Why is the first ionization energy of a nonmetal much higher than that of an alkali metal

-Nonmetals have more filled shells, making it harder to remove electrons.

-Nonmetals have smaller atoms, while metals have larger atoms and therefore less attraction of the electrons to the nucleus. The nuclear charge increases from left to right across a period and the shielding effect stays the same. So it is more difficult to remove an electron from a nonmetal than an alkali metal