Why does potassium is less denser than sodium even though density increases down the group in periodic table?

							
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.
						

							
As density is equal to mass/volume 
Thus it depends on both of the factors 
Even though the mass of sodium is less than potassium but the volume of potassium is much more . This makes the density of potassium less than that of sodium 
						

							
In going from Na to K, a new orbital (empty 3d) is added which expands (unusually) the size of the atom. At the same time, the atomic mass increases but the increase in volume is more than the corresponding increase in mass of the atom and so K get a little lower density than Na.
Detail:
The atomic radii of Na, , Rb, Cs are 154, 196, 211, 225 pm. As you can see, the increase in size (196-154 = 42 pm) is more than the size difference between K and Rb (196-211 =15 pm) and between Rb and Cs (225-211 =14 pm). So there a relatively higher expansion in size when we go from Na to K.
						

							
Why K has low density then Na although density increases down the group?
Answer:
As density is equal to mass per unit volume then in others case the increase in mass is greater then that of increase in volume so mass increases is dominant over volume increase so density is increased.
But one acception is present thaf Why the density of Na is greater then K so in this case increase in volume is greater then increase in mass so K is less density then Na.