(a) Sigma (σ) bond and pi (π) bond.
Sigma (σ) bond: This type of covalent bond is formed by the end to end overlapping of half-filled atomic orbitals along the internuclear axis. The overlap is also known as head on overlap or axial overlap. The electrons constituting sigma bond are called sigma electrons.
Pi (π) bond: This type of covalent bond is formed by the lateral or sidewise overlap of half-filled atomic orbitals. The atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axis.
(b) 10 σ bond in propane
11 σ bond and 1π bond in 2-butene