Sigma and pi bonds are types of covalent bonds that differ in the overlapping of atomic orbitals. Covalent bonds are formed by the overlapping of atomic orbitals. Sigma bonds are a result of head-to-head overlapping of s orbitals whereas pi bonds are formed by the lateral overlap of two atomic orbitals.
Various bond parameters such as bond length, bond angle, and bond enthalpy depend on the way the overlapping of atomic orbital takes place. This overlap occurs in two major ways, giving rise to two primary types of covalent bonds, i.e. sigma and pi bonds.
Sigma Bonding – The σ Bond
This type of covalent bond is formed by head-on positive (same phase) overlap of atomic orbitals along the internuclear axis. Sigma bonds are the strongest covalent bonds, 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. They can be formed via the following combinations of atomic orbitals.
In this kind of overlapping, one ‘s’ orbital from each participating atom undergoes head-on overlapping along the internuclear axis. An s orbital must be half-filled before it overlaps with another.
The overlapping of two s orbitals resulting in a sigma bond is illustrated above. This type of overlap occurs in H2 molecules, where each hydrogen atom has a half-filled s orbital.
Here, one half filed s orbital overlaps with one half-filled p orbital along the inter-nuclear axis, forming a covalent bond. This condition is illustrated below.
This type of overlapping can be observed in ammonia. An NH3 molecule features 3 sigma bonds, formed by the overlap of the 2px, 2py, and 2pz orbitals belonging to the nitrogen atom and the 1s orbitals of the three hydrogen atoms.
In this condition, one half filled p orbital from each participating atom undergoes head-on overlapping along the internuclear axis. This type of overlapping is illustrated below.
A Cl2 molecule features a p-p overlap of the 3pz orbitals of two chlorine atoms. It is important to note that the head-to-head overlapping of two p orbitals gives a sigma bond whereas the lateral overlap of these orbitals leads to the formation of pi bonds.
Pi Bonding – The π Bond
Pi bonds are formed by the sidewise positive (same phase) overlap of atomic orbitals along a direction perpendicular to the internuclear axis. During the formation of π bonds, the axes of the atomic orbitals are parallel to each other whereas the overlapping is perpendicular to the internuclear axis. This type of covalent bonding is illustrated below.
Pi Bonds are generally weaker than sigma bonds, owing to the significantly lower degree of overlapping. Generally, double bonds consist of one sigma and one pi bond, whereas a typical triple bond is made up of two π bonds and one σ bond. It is important to note that a combination of sigma and pi bonds is always stronger than a single sigma bond.
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