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CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

σ bond (sigma bond)

A covalent bond formed by head-on overlap of two orbitals along the line joining the nuclei, so the electron density lies on the bond axis and is symmetric around it. Every single, double or triple bond contains exactly one σ bond; it allows free rotation and is stronger than a π bond.

σ bond: head-on overlapTwo p orbitals on neighbouring atoms point straight at each other along the bond axis; they overlap in the region between the two nuclei, which is where the σ bond's electron density sits.bond axisABoverlap on the axiselectron density between the nuclei → σ bond(s + s, s + p and end-on p + p all give σ)

Head-on overlap: the shared density sits on the bond axis.

Example

In ethane, CHX3−CHX3\ce{CH3-CH3}, all 7 bonds are σ bonds: six C sp3sp^3 + H 1s1s and one C sp3sp^3 + C sp3sp^3. That is why the two CHX3\ce{CH3} groups can spin freely about the C–C bond.

See alsoπ bond (pi bond)Orbital overlapHybrid orbitalMultiple bond