Why do ionization energies decrease from the top to the bottom of the group on the periodic table of elements?
Ionization energies increase across a Period (from left to right), and decrease down a Group. It would be useful to remember that mantra.
Two factors are important in consideration of ionization enthalpies. 1. Shielding by other electrons (especially by electrons in inner shells). And 2. Nuclear Charge.
Across a Period nuclear charge increases sequentially, and the electrons in incomplete shells shield this nuclear charge very ineffectively. Atomic radius decreases markedly across a Period.
Once a shell is complete, the full shell can shield the increased nuclear charge to some extent. The valence, the outermost electrons, experience significantly less effective nuclear charge, and atomic radii increase down a Group.