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.
An alloy of cu and gold crystallizes in a cubic lattice in which the gold atoms occupy the lattice points at the corners of cube and copper atoms occupy the centre of each face. The formula of this compound is