Lead crystallizes in a face-centered cubic structure with a density of 11.34 g/cm3. Polonium crystallizes in a simple cubic structure with a density of 9.3 g/cm3. Explain why lead has a greater density than polonium even though polonium atoms are heavier, and smaller than lead atoms.

Explanation:

In a face-centered cubic structure, each unit cell has 4 atoms. Each of the 8 corner atoms contribute 1/8 part to the unit cell and each of the 6 atoms at face-centres contribute 1/2 part to the unit cell. So, each unit cell has net 4 atoms.

In a simple cubic structure, there are 8 atoms, one at each corner. So, each one of them contribute 1/8 part to the unit cell. So, the unit cell has only 1 atom effectively.

Hence, each cell units of lead has 4 times as atoms as in the unit cells of polonium.

Due to this, density of lead is higher despite the fact that it's atomic mass is smaller and the size is bigger than that of a polonium atom.