Covalent Bond Formation


 Covalent Bond Formation

Covalent Bond

A covalent bond may be defined as the bond formed by mutual sharing of electrons between the combining atoms.

Examples:
1. Formation of chlorine molecule
Chlorine atom has seven electrons in the valency shell. In the formation of chlorine molecule, each chlorine atom contributes one electron and then the pair of electrons is shared between two atoms. Both the atoms acquire stable configuration of argon.
2. Formation of HCl molecule
Both hydrogen and chlorine contribute one electron each and then the pair of electrons is equally shared. Hydrogen acquires the configuration of helium and chlorine acquires the configuration of argon.
3. Formation of O2 molecule
Each oxygen atom contributes two electrons and two pairs of electrons are then shared equally. Both the atoms acquire configuration of neon.
4. Formation of N2 molecule
Nitrogen atom has five valence electrons. Both nitrogen atoms achieve configuration of neon by sharing 3 pairs of electrons, i.e., each atom contributes 3 electrons.

Conditions for formation of covalent bonds
The following condition favours the formation of covalent bond:

Electronegativity Difference between Atoms

An atom will not transfer the electron or electrons to the other atom if the electronegativity difference between the two atoms is zero or very small (less than 1.6).Such atoms prefer to share electrons, i.e., form covalent bonds.

Characteristics of covalent compounds


1. Physical state
(a) Under the normal conditions of temperature and pressure, these exist as gases or liquids of low boiling points.
(b) This is due to the fact that very weak forces of attraction (Vander walls forces) exist between discrete molecules.
(c) Some exist as soft solids if their molecular masses are high. Sulphur, phosphorus, iodine are soft solids.
2. Melting and boiling points
(a) With the exception of few which have giant three dimensional
structures such as diamond, carborundum (SiC), silica (SiO2), others have relatively low melting and boiling points.
(b) This is due to the presence of weak attractive forces between the molecules.
3. Electrical conductivity
(a) In general covalent substances are bad conductors of electricity.
(b) Substances which have polar character like HCl in solution, can conduct electricity.
(c) Covalent solids having giant molecules, are bad conductors since they do not contain charged particles or free electrons.
(d) Graphite can conduct electricity since electrons can pass from one layer to the other.
(e) Some show conductivity due to self ionisation.
4. Solubility
(a) Polar compounds (i.e. EN 0.9 to 1.8) are soluble in polar solvent.
Polar solvents are = H2O, CHCl3, Alcohol.
(b) Nonpolar compounds are soluble in non-polar solvents.
 Non-polar solvents are CCl4, Benzene, CS2.


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