomic radii in the alkali group of the periodic table.

Answer 1

As you go down the group (alkali or other), the number of energy levels (shells) increases, but the number of valence electrons remains the same. The fact that you are adding shells as you move down the group helps to explain why the atomic radii increase.  On the other hand, as you increase the number of shells, they get further away from the nucleus, and the attraction of the valence electrons to the nucleus is less as you move down the group.  Since ionization energy is the energy needed to remove the valence electron(s), it becomes less as you move down the group.  This explains the opposing trends.