Question

why is potassium lighter than sodium​

Answer 1

Answer of this question

because the density of Na is greater than that of K.

We know that density (mass/Volume) down the group generally increases. This is because of the reason that the atomic masses of the elements increase and the effect of increase in atomic mass is more pronounced as compared to increase in atomic size.

But there is an exception in the density trend between sodium and potassium that sodium is denser than potassium. This happens because of there is an abnormal increase in atomic size and atomic volume as we move from sodium to potassium. When we more from Na to K, effect of increase in volume is more pronounced as compared to effect of increase in atomic mass. Due to this potassium is less dense than sodium.