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In chemistry, sigma (σ) bonds are covalent bonds in which electron density is most heavily concentrated along the internuclear axis of two nuclei. In almost all cases, sigma bonds are single bonds. Sigma bonds are not to be confused with pi (π) bonds, which are generally weaker and do not occur along the internuclear axis.
Sigma bonds occur along the internuclear axis of two nuclei. The internuclear axis is the straight line that directly connects the nuclei of two different atoms. A sigma bond is formed when the electron density is concentrated symmetrically around this axis.
An understanding of atomic orbitals is necessary to understand the basics of how sigma bonds can be formed between atoms. The simplest example of a sigma bond can be found in the overlap of the s orbitals in hydrogen (H2). The overlap of an s and a p orbital also can result in a sigma bond, as can the overlap of two p orbitals. Even the overlap of a p orbital and a hybridized sp orbital can result in a sigma bond, as long as the electron density is concentrated around the internuclear axis of the atoms.
A good example of the overlap of an s orbital and a p orbital can be found in hydrogen chloride (HCl). Diatomic bromine (Br2) forms a sigma bond by the overlap of two 4p orbitals. In the chemical beryllium fluoride (BeF2), the bonds are formed by the overlap of the 2p orbitals in fluorine with the large lobes of the sp hybrid orbitals of beryllium.
When p orbitals overlap away from the internuclear axis, a pi bond is formed. Pi bonds are covalent bonds that are generally not as strong as sigma bonds. They are most commonly associated with double bonds and triple bonds.
As a general rule, a double bond consists of one sigma bond and one pi bond and a triple bond consists of one sigma bond and two pi bonds. Because pi bonds are generally weaker, double bonds will be stronger than single bonds, but the total strength will be less than twice the strength of the single bond. A common mnemonic device used by students to remember the association of sigma and pi bonds to single and multiple bonds is "sigma is singular and pi is plural."
The chemical formaldehyde can be used as an example in showing both kinds of bonds present in a single molecule. Three sigma bonds exist in formaldehyde; there is one between the carbon and oxygen atoms and one between each hydrogen atom and the carbon. A pi bond also exists between the carbon and the oxygen but, unlike the other bonds, it does not lie along the internuclear axis.