Question

How is the atomic volume of sodium is related to magnesium and potassium?

Answer 1

sodium and potassium belong to the same group( Alkali metals )
sodium and potassium belong to same period

Answer 2

Sodium and Potassium belong to the same group and Sodium and Magnesium belong to the same period.

Along the group, the atomic volume increases as the size increases due to new valence shell. Sodium has a configuration of 2,8,1 and Potassium has 2,8,8,1. Due to this extra shell the volume of Potassium is higher than Sodium.

Along the period, the volume decreases due to effective nuclear charge increases. So the volume decreases. That means Magnesium has less volume than sodium.