Video Transcript
Which of the following elements has the smallest atomic radius in the alkali metal
group? (A) Potassium, (B) rubidium, (C) lithium, (D) cesium, (E) sodium.
The alkali metals are found in group one of the periodic table. They include the elements lithium, sodium, potassium, rubidium, and cesium. The question wants us to understand the trend in atomic radius down a group. We know that as we go down a group, the mass of the elements increases. This occurs because the atoms have more protons, neutrons, and electrons. However, the atomic radius trend is not as simple as there are more subatomic
particles in the atom. So the radius increases.
Down a group, electrons are located in increasingly higher energy shells. We can see from these diagrams of the electron configuration of lithium, sodium, and
potassium that down a group, there are an increasing number of electron shells that
are occupied by electrons. Higher energy electron shells are located further from the nucleus. So the atomic radius generally increases down a group in the periodic table.
So the alkali metal that will have the smallest atomic radius is lithium. Answer choice (C) is the correct answer.
