Caleulate the packing efficiency in a unit cell of Cubic Close Packing(CCP) structure. 

Explanation

For ccp structure
`a = (4r)/(sqrt(2)) = 2 sqrt(2) r or r = (a)/(2 sqrt(2))`
Packing efficiency = `("Volume occupied by four spheres in the unit cell" xx 100 %)/("Total volume of the unit cell")`
`= (4 xx (4)/(3) pi r^(3) xx 100)/((2 sqrt(2) r)^(3))%`
`= ((16)/(3) pi r^(3) xx 100)/(16 sqrt(2) r^(3))%`
= 74%