Why alkali metal density increase going down the group?

Answer 1

Atomic-volume of alkali metals increases down the group ,But increase in mass dominates over increase in volume ,since density=(mass)/(volume) ,density increases down the group

Answer 2

The alkali metal density increases going down the group because the atoms have more electron shells as you go down the group, resulting in a larger atomic size. The increased number of electron shells leads to weaker attractive forces between the nucleus and the outermost electron, causing the atoms to be less compact and have lower densities.