A face-centred cubic (fcc) unit cell contains atoms at all the corners and at the centre of all the faces of the cube.
It can be seen that each atom located at the face-centre is shared between two adjacent
unit cells and only1/2 of each atom belongs to a unit cell.
The number of atoms present at corners per unit cell= 8 corner atoms x 1/8 atoms per unit cell = 1
The number of atoms present at faces per unit cell
= 6 atoms at the faces x 1/2 atoms per unit cell
= 6 x 1/2 = 3 atoms
Therefore, total number of atoms per unit cell = 4 atoms.
When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal.
The Cl– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites and impart colour.
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