Of these elements, which has the greatest first ionization energy? Why? Ce, C, Cl, Ca, Cs.
Quite possibly chlorine.
Ionization energy is the energy associated with the reaction:
Ionization energy increases across a Period (from left to right), and decreases down a Group. Why does this trend make sense intuitively?
As physical scientists we should always consider the data. See this link.
Note that we consider atomic (NOT molecular) properties.