# The Solid State

Question 65

## How in the  crystal of zinc sulphide, zinc occupies tetrahedral voids. What is the coordination number of zinc?

Solution
The zinc(II) ions, which are smaller than the sulfide ions, are inserted into tetrahedral holes and push the sulfide ions apart so that no two sulfide ions are in contact with each other. The resulting structure has (4,4)-coordination. Observe that none of the octahedral holes are occupied.
The coordination number of zinc isfour.
Question 66

## What is  point defects in solids.

Solution
Point defects can be define as  the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance.
Point defects can be classified into three types : (i) stoichiometric defects (ii) impurity defects and (iii) non-stoichiometric defects.
Question 67

## What type of compounds exhibit Schottky defect?

Solution
schottky is basically vacancy defect. it can be shown by ionic substance in which cation and anion are of almost similar sizes. for example NaCl, KCl etc.
Question 68

## The unit cell of a substance has cation A+ at the corners of the unit cell and anions B- the centre. What is the simplest formula of the the substance?

Solution
Number of cations A+ at the corners of the unit cell is
$=\frac{1}{8}×8=1.$
Number of anions Bat the centre of unit cell is=1
Therefore, simplest formula of the substance = AB.