Chemistry Revision of Chemical Bonding For NDA part-3
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Covalent Bonds

The term "covalent bond" came into use in 1939 first given by Langmuir :

A covalent bond is a chemical link between two atoms or ions where the electron pairs are shared between them. A covalent bond may also be termed as molecular bond.

Formation of `H_2` molecule `H • + • H -> H: H` or` (H - H)`

`=>` Single, double and triple bonds are formed by the sharing of one, two and three electron pairs respectively.

Covalent bonds are of two types


(i) Non-polar Covalent Bonds : These are formed between two identical atoms or atoms having the same electronegativity. The electron pair is equally shared between them, such a bond is called non-polar covalent bond. e.g. `Cl_2, O_2` etc.

(ii) Polar Covalent Bonds : These are formed between different elements or elements having different electronegativity. In this bond the electron pair is attracted towards more electronegative atom, hence one atom have partial negative charge and other atom have partial positive charge. e.g. `HBr`, `HCl` etc.

Characteristics of Covalent Compounds

The main characteristics are as following

(i) In general covalent compounds are gases or liquids. Some of the compounds are soft solids.

(ii) Soft or brittle solid forms: The weak intermolecular forces make the solid form of covalent molecular compounds easy to distort or break.

(iii) These compounds are slightly soluble in water but highly soluble in organic solvents such as alcohols, ethers etc.

(iv) Those cova cnt molecules which form hydrogen bonds with water molecules becomes fairly soluble in water. e.g. lower alcohols, lower carboxylic acids etc.

(v) Poor electrical and thermal conductivity : Covalent molecular compounds do not conduct electricity well.

(vi) Low melting points and boiling points : A relatively small amount of energy is required to overcome the weak attractions between covalent molecules, so these compounds melt and boil at much lower temperatures than metallic and ionic compounds do.

(vii) Due to low melting and boiling points, covalent solids arc more volatile than ionic solids and therefore have high vapour pressure.

(viii) They are generally rigid and directional hence show isomerism.

(ix) They undergo chemical reactions which are quite slow because these reactions involve breaking of older bonds and formation of new bonds.

Dipole Moment

This is the property which measures the extent of polarity in the bond.

Dipole moment = Magnitude of charges (e) x distance (d)

`=>` Dipole moments of diatomic molecules is zero because they are non-polar. e.g. `H_2, Cl_2, F_2, O_2, N_2` etc.

`=>` Dipole moments of `HF, HCl, HBr` and `HI` are very large due to their high electronegativity differences.

`=>` Molecules like `CO_2, CS_2` and `BeF_2` are linear and have polar covalent bonds. The bond moments of two bonds being equal but opposite in direction cancel each other so their dipole moment is zero.

`=>` The dipole moment of `CH_4, CCl_4, BF_3` are zero due to their symmetrical structure.

 
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