Illustrated Glossary of Organic Chemistry

Sigma bond (σ bond): A covalent bond formed by overlap of atomic orbitals and/or hybrid orbitals along the bond axis (i.e., along a line connected the two bonded atoms).

Hydrogen 1s orbitals
Hydrogen molecule (H2)
The sigma bond in the a hydrogen molecule (shown in red) is formed by overlap of a pair of 1s orbitals, one from each hydrogen atom.

2 x
+
 6 x


Carbon sp3 orbitals
     
Hydrogen 1s orbitals
C-H sigma bonds in ethane (H3C-CH3)
The carbon-carbon sigma bond in ethane is formed by overlap of a pair of carbon sp3 hybrid orbitals, one from each carbon atom. In a similar manner, the carbon-hydrogen sigma bond is formed by overlap of a carbon sp3 orbital with a hydrogen 1s orbital.