Answer. There are a total of 118 elements known today. The number of organic compounds (carbon compounds) is more than ten million. This number is far more than the number of compounds of all the remaining elements taken together. The existence of such a large number of organic compounds is due to the following reasons:
(i) The main reason for the existence of a large number of organic compounds is that carbon atoms can link with one another by means of covalent bonds to form long chains or rings of carbon atoms. The chains can be straight or branched. The ability of carbon atoms to link with other carbon atoms to form long chains and large rings is called catenation.
Two basic conditions for an element to exhibit catenation are:
(a) Element should have valence two or more than two.
(b) Bonds made by an element with its own atoms should be stronger than the bonds made by the element with other atoms especially oxygen.Both silicon and carbon have similar electronic configurations but carbon shows catenation whereas silicon does not. It is mainly due to the reason that C-C bonds are much stronger (355 kJ mol-1) than Si-Si (200 kJ mol-1) bonds. On the other hand, Si - O bonds are much stronger (452 kJ.mol-1) than C-0 bonds (351 kJ mol-1). Hence, silicon occurs in the form of silica and silicates in nature.
Another reason for the abundance of organic compounds is the phenomenon of isomerism.
The compounds are said to be isomers if they have the same molecular formula but different arrangement of atoms in molecules or different structural formula.
Isomerism also adds to the possible number of structures, e.g., molecular formula C5H12 can be represented by three different structures. Thus, C5H12 has three isomers, as shown below:
CH3
H3C -CH2-CH2-CH2-CH, H3C-CH-CH2-CH3 H3C-C-CH3 n-pentane n-pentane CH3 CH3
isopentane neopentane
Number of isomers increases with the increase in number of carbon atoms in the given molecular formula.
(iii) Due to its very small size, carbon can form very strong covalent bonds with other carbon atoms, hydrogen, oxygen, nitrogen and halogens. This enables it to form a large number of compounds.
(iv). In order to satisfy its tetravalancy, carbon can make multiple bonds (i.e., double and triple bonds). This further adds to the possible number of structures. . For example, two carbons in ethane are linked by a single covalent bond, by a double covalent bond in ethylene and a triple covalent bond in acetylene.