What is the Difference Between Sigma and Pi Molecular Orbitals

The key difference between sigma and pi molecular orbitals is that sigma molecular orbital forms from the overlapping of atomic orbitals in a head-to-head direction along the internuclear axis, whereas pi molecular orbitals form from the overlapping of atomic orbitals in a parallel direction.

Sigma and pi molecular orbitals are two types of molecular orbitals that contribute to the actual structure of a chemical compound. The molecular orbitals are responsible for the formation of single and double or triple bonds, respectively.

CONTENTS

1. Overview and Key Difference
2. What are Molecular Orbitals
3. What are Sigma Molecular Orbitals 
4. What are Pi Molecular Orbitals 
5. Sigma vs Pi Molecular Orbitals in Tabular Form
6. Summary – Sigma vs Pi Molecular Orbitals

What are Molecular Orbitals?

Atoms join together to form molecules. When two atoms move closer together to form a molecule, atomic orbitals overlap and combine to become molecular orbitals. The number of newly formed molecular orbitals is equal to the number of combined atomic orbitals. Furthermore, the molecular orbital surrounds the two nuclei of the atoms, and electrons can move around both nuclei. Similar to atomic orbitals, molecular orbitals maximally contain 2 electrons, which have opposite spins.

Moreover, there are two types of molecular orbitals as bonding molecular orbitals and antibonding molecular orbitals. Bonding molecular orbitals contain electrons in the ground state, while antibonding molecular orbitals contain no electrons in the ground state. Furthermore, electrons may occupy antibonding orbitals if the molecule is in an excited state.

What are Sigma Molecular Orbitals?

Sigma molecular orbitals are types of hybrid orbitals that form from the overlapping of two atomic orbitals from head-to-head along the internuclear axis. Typically, the first covalent bond between two atoms is always a sigma bond. Overlapping of two atomic orbitals in the inter-nuclear axis forms a sigma covalent bond. In a sigma molecular orbital, the electron density at the middle of the molecular orbital is high if the two atoms from which the atomic orbitals overlap are identical.

When considering hydrogen as an example, sigma molecular orbital forms from the overlapping of two 1s atomic orbitals coming from each hydrogen atom. We can abbreviate this sigma bond as σ. Here, the shared electron density occurs directly between the bonding atoms along the bonding axis. This makes a more stable interaction between two bonded atoms along with the bonding electrons compared to the stability of the separate atoms. Typically, a sigma bond is the first bond that forms between two atoms.

What are Pi Molecular Orbitals?

Pi molecular orbitals are types of hybrid orbitals that form from the overlapping of two atomic orbitals in the parallel direction. Here, the bonding electron density occurs above and below the internuclear axis. Further, we cannot observe electrons at the bonding axis. This type of interaction between two atoms forms a more stable arrangement than the stability of separated free atoms. Usually, electrons tend to occupy this type of molecular orbitals when enough electrons are present in the atoms. Pi molecular orbitals always form as second or third molecular orbital formation regarding the bonding of two atoms because sigma molecular orbital is the first molecular bond to form between two atoms.

Moreover, the number of atoms that are contributing to a p atomic orbital of the pi system is always equal to the number of pi molecular orbitals present in a chemical bond. Typically, the lowest pi molecular orbital has zero vertical nodes existing. Here, the successive pi molecular orbitals get one additional vertical node upon increasing the energy. We can abbreviate a pi molecular orbital as π.

What is the Difference Between Sigma and Pi Molecular Orbitals?

Sigma and pi molecular orbitals are two types of molecular orbitals that contribute to the actual structure of a chemical compound. The key difference between sigma and pi molecular orbitals is that sigma molecular orbital forms from the overlapping of atomic orbitals in a head-to-head direction along the internuclear axis, whereas pi molecular orbitals form from the overlapping of atomic orbitals in a parallel direction.

The following infographic lists the differences between sigma and pi molecular orbitals in tabular form for side by side comparison.

Summary – Sigma vs Pi Molecular Orbitals

A molecular orbital is a type of orbital that forms from the overlapping of atomic orbitals. The key difference between sigma and pi molecular orbitals is that sigma molecular orbital forms from the overlapping of atomic orbitals in a head-to-head direction along the internuclear axis, whereas pi molecular orbitals form from the overlapping of atomic orbitals in a parallel direction.

Reference:

1. “What Is a Sigma Molecular Orbital?” BYJUS, 24 Dec. 2020.
1. “15.3: Pi Molecular Orbitals of Benzene.” Chemistry LibreTexts, Libretexts, 14 July 2020.

Image Courtesy:

1. “Hydrogen Molecule” By ChiralJon (CC BY 2.0) via Flickr
2. “MO diagram pi orbitals” By V8rik at en.wikipedia (CC BY-SA 3.0) via Commons Wikimedia