Multiple Bonds
A typical double bond consists of one sigma bond and one pi bond; for example, the C=C double bond in ethylene. A typical triple bond, for example in acetylene, consists of one sigma bond and two pi bonds in two mutually perpendicular planes containing the bond axis. Two pi bonds are the maximum that can exist between a given pair of atoms. Quadruple bonds are extremely rare and can be formed only between transition metal atoms, and consist of one sigma bond, two pi bonds and one delta bond.
A pi bond is weaker than a sigma bond, but the combination of pi and sigma bond is stronger than either bond by itself. The enhanced strength of a multiple bond versus a single (sigma bond) is indicated in many ways, but most obviously by a contraction in bond lengths. For example in organic chemistry, carbon–carbon bond lengths are 154 pm in ethane, 134 pm in ethylene and 120 pm in acetylene. More bonds make the total bond shorter and stronger.
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| ethane (1 σ bond) | ethylene (1 σ bond + 1 π bond) | acetylene (1 σ bond + 2 π bonds) |
Read more about this topic: Pi Bond
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