Sigma and Pi Bond

Do you know what sigma and pi bonds are? In Chemistry, sigma (σ) and pi (π) bonds are covalent bonds resulting from overlapping atomic orbitals. They are crucial in understanding the structure and properties of molecules. In this section, we will discuss sigma and pi bond definition, structure, and formation. As we know, Sigma (σ) and pi (π) bonds are types of covalent bonds that form between atoms in molecules. They are defined based on the way the orbitals overlap during bond formation. Further, we will discuss each in detail.

1. 1.0Sigma (σ) Bond

A sigma (σ) bond is a type of covalent bond formed by the head-on, positive (same phase) overlap of atomic orbitals along the internuclear axis. It is the strongest type of covalent bond due to the direct overlap of the participating orbitals, which maximises the electron density along the bond axis. The electrons participating in a sigma bond are called σ electrons.

Bond Characteristics of Sigma Bonds

Overlap Nature:

• Sigma bonds result from the direct head-on overlap of atomic orbitals.
• The orbital overlap occurs along the internuclear axis, directly connecting the nuclei of the bonding atoms.

Strength:

• Sigma bonds are stronger than pi bonds because of the direct overlap of orbitals, leading to a more effective sharing of electrons.
• They are generally the strongest type of covalent bonds.

Rotational Freedom:

• Sigma bonds allow free rotation around the bond axis.
• This rotation is possible because the electron density is concentrated along the internuclear axis.

Formation of Sigma Bonds

1. s-s Overlapping in sigma bond:

• Sigma bonds can form through the overlap of two s orbitals.
• ss overlapping examples: In forming a sigma bond between two hydrogen atoms (H₂), each hydrogen atom contributes its 1s orbital for the sigma bond.

2. s-p Overlapping in sigma bond:

• Sigma bonds can form between an s orbital and a p orbital.
• Example: In forming a sigma bond between hydrogen and chlorine in HCl, the hydrogen 1s orbital overlaps with the chlorine 3p orbital.

3. p-p Overlapping in sigma bond:

• Sigma bonds can form between two p orbitals.
• Example: In forming a sigma bond between two carbon atoms in ethane (C₂H₆), the two carbon atoms contribute sp³ hybridised orbitals for sigma bond formation.

To summarize: 

• A sigma bond is formed by the head-on overlap of atomic orbitals along the axis connecting the nuclei of the bonding atoms. It is the strongest type of covalent bond.
• In a sigma bond, the electron density is concentrated along the internuclear axis, directly between the two nuclei.
• Sigma bonds can occur in single bonds (σ), double bonds, and triple bonds.
• In a simple example of a single bond between two hydrogen atoms (H₂), the sigma bond is formed by the overlap of two hydrogen 1s orbitals:

2.0Pi (π) Bond

Bond Characteristics of the pi Bond:

• Pi bonds are weaker than sigma bonds.
• They do not allow free rotation around the bond axis.
• Pi bonds are typically found in double bonds (one sigma, one pi) and triple bonds (one sigma, two pi).

Formation of Pi Bond: 

Pi bonds are formed by the side-to-side overlap of parallel p orbitals that are perpendicular to the internuclear axis.

Representation: In a Lewis structure, a double bond (σ + π) is represented by a double line (e.g., C₂H₄: C=C), and a triple bond (σ + 2π) is represented by a triple line (e.g., C₂H₂: C≡C).

In a molecule with multiple bonds, the first bond formed is always a sigma bond, and any additional bonds involve the formation of pi bonds.

Example: Ethene (C₂H₄)

• The carbon-carbon double bond in ethene involves one sigma bond and one pi bond.
• The sigma bond is formed by the head-on overlap of two sp² hybridised orbitals, while the pi bond results from the side-to-side overlap of the unhybridised p orbitals.           

Understanding the nature of sigma and pi bonds is essential for predicting molecular geometries, reactivity, and other properties in organic and inorganic chemistry.

3.0Difference Between Sigma and Pi Bond