Question

Define covalent solid. An ionic compound AxBy occurs in fcc type of crystal structure. The ion B is at centre of each and the ion A occupies corners of the cube Calculate the value of x and y. Write the formula of the ionic compound.

Answer 1

Covalent Solid while A network solid or covalent network solid is a chemical compound (or element) in which the atoms are bonded by covalent bonds in a continuous network extending throughout the material. In a network solid there are no individual molecules, and the entire crystal or amorphous solid may be considered a macromolecule.

Given A atom occupy all corners and alternate edge centre. Therefore

in one unit cell total number of corner =8

And number of adjacent edge centres=4

We know contribution of corners to unit cell is 1/8 th and that by edge centre is 1/4.

Therefore,

A atoms in a unit cell=( 1/8)*8 + 4*(1/4)= 2

Similarly for B atoms in face centre

Contribution of a face centre to unit cell is 1/2

Total face=6

Total atoms of B in a unit cell=6*(1/2)=3

Therefore unit cell has 2 A atoms and 3 B atoms so formula is A2B3