Section-D: Comprehension Type
In crystalline solids atoms or molecules are arranged in a regular and long range order fashion in a three dimensional pattern. These have sharp melting point, flat faces, sharp edges, bounded by well defined planes. A large number of unit cells, each of which possess a definite geometry bounded by plane faces give rise to the formation of a crystal. A point at the corner of unit cell contributes for 1/8 of each such point to unit cell. A point along an edge contribute for ¼ of each such point to unit cell. A body centred point contributes for 1 each such points to unit cell. Co-ordination number is the number of nearest neighbours that each ion is surrounded by an oppositely charged ions. Radius of unit cell in sc, fcc and bcc is and where a is edge length of cell.
A mineral having the formula AB2 crystallizes in the cubic closed packed lattice, with A2+ atoms occupying the lattice points and B− tetrahedral voids. The co-ordination number of A, B and fraction of the tetrahedral sites occupied by B atom respectively are