Answer:
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.
Types of molecular solids:
Molecular solids are of three types
(a) Non-polar molecular solids
(b) Polar molecular solids
(c) Hydrogen bonded molecular solids
(a) Non-polar molecular solids: Molecular solids which consist of either atoms or molecules formed by non-polar covalent bonds are called non-polar molecular solids.
Example: Ar, He, H2, Cl2, I2, Naphthalene, etc.
In these solids constituent atoms or molecules are held by weak London-dispersion forces.
(b) Polar molecular solids: Molecular solids which consist of polar covalent molecules are called polar molecular solids.
Examples: Solid SO2, solid HCl, etc.
In these solids molecules are held together by relatively strong dipole-dipole interaction. Therefore, melting points of these solids are higher than non-polar molecular solids.
(c) Hydrogen bonded molecular solids: Molecular solids in which molecules are held together by intermolecular hydrogen bonds are called hydrogen bonded molecular solids.
Example:
Properties of molecular solids:
(a) These are soft solids.
(b) These are insulators of heat and electricity.
(c) They have low melting points.
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