Question

why k is lighter than Na?

Answer 1

it is because of the density

Answer 2

As we know, on moving down the group, generally there is increase in density. But moving from sodium to potassium, there is increase in charge by 8 units, but atomic radius of potassium increases much due to presence of 3d subshell but in case ofsodium, there is no presence of d subshell
hope it helps you