Answer:
On the basis of nature of intermolecular forces crystalline solids have been classified into four categories.
(i) Ionic (ii) Metallic
(iii) Covalent (iv) Molecular.
(i) Ionic solids: Solids in which constituent particles are held by ionic bonds are called ionic solids. Ionic solids are made of ions (cations and anions). In ionic solids cations and anions are bound by strong electrostatic forces of attraction (coulombic force). Ionic solids are formed by three dimensional arrangements of cations and anions.
Example: NaCI, ZnS, etc.
(ii) Metallic solids: Solids in which constituent particles are held by metallic bond are called metallic solids.
Examples: Iron, copper, silver, sodium, magnesium, aluminium, etc.
Metallic solids are made of metal ions and electrons. These are formed by three dimensional arrangements of metal ions surrounded by electrons.
(iii) Covalent solids: Solids in which constituent particles are held together by a network of covalent bonds throughout the crystal are called covalent solids.
Examples: Diamond, graphite, silica (quartz), carborundum (silicon carbide).
These are also called network solids. Covalent solids are made of non-metallic atoms. These atoms are bound by forming covalent bonds between adjacent atoms throughout the crystals. These solids have giant structures.
(iv) Molecular solids: Solids in which constituent particles are held together by weak van der Waals forces are called molecular solids.
Molecular solids are made of discrete covalent molecules.
Examples: Iodine, naphthalene, phosphorus, sulphur, camphor, etc.
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