1.

Find the number of atoms per unit cell in the following crystal structures :

Answer»

Solution :(1) Number of atoms in body-centred CUBIC (bcc) crystal: In this unit cell, there are 8 atoms at 8 corners and one additional atom at the body centre. Each CORNER butes 1/8th atom, to the unit cell, HENCE due to 8 corners.
Numberof atoms `= 8 xx (1)/(8)`
=1 atom.
An atom at the body centre whollybelongto the unit cell.
`THEREFORE`Total numberof atoms presentin bcc unit cell`= 1+1 = 2`

(2) Number of atoms in face-centred cubic (fcc)crystal:
In thisunit cell, thereare8 atomsat 8 corners and 6 atoms at 6 facecentre.
Each corner contributes 1/8th atom to the unit cell, hence due to 8 corners.


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