potassium is less denser than sodium. why?

Dear user,

Your query is quite obvious as you have studied that the densities of alkali metal group elements increase on moving down a group. But while moving down from sodium to potassium, the reverse is observed.

Well this exceptional behaviour can be answered as follows.

Basically, in the case of sodium and potassium the increase in shell size outweighs the pull of the nucleus by the outer shell electron.  So, the increase in atomic size is counterbalanced by increase in mass on moving from sodium to potassium. This explains lesser density of potassium than that of sodium.

Hope, this answers your question.

Cheers!!