Question

Why potassium is lighter than sodium?

Answer 1

This is because the density of sodium is greater than of potassium, we know that density down the group generally increase this is it because of the reason that the atomic mass of element increase and the effect of increase in atomic mass is more pronounced as compared to increase in atomic size.

But there is a exception in density Trends between Sodium and potassium and sodium is denser than potassium this happens because of there is an abnormal increase in atomic size and atomic volume has been remove from sodium to potassium . we can move from sodium to potassium effect of increase in volume is more pronounced as compared to effect of increase in atomic mass.

Due to this Potassium is less than denser than sodium.