The regular three dimensional arrangement of points in a crystal is known as crystal lattice and the smallest repeating pattern in the lattice is called unit cell. The unit cells are characterised by the edge lengths a, b, c and the angles between them `alpha, beta and gamma` respectively. Based on this, there are seven crystal systems. In a cubic unit cell:
`a=b=c and alpha = beta=gamma=90^(@)` The number of points in simple, body centred and face centred cubic cells are 1,2 and 4 respectively In both the hcp and ccp of spheres, the number of tetrahedral voids per sphere is two while the octahedral voids is one.
In a face centred cubic cell, an atom at the face contributes to the unit cell

Options

A. 1 part
B. `1//2` part
C. `1//4` part
D. `1//8` part

Answer

B

Explanation

In fcc unit cell, an atom at the face centre is in contact with two unit cells. It contribution to the unit cell `=1//2`