CBSE Class 12 Chemistry Notes: Solid States – Classification of Crystalline Solids

CLASSIFICATION OF CRYSTALLINE SOLIDS:-

Based upon nature of constituent particles and binding forces present in them:-

1. Ionic Solids:- 

• In these solids constituent particles are positive and negative ions. (cation or anions). They are held together by strong columbic electrostatic forces of attraction examples are NaCl, CaCl2, MgCl2, BaCl2 etc.

Characteristics of ionic solids:-

1. They have high melting and boiling points
2. They are soluble in polar solvents but are insoluble in non polar solvents.
3. Due to strong electrostatic forces of attraction they are closely packed hence hard but they are brittle.

2. Molecular Solids:-

• In these solids the constituent particles are molecules on the nature of molecules they can further subdivided into following three types:-

1. Non polar molecular solids:-

• The crystalline solids in which constituent particles are atoms of noble gases [helium, neon] or non polar molecules like [H2, Cl2, I2]

Their characters are:-

1. They are soft due to weak intermolecular forces.
2. They are non conductors of electricity. They have low melting and boiling points.

2. Polar molecular solids:-

• The crystalline solids in which constituent particles are polar molecules like HCl, SO2 etc. the intermolecular forces of attraction are dipole – dipole forces of attraction.

Thus their characters are:-

1. They are soft; they are non conductors of electricity.
2. Their melting and boiling points are high then non polar solids. They exists gases or liquid at room temperature.

3. Hydrogen bonded – molecular solids:-

• In these solids the constituent particles are which contain hydrogen atom linked to high electronegative atoms as N, O, F

Their characters are:-

1. They exists as volatile liquids or gases at room temperature.
2. They are non conductor of electricity.
3. Their melting and boiling points are high.

3. Covalent or network solids:-

• In these crystalline solids the constituent particles are non metal atoms linked to adjacent atom by covalent bond throughout the crystal. They forms a network of covalent bonds and exists as giant molecules. Example: Diamond

Their main characteristics are:-

1. As covalent bond is strong and directional in nature, these solid are very hard and brittle.
2. They have extremely high melting points and decompose before melting.
3. They are insult of and do not conduct electricity one exception is graphite which is covalent solid but is soft and also a good conductor of electricity.

4. Metallic solids:-

• In natural the constituent particles are positively charged metal ions – and free electrons.
• They are formed of metal atoms which lose their valance electrons to left behind positively charged ions.
• These metal atoms are surrounded by the sea of electrons each metal atom contributes one or more electrons to this sea of electrons.
• The electrons are simultaneously attracted by the +ve ions and holds these +ve ions ether

Metallic bond:-

• The force that holds the metal ion together in the crystal is called metallic bond.

Properties of metallic solids:-

1. They possess high electrical and thermal conductivity.
2. They possess lusture and colour in some case due to presence of sea of free electrons.
3. They are highly malleable and ductile.
4. They are closely packed. They exhibit high melting points and high densities.

QUESTIONS:-

1. Why glass of windows in buildings is thicker at bottom at top and why it is milky?
   Ans. Glass is an amorphous solid which is a supercooled liquid of high viscosity. Thus it possesses fluidity. Due to this it is thicker at bottom on the other hand its milkness is due to fact that it undergoes heating during day and cooling at night thus it aquires some crystalline character.
2. Why do the solids have a definite volume?
   Ans. The constituent particles of a solid have fixed positions and are not free to move about. They possess rigidity. Hence they have a definite volume.
3. Ionic solids conducts electricity in molten state but in solid state explain?
   Ans. In the molten state ionic solids dissociated to give free ions and hence conducts the electricity but in solid state the ions are not free but remains held together by strong electrostatic force of attraction thus they cannot conduct electricity.

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