Why does a Cs atom have lower ionization energy than a Li atom?
Because the valence electron of caesium is further removed from the nuclear charge as compared to that of the lithium atom.
We are looking at the equation:
Obviously you have to pump in energy. This energy INCREASES across a Period (from left to right) and DECREASES down a Group. As nuclear charge increases, the nuclear attraction is greater. You should look at some values of this.