Sigma Bond vs. Pi Bond — What's the Difference?

Sigma (σ) bonds are the strongest type of covalent chemical bond, formed by the head-on overlapping of atomic orbitals. These bonds are found in all single covalent bonds and are characterized by their symmetrical distribution of electron density around the axis connecting the bonded nuclei. Pi (π) bonds, on the other hand, are formed from the side-on overlapping of p-orbitals, with electron density concentrated above and below the plane of the nuclei. They are typically found in double and triple bonds, alongside sigma bonds.

Sigma bonds allow for free rotation of the bonded atoms around the bond axis due to their cylindrical symmetry. This property gives molecules with sigma bonds flexibility in their structures. In contrast, pi bonds restrict this rotation because of their parallel p-orbital overlap, which results in fixed planar structures.

The formation of a sigma bond involves either s orbitals, p orbitals, or a combination of both. This versatility makes sigma bonds more common in organic and inorganic molecules. Pi bonds are limited to p orbitals and are thus less versatile in how they can be formed.

Sigma bonds are generally stronger than pi bonds due to the direct overlap of orbitals, leading to a greater density of shared electrons between the bonded atoms. The side-on overlap in pi bonds is less effective at sharing electrons, making them weaker in comparison.

From a chemical reactivity standpoint, pi bonds often participate in reactions because their electrons are more loosely held and thus more reactive. Sigma bonds, being stronger and more stable, are less likely to be involved in chemical reactions.