We have studied that the ionic bond is formed between electropositive element with low ionisation energy and an electronegative element with high electron affinity. The bond results by the transference of electrons from one atom to another so that they acquire stable octets. The gain or loss of electrons cannot take place between atoms of same element or those which have similar electronegativities or high ionisation energy. Such atoms combine chemically by mutual sharing of electrons so as to attain noble gas configuration.
A force which binds atoms of same or different elements by mutual sharing of electrons is called a covalent bond the term covalent bond was introduced by Langmuir in 1919
The atoms involved in bond formation contribute equal number of electrons for sharing. The shared electrons become a common property of both the atoms and constitute a bond between them. The shared pair of electrons which is responsible for the formation of bond is also called bond pair of electrons. Let us consider the formation of Cl2 molecule.
Each chlorine atom, Cl (2, 8, 7), has seven electrons in Its valence-shell and needs one more electron to acquire octet. both the chlorine atoms contribute one electron each to share two electrons. The bond can be represented by putting a line between the atoms instead of a bond pair.
Similarly, in the formation of HCl molecule, both H and Cl atoms contribute one electron each which is shared by Both. In this way H attains configuration of helium while Cl Attains configuration of argon.
Multiple Covalent Bonds. When the atoms share one Electron pair, the bond formed is called single covalent bond. however, if two electron pairs are shared by the atoms, the bond formed is called double covalent bond. Similarly, when the atoms share three electron pairs, the bond is called triple covalent bond. The double and triple covalent bonds are collectively called multiple covalent bonds. Let us now study the examples for double and triple bonds.
Formation of oxygen (02) molecule. Oxygen atom (8O) Has six electrons in its valence shell (2, 6). It has a tendency To attain the configuration of Ne (2, 8). The two oxygen atoms, Therefore contribute 2 electrons each and share four electrons, i.e;2 bond pairs. Thus, the oxygen atoms in oxygen molecule are held by a double covalent bond.
Formation of nitrogen molecule. Nitrogen atom (7N) has have electrons in its valence shell (2, 5). It also tends to acquire The configuration of Ne (2, 8). The two nitrogen atoms, Therefore contribute 3 electrons each to share three pairs of electrons. Thus, the nitrogen atoms in N2 molecule are held by a triple covalent bond
The covalency of an element may be defined as the number of electrons which its atom contributes for sharing while forming covalent bond. For example, the covalency of chlorine is 1 ; oxygen is 2 and that of nitrogen is 3.
It may be noted that covalent bonded substances exist as discrete molecules. therefore, they are also called molecular substances.