Question

Rules of hybridization say that hybrid orbitals form only sigma bonds. However, unsaturated hydrocarbons like (ethylene and acetylene) possess multiple bonds between carbons.

Answer 1

Rules of hybridization say that hybrid orbitals from only sigma bonds. This statement is true but ethylene and acetylene form multiple bonds.

In the case of ethylene, it shows $s{p}^2$ hybridization. Thus, only two p-orbital takes part in hybridization and form a sigma bond.

The one unhybridized p-orbital lie perpendicular to $s{p}^2$ hybrid orbital and form a double bond with unhybridized p-orbital of other carbon atoms by lateral sharing of electrons.

Similarly, in acetylene, two p-orbitals remain unhybridized and form two $\pi$ bonds with each other.