In chemistry, sigma bonds (σ bonds) are chemical bonds of the covalent type, where a single lobe of one involved electron orbital overlaps a single lobe of the other involved electron orbital. The orbital has no node planes which go through both atoms. Sigma bonds are the strongest type of covalent bonds. The most intense parts of the involved electron orbitals overlap. Electrons in sigma bonds are sometimes referred to as sigma electrons. The Greek letter σ in their name refers to s orbitals, since the orbital symmetry of the sigma bond is the same as that of the s orbital when seen down the bond axis.

The simplest sigma bond is between the two s orbitals in molecular hydrogen (H₂). There are more exotic sigma bonds, such as between metals and olefins (alkenes), where there is a sigma bond between a d orbital of the metal and a pi bond of the olefin. In recognizing sigma bonds in chemical chains of elements, they are the single lines, or one of the double or triple bonds. (i.e. H₂O H-O-H has two sigma bonds. CO₂ O=C=O has two sigma bonds and two other bonds.)

See also

• Pi bond
• Delta bond
• Molecular geometry

رابطة سيجما | Liaison σ | Σ結合 | Orbital sigma