<P> Pi bonds result from overlap of atomic orbitals that are in contact through two areas of overlap . Pi - bonds are more diffuse bonds than the sigma bonds . Electrons in pi bonds are sometimes referred to as pi electrons . Molecular fragments joined by a pi bond cannot rotate about that bond without breaking the pi bond, because rotation involves destroying the parallel orientation of the constituent p orbitals . </P> <P> For homonuclear diatomic molecules, bonding π molecular orbitals have only the one nodal plane passing through the bonded atoms, and no nodal planes between the bonded atoms . The corresponding antibonding, or π * ("pi - star") molecular orbital, is defined by the presence of an additional nodal plane between these two bonded atoms . </P> <P> 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 . </P> <P> 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 about 154 pm in ethane, 134 pm in ethylene and 120 pm in acetylene . More bonds make the total bond shorter and stronger . </P>

Number of pi bonds in a triple bond