Dear student 
Potassium has vacant d orbital that makes it less denser than sodium.
WITH REGARDS 
MAHIMA 
ASKIITIANS FORUM EXPERT 
						

							
Density increases down the group from lithium to caesium, then why is pottasium lighter than sodium?
Though there is a trend in increasing density down the groups in the periodic table, there is an exception in the density trend between sodium and potassium. The exception is that although the atomic mass increases and the number of protons increase for potassium, its density is less than that for sodium.
 
Basically, as you go down a group the elements are heavier because they contain more protons and neutrons in their nuclei. But working against this is the fact that the increased nuclear charge tends to pull all the electrons closer, resulting in a smaller atomic radius and hence a higher density.
 
Density down a group generally increases, with the notable exception of potassium being less dense than sodium.
 
Density is inversely proportional to the volumes of the atoms and directly proportional to their masses. Basically, in the case of sodium and potassium the increase in shell size outweighs the pull of the core on the outer shell electron and so potassium is less dense than sodium. When we move 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.