Question

which of the following elements has the smallest first ionization energy?
lithium
sodium
potassium
rubidium

Explanation:

First - ionization energy decreases down a group in the periodic table. Lithium (Li), sodium (Na), potassium (K), and rubidium (Rb) are all in Group 1. Rubidium is the lowest among them in the group, so it has the smallest first - ionization energy due to its outermost electron being farther from the nucleus and less tightly held.

Answer:

rubidium