Sigma Bond and Pi Bond

Chemical bonding is a basic concept in chemistry that describes how atoms are brought together to create molecules. Of all the various forms of chemical bonds, sigma bonds and pi bonds are two of the main forms of covalent bonding. Although both stabilise and contribute to the properties of molecules, they are quite distinct in their structure, strength, and nature. The importance of these bonds is such, it explains the molecular geometry, creativity and even molecules’ physical properties; and hence will be our topic of discussion today. 

1.0Covalent Bonding and Orbitals

Before getting into the insight of sigma and pi bonds, let’s take a quick look at what exactly the covalent bonds and atomic orbitals are. A covalent bond is a type of chemical bond that is formed via the sharing of electron pairs between atoms. These shared pairs of electrons in atoms give them the electronic configuration of a noble gas, which is important for an atom’s stability. 

Now, coming to the orbitals, these are basically the regions where the probability of finding an electron is the highest. Electrons in this region move in a certain path, giving them a particular shape, according to the number of electrons, such as: 

• s-orbital: An s-orbital is spherical and can hold up to 2 electrons. 
• p-orbitals: These are dumbbell-shaped orbitals and are divided into three sets of p-orbitals (px, py, pz) of 2 electrons each. 
• d-orbitals: d-orbitals are more complex with a clover shape. d-orbitals start showing up in the 3rd shell with five d orbitals per energy level. 
• f-orbitals: These start with the fourth shell and have a more complex shape. 

When atoms come closer to each other to form bonds, these orbitals can overlap with respect to an imaginary line joining the centres of nuclei of two bonded atoms, the internuclear line. The nature of overlap decides the type of bond formed, either sigma or pi. 

2.0Sigma Bond Definition: What is a sigma bond?

A sigma bond can be defined as the strongest form of chemical bond in covalent bonding. It is formed by the head-on (end-to-end) extensive overlapping of atomic orbitals, leading to a high electron density between the bonded atoms. The sigma bonds of atoms are identified for their ability to provide stability, which ultimately facilitates forming a molecule. The sigma bonds are denoted with the sign “ \sigma”. 

3.0Properties of Sigma Bonds

There are a number of properties of sigma bonds that help in the identification of these bonds. Some of these properties are: 

• Stronger than pi bonds: Sigma bonds are more stable and stronger owing to the direct overlap of the orbitals.
• Permit free rotation: Owing to the symmetrical electron density along the bond axis, the atoms linked by a sigma bond are able to rotate freely with respect to one another without disrupting the bond.
• In single bonds: All single bonds between atoms, including C-H, O-H, and N-H, have a sigma bond involved.

4.0Pi Bond Definition: What is a pi bond? 

A Pi bond, denoted with “ \pi ”, is another type of covalent bond that is formed by the side-by-side (lateral) overlapping of unhybridised p-orbitals. This means that in pi bonds, electron density can be found above and below the axis connecting the two atomic nuclei. These bonds are generally weaker when compared to sigma bonds due to the manner of overlapping of their orbitals. 

5.0Properties of Pi Bonds

Here are some key properties of Pi bonds: 

1. Weaker than sigma bonds: Pi bonds have weaker electron density and are more prone to breaking than sigma bonds.
2. Hinder rotation: The side-to-side overlap of p-orbitals leads to a stiff structure, hindering the rotation of atoms about the bond axis.
3. Existed in multiple bonds: Pi bonds exist only in double and triple bonds, always accompanied by a sigma bond. This is the reason why molecules with multiple bonds (double or triple bonds) are less stable than their counterparts. 
4. Electron density above and below the internuclear axis: In contrast to sigma bonds, in which electron density is found in between the two nuclei, pi bonds feature electron density distributed above and below the axis that links the nuclei.

6.0Formation of Sigma and Pi Bonds

Formation of Sigma Bonds

The formation of a sigma bond occurs when atomic orbitals overlap in such a way that all the electrons in the orbital align along the internuclear axis. These orbitals can overlap as: 

• s-s overlap: Two s orbitals of two different atoms overlap to produce a strong sigma bond, such as in the molecule of H₂.
• s-p overlap: One s orbital and one p orbital of two different atoms overlap and produce a sigma bond. An example of such overlap is the HCl molecule. 
• p-p overlap: Overlap of two p orbitals produces a sigma bond, like in the molecule of F₂.

Formation of Pi Bond 

Pi bonds are always created together with a sigma bond. In a double and a triple bond, a sigma bond is created initially, and subsequently, one or more pi bonds are created. Pi bonds exist between unhybridised p-orbitals, which occur in higher atomic orbitals.

7.0Sigma Bond vs Pi Bond