Which of the alkali metals has the lowest density ?

Options

A. Na
B. K
C. Rb
D. Cs

Answer

B

Explanation

As we move down the group, both the atomic mass and atomic size increase. But the increase in atomic mass more than compensates the increase in size. Consequently, the densities of alkali metlas increase from Li to Cs with deviation at K which is lighter than Na due to an abnormal deviation at K which is lighter than Na due to an abnormal increase in atomic size of K on account of the d-subshell.