What is density?
Density: Mass per unit volume of any materials are known as the density.
Metals have high density but Sodium and Potassium have low density because cohesive force between atoms of these elements are very less. This cohesive force can hold the atoms of metals. Atoms of metals are together, closely packed and have small atomic radii with high mass. There fore mass per unit volume of metals i.e. density is high. exception : Sodium and potassium.