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Sigma- and pi-bonds
 
This model illustrates the formation of single, double and triple bonds between two atoms of hydrogen in ethane, ethene (ethylene) and ethyne (acetylene) molecules. Two electrons that are located on the two sp3- hybridized orbitals of the carbon atoms (one electron per orbital) form a single bond. Individual molecular sections can rotate around this bond, which is called a σ-bond, and this constitutes an example of the so-called rotational isomerism. A double bond has essentially the same structure as a σ-bond, but it also includes an additional bond formed by two other electrons, originally located on the non-hybridized p-orbitals of a carbon atom (the so-called π-bond). Individual molecular sections cannot rotate around a π-bond, and ethane molecules have "flat" configurations, as determined by the locations of their sp2 - hybridized orbitals.

In a triple bond, one more π-bond is added to already existing σ-bond and π-bond. The linear structure of sp-hybridized orbitals accounts for the linear structure of acetylene molecules.

 
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