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.
()
S-P overlapping
This sort of overlap takes place between half-full s-orbitals of one atom and half-full p-orbitals of another.
()
P–P overlapping
This sort of overlap exists between half-filled p-orbitals of the two atoms that approach.
()
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.
()
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
|
Parameter |
Sigma Bond |
Pi Bond |
|
Formation of Bonds |
Sigma bonds are formed by the axial overlap of half- filled atomic orbitals. |
Pi bonds are formed through the lateral overlap of the half – filled atomic orbitals. |
|
Overlapping Orbitals |
In sigma bonds, orbitals may overlap: two hybrid orbitals, one hybrid and one pure orbital or two pure orbitals. |
For pi bonds, two pure (i.e., unhybridised) orbitals are always alternating orbitals. |
|
Existence |
It exists independently. |
Pi-bond always exists along with sigma bonds. |
|
Rotation of Two Carbon Atoms |
Free rotation is seen in sigma bonds. |
Free rotation is restricted. |
|
Bond Strength |
The strength of sigma bonds is more than pi bonds. |
Pi bonds are less strong than sigma bonds. |
|
Bond Forming Order |
Sigma bonds are formed first when atoms come closer. |
Pi bonds are formed after the formation of sigma bonds. |
|
Number of Bonds |
There is only one sigma bond between two atoms. |
There can be two pi bonds between two atoms. |
|
Control of Geometry in Polyatomic Molecules |
Sigma bonds are involved in the control of geometry in polyatomic molecules. |
Pi bonds are not involved in the control of geometry in polyatomic molecules. |
|
Number of bonds in a double bond |
There is one sigma bond in a double bond. |
There is only one pi bond observed in a double bond. |
|
Number of bonds in a triple bond |
There is one sigma bond in a triple bond. |
There are two pi bonds in the triple bond. |
|
Symmetry of the charge |
Sigma bond has cylindrical charge symmetry around the bond axis. |
Pi bond has no symmetry. |
|
Reactiveness |
Sigma bonds are more reactive. |
Pi bonds are less reactive. |
|
Shape determination |
Shape of a molecule is determined by the sigma bond. |
Shape of a molecule is not determined by the 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).