Reason for formation of large number of compounds

1.  Atoms combine to form molecules to complete their octet and attain lower energy state so as to become more stable. 
2.  Elements on the left hand side of carbon have more tendency to lose electrons. 
3. • Elements on the right hand side of carbon have more tendency to gain electrons.
4.  • As carbon is in the middle of the second period so it has equal tendency to loose or gain 4 electrons. But generally it is seen that carbon neither gains nor looses electrons, instead it shares electrons with other atoms like H, F, Cl, Br, I, N, O, S, P, etc and forms covalent compounds.
5.  • Due to sharing of electrons with other atoms many different functional groups are formed resulting in large number of organic compounds.
6.  • Carbon not only shares its electrons with other atoms but it can also share with other carbon atom thereby forming long chain carbon compounds having single, double or triple bond. This property is called catenation.
7.  • Hence position of carbon in the periodic table which includes its size, electronegativity, electron affinity, property of catenation etc is the reason behind the formation of large number of organic compounds.