Sigma Bond and Pi Bond

The overlapping of atomic orbitals distinguishes sigma and pi bonds from each other. The overlapping of atomic orbitals forms covalent bonds. The head-to-head overlapping of atomic orbitals forms sigma bonds, whereas the lateral overlap of two atomic orbitals forms pi bonds. Both names, sigma and pi, are derived from the Greek letters.

Various bond properties, including bond length, bond angle, and bond enthalpy, are influenced by how atomic orbitals overlap. This overlap happens in two ways, resulting in two different types of covalent bonds: sigma and pi bonds.

Generally, sigma bonds are stronger than pi bonds. Both are used extensively to predict the behaviour of molecules in molecular orbital theory.

Sigma(σ) Bond

Sigma Bond Definition

The covalent bond formed by the axial overlap of atomic orbitals is called a sigma bond. For example, the methane molecule contains 4 C-H sigma bonds. This type of covalent bond is formed by the overlap of bonding orbitals along the internuclear axis from end to end (head-on). This is called head overlapping or axial overlapping. Any of the following types of combinations of atomic orbitals may form this.

S-S overlapping

In this case, two half-filled s-orbitals are interacting along the internuclear axis, as shown below.

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S-P overlapping

This sort of overlap takes place between half-full s-orbitals of one atom and half-full p-orbitals of another.

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P–P overlapping

This sort of overlap exists between half-filled p-orbitals of the two atoms that approach.

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Pi (π) Bond

Pi Bond Definition

Pi bonds are formed when atomic orbitals intersect in a sideways positive (same phase) direction perpendicular to the internuclear axis. The axes of the atomic orbitals are parallel to one other during bond formation, whereas the overlapping is perpendicular to the internuclear axis.

Throughout pi-bond formation, the atomic orbitals converge so that their axes appear parallel to each other and perpendicular to the central axis. The side-overlapping orbitals consist of two types of saucer-charged clouds above and below the surface of the atoms involved.

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Strength of Sigma and Pi Bonds

Essentially, a bond's strength depends on the extent to which it overlaps. The duplication of orbitals arises to a greater degree in the case of a sigma bond. Therefore, it is stronger than the pi bond, where the extent of overlap occurs to a lesser extent. Further, it is important to note that pi bond(s) are produced in addition to a sigma bond in the formation of multiple bonds between two atoms of a molecule.

Difference Between Sigma Bond and Pi Bond

Importance

Multiple bonds are seen in covalent compounds (double or triple bonds). In single bonds only a sigma bond is present, but multiple bonds have both sigma and pi bonds. A double bond has one sigma and one pi bond, while a triple bond has one sigma and two pi bonds.

Multiple bonds influence a molecule's electrical characteristics and can change its physical properties such as the boiling point and melting point. Multiple bonds are also beneficial for understanding nuclear magnetic resonance spectra (NMR).