1. Co-ordination number
Number of spheres touching one Sphere in a close packed structure is called co-ordination number of a lattice
2. Unit cell
The smallest repeating pattern in a crystal lattice is called a unit cell.
Characteristics of unit cell
Are unit cell has a characteristic dimensions along the three edges a,b,c and interfacial angles alfa ( between edges b and c), beta ( between a and c) and gamma (between a and b).
Types of unit cells
(i) Primitive Unit Cell
A unit cell in which the constituent particles are present only on the corners.
(ii) Centred Unit Cell
The unit cell in which the constituent particles are present on the edges , faces or for the body centre in addition to to the corners.
Types of primitive unit cells and there possible variations.