Potassium crystallizes in a body centred cubic lattice. What is the approximate number of unit cells in 4.0g of potassium? Atomic mass of potassium =39

Options

A. `3.09xx10^(22)`
B. `2.09xx10^(21)`
C. `4.10xx10^(21)`
D. `3.09xx10^(20)`

Explanation

There will be eight atoms at corners of the cube and one atom at the body centre.
`therefore` Number of atoms per unit cell `=(8xx(1)/(8))+1=2`
`therefore` Number of unit cells in 4.0 g of potassium `=(4)/(39)xx(6.023xx10^(25))/(2)=3.09xx10^(22)`
Hence 'A' is the correct Choice.
In CCP unit cell there are 8 tetrahedral voids and 4 octahedral voids.
No. of tetrahedral voids in an unit cell=No. of octahedral voids '2