What is the predicited order of first ionization energies from highest to lowest for lithium sodium potassium and rubidium

The predicted order of ionization energies is Li > Na > K > Rb

Atomic size increases as you go down a Group (see image). We are adding electrons to increasingly larger shells.

The valence electrons are further from the attraction of the nucleus, so they are less tightly held.

Thus, Li has the highest ionization energy and Rb the lowest.