Give reasons for the following:
(a) Unsaturated hydrocarbons show addition reactions but not saturated hydrocarbons.
(b) Carbon forms only covalent bonds.

Explanation

(a) Unsaturated hydrocarbons contain either one/more double bonds `(gtC=Clt)` or triple bonds `(-C-=C-)` in their molecules. These are unstable and reactive in nature. The saturated hydrocarbons contain only C-C bonds in their molecules and are of stable nature. Therefore, unsaturated hydrocarbons have urge or desire to become saturated or stable. to achieve this, they take part in the addition reactions. for example.
`underset("Ethyne")(H-C-=C-H+H_(2))underset(473K)overset("Nickel (Ni)")tounderset("Ethene")(H-overset(H)overset(|)(C)=overset(H)overset(|)(C)-H), H-overset(H)overset(|)(C)=overset(H)overset(|)(C)=overset(H)overset(|)(C)-H+H_(2)underset(473K)overset("Nickel (Ni)")tounderset("Ethane")(H-underset(H)underset(|)overset(H)overset(|)(C)-underset(H)underset(|)overset(H)overset(|)(C)-H)`
(b) Carbon (Z=6) has electronic distribution 2,4. It is not in a position to either lose four electrons or gain four electrons since energy involved is very high. It shares its four valence electrons with the electrons of other atoms ad takes part in the covalent bond formation.