Question

Why sodium has a higher density than lithium but potassium has a lower density than sodium?

Answer 1

Density=$\frac{Mass}{Volume}$

In alkali metals, atomic weight increases down the group. Lithium has less atomic weight than Sodium. So density of 'Na' is greater than that of 'Li'.

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 low density than Sodium.