(i) Carbon has the ability to form long, branched chains or ring structures with other carbon atoms. This self-linking capacity is referred to as catenation.

Also carbon has a valency of 4 and combines with 4 other monovalent atoms or shares its outermost electrons with different elements through single, double or triple bonds. The bonds formed are highly strong and stable. Hence carbon forms a large variety of compounds.

(ii) Compounds of carbon which are linked by only single bonds between the carbon atoms are called saturated compounds.

Compounds of carbon having double or triple bonds between the carbon atoms are called unsaturated compounds.

(iii) Unsaturated compounds are more reactive than saturated compounds because of the presence of double or triple bonds present in them. These bonds tend to break in the presence of some reacting species to become saturated compounds which are highly stable. The relative stability of hydrocarbons in the decreasing order is given by:

alkanes > alkenes > alkynes