University:
University of Alberta
Course:
CHEM 101 | Introductory University Chemistry I
Academic year:

2024
Views:

92

Pages:

4
Author:

Neena N.

Sigma and pi bonding Covalent bonds are formed when two atoms share electron pairs, and these bonds can be classified as sigma (σ) bonds or pi (π) bonds based on the types of overlapping orbitals involved. Sigma (σ) Bond A sigma bond is formed by the direct head-on or end-to-end overlap of atomic orbitals along the bonding axis. The electron density between the nuclei is concentrated in a cylindrical region. This type of bond allows for maximum electron density between the nuclei of the bonding atoms. In a sigma bond, the electron cloud is symmetrically distributed along the axis connecting the nuclei. Sigma bonds are usually stronger than pi bonds and are typically formed by the overlap of s orbitals, sp hybrid orbitals, or hybridized orbitals with a predominant s character. Example 1 In a simple covalent bond between two hydrogen atoms (H2), each hydrogen atom has one electron in its 1s orbital. When the two hydrogen atoms approach each other, their 1s orbitals overlap, resulting in a sigma bond. The overlapping region forms a molecular orbital known as a sigma molecular orbital. Example 2 In the case of a single bond between two carbon atoms (C-C) in an ethane molecule (CH3CH3), each carbon atom undergoes sp³ hybridization. 3 The sp hybrid orbitals overlap end-to-end to form sigma bonds. Pi (π) Bond A pi bond is formed by the sideways overlap of two parallel p orbitals or unhybridized p orbitals. The electron density is concentrated above and below the plane of the bonding nuclei. This type of bond occurs when parallel p orbitals or unhybridized p orbitals overlap. The overlapping p orbitals create regions of electron density both above and below the plane of the nuclei. Pi bonds are generally weaker than sigma bonds. Example 1 In an ethene molecule (C2H4), each carbon atom 2 2 undergoes sp hybridization. The sp hybrid orbitals form sigma bonds with other atoms, and the remaining unhybridized p orbitals on each carbon atom overlap sideways to create a pi bond. In this example, the sigma bonds are formed by the 2 end-to-end overlap of sp hybrid orbitals, while the pi bond is formed by the sideways overlap of unhybridized p orbitals. The pi bond consists of two regions of electron density, one above and one below the plane of the nuclei. In many cases, covalent bonds involve both sigma and pi bonds. For example, in a double bond between two atoms, one sigma bond is formed by the overlap of two atomic orbitals, and an additional pi bond is formed by the sideways overlap of two parallel p orbitals. In a triple bond, there is one sigma bond and two pi bonds.