Sigma and pi bonds are two types of covalent bonds formed by the overlapping of atomic orbitals. The main difference between them is the way the overlapping of atomic orbitals occurs. Sigma bonds are formed by the direct head-to-head overlap of atomic orbitals along the internuclear axis, while pi bonds are formed by the lateral overlap of two atomic orbitals.

Here are some key points about sigma and pi bonds:

Sigma (σ) Bond

• Formed by head-on positive (same phase) overlap of atomic orbitals along the internuclear axis.
• The strongest covalent bond, owing to the direct overlapping of the participating orbitals.
• The electrons participating in a σ bond are commonly referred to as σ electrons.
• Generally, all single bonds are sigma bonds.

Pi (π) Bond

• Formed by the lateral overlap of two atomic orbitals.
• Generally weaker than sigma bonds, owing to the significantly lower degree of overlapping.
• Consist of one sigma and one pi bond in double bonds, whereas a typical triple bond is made up of two π bonds and one σ bond.
• The overlapping of un-hybridized filled p orbitals form π bonds.
• Their orientation is orthogonal, as well as being displaced somewhat, to the initial sigma bond and parallel to the p orbital of the other atom.
• Unlike normal sigma bonds which can freely rotate, the addition of pi bonds prevents a bond from rotating.

To identify sigma and pi bonds, look at whether its a single, double, or triple bond. Sigma bonds are always the first bond to form, so every single covalent bond is a sigma bond. Pi bonds are the second and third bonds to form, so double and triple bonds have the initial sigma bond and then one and two pi bonds, respectively.