(Polar molecules, Non-polar molecules, etc.)
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A sigma bond has two orbitals that interact end to end to form the bond. Sigma bonds allow rotation of atoms. A pi bond has two orbitals that overlap side to side to form the bond. Pi bonds do not allow the atoms to rotate. When two atoms are bound by more than one covalent bond, the additional orbitals overlap side by side to form pi bonds. In other words, a single bond is always a sigma bond but any additional bond ( such as double or triple) is a pi bond. This is why triple bonds have one sigma bond and two pi bonds.
Sigma bonds are overlapping s-orbitals of two neighboring atoms that create a tubelike shape. Pi bonds consist of two unbonded p-orbitals of neighboring atoms. Atoms tend to bond first with sigma bonds, so single bonds will always be sigma bonds. A double bond consists of a sigma and a pi bond, and a triple bond consists of a sigma and two pi bonds. Pi bonds add rigidity to the bond (because there are two bonds, the atoms cannot "twist" or change their orientation with respect to won another). Pi bonds, while not as strong as sigma bonds, add strength to the bond between two atoms (sigma + pi > just sigma) and therefore shorten the bond length.