Sigma Bond

In-depth explanation of Sigma Bond

A sigma bond forms when orbitals overlap head on along the line joining the two nuclei, the internuclear axis. Its electron density is cylindrically symmetric about that axis, so rotation about the bond is essentially free. Every single bond is a sigma bond, and the first bond formed between any two atoms is always a sigma bond. Sigma bonds can arise from s-s, s-p or p-p end-on overlap or from overlap of hybrid orbitals. Sigma bonds are generally stronger than the pi bonds that accompany them in multiple bonds.

Going deeper: Sigma Bond

Key ideas: Count sigma bonds as exactly one per connected pair of atoms. In a molecule, number of sigma bonds equals number of bonds in a skeleton with all multiple bonds reduced to single. Example: ethene has 5 sigma and 1 pi bond, and ethyne has 3 sigma and 2 pi. Sigma bonds allow free rotation and have cylindrical symmetry.

Common mistakes and connections: Students mistakenly call a double bond two sigma bonds or count pi bonds as sigma. Another slip is saying that rotation is free in any bond with a sigma component: rotation is blocked by the pi component. The number of sigma bonds plus lone pairs equals domains, which gives hybridization. Links to hybridization and VSEPR.

Visualization of Sigma Bond

The H-Cl bond is a sigma bond, formed by head-on overlap of hydrogen 1s with a chlorine 3p orbital along the internuclear axis. The interactive lesson shows this as a live animation or chart.

Worked example and use case

All four C-H bonds in methane are sigma bonds formed from carbon sp3 hybrids and hydrogen 1s orbitals.

Where Sigma Bond fits in Bonding & Molecular Structure

Sigma Bond is topic 51 of 104 in Bonding & Molecular Structure.

Related topics

• Square Pyramidal GeometryAdvanced
• Bond Angle DeviationsIntermediate
• Valence Bond TheoryIntermediate
• Pi BondBeginner
• HybridizationIntermediate
• sp3 HybridizationIntermediate

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