(B) Density:

Density of an element in solid state exhibits a variation with atomic number. The density of the alkaline metals occupies the lowest position in the curve (Figure). This is because of the nature of the atoms. The increase in density down a group in the periodic table is because of the increase number of neutron and proton in the nuclei.

Figure 1. 15. Periodic variation of density of elements.

Figure 1.16. Atomic volume difference.