Question

why potassium is lighter than the sodium ?

Answer 1

this is due 2 density of sodium whish is greater than potassium. sodium is denser than potassium because there is a sudden increase in atomic size and atomic volume as we move frm sodium to potassium. when we move frm sodium to potassium as volume increases atomic mass increases.

Answer 2

evn though K have more protons , neutrons and electrons 
Na encloses these things in a smaller radius and have high density 

mass proportionl to density