Question

Why does sodium have a higher density than Lithium but Potassium has lower density than Sodium?

Answer 1

Though the trend of density down the group in a periodic table is an increase in the densities down the group, there is an inversion in the trend from sodium to potassium. The trend of increase in densities is due to increase in mass numbers. As there is an additional d orbitals in the increased nuclear charge tends to pull all the electrons close.

Complete answer:
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.
$Density = \dfrac{{mass}}{{volume}}$
Lithium has lesser atomic weight than sodium.So, lithium has lesser density than sodium.But in the case of 'K' and 'Na', d-orbitals present in Potassium, which increases the volume of 'K'.Thereby density of 'K' decreases.

So Potassium has a lower density than Sodium.

Note: Density down a group generally increases, with the notable exception of potassium being less dense than sodium. The densities of alkali metals are low and increase down the group. Potassium, however, is lighter than sodium. Lithium is the lightest metal having a density of $0.534{\text{ }}g/cc$ .