How would you arrange the following elements in order of increasing ionization energy: Li, Rb, Cs, Na?
The element with LARGEST atomic number,
Ionization energy is the energy associated with the reaction:
We examine 4 alkali metals,
As the valence electron is farther removed from the nuclear core, the energy required for the given reaction should become progressively less.
As a chemist, as a physical scientist, you should consult a Table of ionization energies in order to see whether this rationalization is supported by the data.