Sigma Bond
A sigma bond (σ bond) is the strongest type of covalent bond, formed by the direct head-on overlap of atomic orbitals along the internuclear axis. It is the first bond formed between two atoms in any covalent bond and allows free rotation around the bond axis. Sigma bonds are present in all single, double, and triple bonds and are responsible for the overall framework and shape of molecules.
Sigma Bond: Orbital Overlap Types
| Orbital Combination | Example | Bond Strength (kJ/mol) | Notes |
|---|---|---|---|
| s–s overlap | H–H (H₂) | 436 | Simplest sigma bond |
| s–p overlap | H–C (methane) | ~414 | Common in organic molecules |
| p–p overlap (head-on) | C–C (ethane) | ~347 | Backbone of hydrocarbons |
| sp³–sp³ | C–C (ethane) | ~347 | Tetrahedral hybridisation |
| sp²–sp² | C–C (ethene, single bond component) | ~614 (total) | One σ + one π in double bond |
| sp–sp | C–C (ethyne, single bond component) | ~839 (total) | One σ + two π in triple bond |
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Related Terms
Pi Bond
A pi bond (π bond) is a covalent bond formed by the lateral (side-by-side) overlap of unhybridised p orbitals above and below the internuclear axis. Pi bonds are always formed in addition to an existing sigma bond, making up the second bond in a double bond and the second and third bonds in a triple bond. Unlike sigma bonds, pi bonds restrict rotation around the bond axis, which is critical for cis-trans isomerism in alkenes.
Covalent Bond
A covalent bond is a type of chemical bond formed when two atoms share one or more pairs of electrons, resulting in a stable arrangement for both atoms. This sharing occurs most commonly between non-metal atoms that have similar electronegativities, allowing each atom to achieve a full valence shell without complete electron transfer. Covalent bonds are the foundation of organic chemistry and molecular biology, governing the structure of molecules ranging from water (H₂O) to complex proteins.
From the Greek letter sigma (σ), the first letter of the Greek word for "overlap" (syndesis). The notation was introduced in molecular orbital theory in the early 20th century to denote bonds with cylindrical symmetry around the internuclear axis.