What is a sigma bond?
The sigma bond is a bond formed by head to head overlap of orbitals. In sigma bonds the electron density is concentrated in between the two atoms. The sigma bond in a molecule is denoted by a single bond. Hence in a molecule, all the single bonds are sigma bonds. For example in ethane (CH3-CH3), there are seven single bonds. So, there are seven sigma bonds in ethane (six C-H and one C-C). Thus saturated hydrocarbons possess only sigma bonds. The formation of sigma bonds is shown below