An element crystallises into a structure which is described by a cubic unit cell having one atom at the centre of each edge. If volume of this unit cell is 24*10^-24 cm³ and the density of the element is 7.2g/cm³ calculate the number of atoms present in 200g of the element.

Number of atoms per unit cell = 2 atoms

Mass of one unit cell = volume x density

Mass = 24 x 10^-24 x 7.2 = 1.73 x 10^-22 gms

This is mass of one unit cell that is 2 atoms.

If 2 atoms are in 1.73 x 10^-22 gms

we have X atoms in 200 gms

We get 

X = 2 x 200/1.73 x 10^-22

X = 2.32 x 10²⁴ atoms