The Solid State

Question 245

How the crystalline solids are classified on the basis of the nature of bonding? Give suitable examples and nature of the forces present in different types of solids.


i) A crystalline solid usually consists of a large number of small crystals, each of them having a definite Characteristic geometrical shape. In a crystal, the arrangement of constituent particles (atoms, molecules or ions) is ordered. It has long range order which means that there is a regular pattern of arrangement of particles which repeats itself periodically over the entire crystal. Sodium chloride and quartz are typical examples of crystalline solids.

ii) Crystalline solid have a sharp melting point.

Crystalline solids can be classified on the basis of nature of intermolecular forces operating in them into four categories.
i) molecular
iv)covalent solids.

i) molecular: Molecules are the constituent particles of molecular solids. These are further sub divided into the following categories:

a) Non polar Molecular Solids: They comprise of either atoms, for example, argon and helium or the molecules formed by non polar covalent bonds for example H2, Cl2.

b) Polar Molecular Solids: The molecules in which solids are held together by relatively stronger dipole-dipole interactions.for example HCl, SO2, etc.

c) Hydrogen Bonded Molecular Solids: The molecules of such solids contain polar covalent bonds between H and F, O or N atoms. Strong hydrogen bonding binds molecules of such solids like H2O (ice).

ii) Ionic solid : Ions are the constituent particles of ionic solids. In ionic solid cations and anions bound by strong coulombic (electrostatic) forces. for example NaCl, KCl etc.

iii) metallic solid: These solids contain metal atoms as constituent particles. As metals have a good tendency to lose their valence electron and change in to positively charged metal ions (kernel). These electrons can easily move throughout the whole crystal and form the sea of free electrons. for example iron, calcium etc.

iv) covalent solids: A wide variety of crystalline solids of non-metals result from the formation of covalent bonds between adjacent atoms throughout the crystal. They are also called giant molecules. for example diamond, silicon carbide, etc.

Question 246

Write two important features, coordination numbr of the ions and number of formula units per unit cell for the following crystals:
(i) Cesium chloride, (ii) Zinc sulphide, (iii) Calcium fluorite, (iv) Sodium oxide.


i) CsCl is simple cubic cell. Cesium ion is surrounded by eight chloride ion which are also disposed towards the corner of a cube therefore both type ions are in equivalent positions and the stoichiometry is 1:1 . the coordination of CsCl is 8:8.

ii) Zinc sulfide is a FCC unit cell. The net number of zinc cation per unit cell is four, and the net number of sulfide anions per unit cell is four therefore, the ratio of ZnS ion in the cell is 1:1.

iii)Calcium fluoride is a FCC unit cell. The net number of Calcium cation per unit cell is four and the net number of fluoride anion is eight. Therefore the ratio of CaF2 ion in cell is 1:2 

iv) Na2O has the structure opposite to CaF2. In
this case coordination number of Na+ ions is 4 and that of O2- ion is 8. Thus Na2O has 4:8 coordination.

Question 247

Explain the reasons why:
(a) Frenkel defect is not found in pure alkali halides?
(b) Zinc oxide appears yellow on heating?
(c) Solid containing F-centres are para magnetic?
(d) Uncharged atoms or molecules never crystallize in a simple cubic structure?
(e) A given element will have the same density in both a hexagonal close-packed structure and a cubic close-packed structure?


a) Frenkel defect is not found in the pure alkali metal halides because this defect is due to vacancy of ion ,which is shifted in interstitial spaces and because the ions are too large and cannot get into interstitial sites.

b) ZnO becomes yellow on heating due to frankel defect, when ZnO is heated, it loses oxygen reversibly and turns yellow in colour.

Zno + heat -->Zn2+ +1/O2 +2e-

The Zn2+ ions formed the vacant voids in the crystal to form non- stoichiometric solid. The released electrons are trapped in its neighborhood. It is therefore non-stoichiometric ZnO is yellow and shows increased conductivity.

c) The free electrons trapped in the anion vacancies are called as F-centers. Solids containing F-centres are paramagnetic because the electrons occupying the vacant sites are unpaired.

d) Uncharged molecules and atoms are packed more efficiently in closed-packet structures. Hence theses uncharged molecules and atoms do not crystallize in simple cubic structures.

e) In both the structure, the fraction of the total volume occupied is 0.74. The two structure have the same coordination number of 12.

Question 248

What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 1/3rd of tetrahedral voids?


Number of tetrahedral voids formed = 2 x Number of atoms of element Y

Number of atoms of element Y in the ccp unit cell = 4

Number of tetrahedral voids formed = 2 x 4 = 8

Number of tetrahedral voids occupied by atoms of X = 1/3x8

Ratio of the numbers of atoms of X and Y = 8/3: 4 =2: 3

Hence, the formula of the compound is X2Y3.

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