Q. Define co-ordination number.

Ans.CO-ORDINATION NUMBER: The number of atoms which are directly surrounding a given atom is known as coordination number. Number of atoms which are touched to a given atoms.

Simple Cubic Structure: In the simple cubic structure one atom is on each corner of a unit cell and any corner atom has four nearest neighbours in the same plus two nearest neighbours are exactly above and other exactly below of that corner atom, thus total number of neighbouring atoms are six. Hence co-ordination number is 6 for SC structure.

Body cubic center structure: In this BCC structure there are 8 atoms, one each at the corner of a unit cell and one atom is at the center of the cube. thus for any corner atom of the unit cell, the nearest atoms are the body centered atoms and the corresponding to a corner  atom of the unit cell , there are 8 unit cell in neighbour , which are having 8 body centered atoms.

     Thus co-ordination no. of BCC=8

Face centered cubic: The co-ordination no. of such structure is12.hence the nearest neighbors of any corner atoms are the face centered atoms of surrounding unit cell. Any corner atom has four such atoms in its own plane, four in a plane above it and four in a plane below it thus total no. of neighboring atoms are 12,it means co-ordination no. of FCC is 12.

Hexagonal closed packed: Each atom touches three atoms in the layer below its plane, six atoms in its plan, and three atoms in the layer above. Hence co-ordination no. of this structure is 12.