Jerusha Mamre N.

Hello Amit,

Covalent bonding is the type of bonding between two atoms that occurs by sharing electrons as opposed to ionic /electrovalent bonding, wherein atoms are bound by losing/gaining electrons to fulfil their valency. It occurs when the electrons cannot be transferred entirely (like ionic bonding). Sharing of 1 or more electrons forms covalent bonds between two atoms. It gives rise to homoatomic or heteroatomic covalent molecules. Homoatomic covalent molecules are formed by sharing electrons amongst the atoms of the same element. At the same time, heteroatomic covalent molecules are formed when electrons are shared between atoms of different elements. The simplest examples of covalent molecules are H2, O2, F2, Cl2, N2, etc. You will notice that almost every homoatomic molecule/two atoms of the least electronegativity difference are made up of covalent bonds. 

Let us consider H2. Each Hydrogen atom has 1 electron, but the duplet valency, at least, must be fulfilled by bonding, and therefore, Hydrogen doesn't exist in monoatomic condition; it is always found as a diatomic molecule. Hence, if 2 Hydrogen atoms contribute their single valence electron towards the bond, both their valencies are fulfilled, and a covalent bond is formed between them. Since the number of electrons shared by each atom is 1, therefore 2 in total, we say that a 'shared pair of electrons form this bond. Likewise, you can observe some molecules with more than one shared pair of electrons, e.g. O2 has two shared pairs of electrons(double bond), N2 has three shared pairs of electrons (triple bond).

Sharing of electrons can occur in two ways. When both atoms contribute towards the bond formation, it is just the simple covalent bond explained above, e.g. H2. Still, when only one atom contributes to the shared pair, and the other atom doesn't contribute any electrons, it is called 'coordinate covalent/ dative bond'. eg. NH4+ ion(ammonium ion).

In a Carbon atom, there are four valence electrons; it needs four more electrons for its valency to be fulfilled. It can neither lose all 4 electrons nor gain 4 electrons completely from another atom to attain a stable octet configuration. Therefore, it shares its 4 electrons with four other atoms (either carbon atoms/ hydrogen and hydrogen-like atoms, which can contribute electrons towards sharing). If sharing occurs between a single Carbon atom and 4 Hydrogen atoms, there will be four shared pairs, and each pair is formed by 1 electron each from the carbon atom and 1 Hydrogen atom. The molecule formed is CH4(methane), a 'hydrocarbon', so-called Hydrogen and carbon atoms. 

Covalent bonding is essential and very advantageous to Carbon because, through this, Carbon can attain an octet configuration, form multiple bonds(single, double, triple), and even form long carbon atoms.

*(this is one of the main reasons. The process is called 'catenation')* or cyclic systems (carbon rings) and can form many hydrocarbons of different possible compositions and many possible structures for a single molecular formula. And all this is possible only with a single element, Carbon. And hence, it is relevant since all organic compounds in nature are made up of hydrocarbons of different compositions and structures.

Now, you might say, 'even Hydrogen, Oxygen also from covalent bonds, why are they not given so much importance?' You see, after 2 Hydrogen atoms are bound, there is no further scope to form another covalent bond or form long chains. Similarly, oxygen molecules can have double bonds only, so they can't bond further with other atoms. But, since Carbon can form single and multiple bonds, they have scope to bind to other atoms of their kind and different elements. That makes it even more special because now we can have functional groups as well (we can have heteronuclear and heterocyclic compounds - which make up many drugs in today's medicinal chemistry).

I hope this makes everything clear!