Sigma bonds are covalent bonds with electron density concentrated along the internuclear axis. They occur when atomic orbitals overlap symmetrically around the axis. Pi bonds are weaker and occur when orbitals overlap away from the axis. Double and triple bonds have one sigma bond and one or two pi bonds respectively. Formaldehyde has three sigma bonds and one pi bond.
In chemistry, sigma bonds (σ) are covalent bonds in which the electron density is most concentrated along the internuclear axis of two nuclei. In almost all cases, sigma bonds are single bonds. Sigma bonds should not 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 directly connecting the nuclei of two different atoms. A sigma bond forms 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 form between atoms. The simplest example of a sigma bond can be found in the overlap of s orbitals in hydrogen (H2). The overlap of one s and ap orbital can also result in a sigma bond, as can the overlap of two p orbitals. The overlap of an ap orbital and a hybridized sp orbital can also result in a sigma bond, provided the electron density is concentrated around the internuclear axis of the atoms.
A good example of an orbital overlap if an ap orbital can be found in hydrochloric acid (HCl). Diatomic bromine (Br2) forms a sigma bond through the overlap of two 4p orbitals. In the chemical beryllium fluoride (BeF2), 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. Since 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 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 types of bonds present in a single molecule. There are three sigma bonds in formaldehyde; there is one between the carbon and oxygen atoms and one between each hydrogen atom and the carbon. There is also a pi bond between carbon and oxygen but, unlike the other bonds, it does not lie along the internuclear axis.
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