Q.Define unit cell.

A crystal lattice is the regular, repeating three-dimensional arrangement of a crystal's atoms, ions or molecules. The smallest structural block that, repeated in all three directions, reconstructs the entire lattice is called the unit cell -- a crystal can be thought of as a huge number of unit cells stacked together, each in direct contact with its nearest neighbours and all identically oriented.

A unit cell is fully described by six numbers: three edge lengths (lattice constants) a, b and c, and the three angles between those edges -- alpha (between b and c), beta (between a and c), and gamma (between a and b).

The coordination number of a particle is the number of nearest-neighbour particles surrounding it in the crystal. This single idea recurs throughout the whole unit as the way of comparing how tightly different arrangements are packed: simple cubic = 6, body-centred cubic = 8, and both hcp and ccp = 12 (the maximum possible for identical spheres, hence the most efficient packing).

A primitive unit cell has lattice points only at its 8 corners; a non-primitive unit cell carries extra lattice points on a face (face-centred) or fully inside the cell (body-centred).