Question

question 10 of 25
which of these elements has the smallest first ionization energy?

a. na
b. li
c. rb
d. k

Explanation:

First ionization energy (the energy to remove the outermost electron) generally decreases down a group in the periodic table because as the atomic radius increases (more electron shells), the outermost electron is farther from the nucleus and experiences less effective nuclear charge, making it easier to remove.

Sodium (Na), Lithium (Li), Rubidium (Rb), and Potassium (K) are all in Group 1 (alkali metals). The order of these elements from top to bottom in the group is Li, Na, K, Rb. As we go down the group (from Li to Rb), the atomic radius increases, and the first ionization energy decreases. Among these, Rb is the lowest in the group (after K), so it has the smallest first ionization energy.

Answer:

C. Rb