• Question: What is a sigma bond and a pi bond?? ~very confused student

    Asked by Plasma_based_lifeform to Anne, Arthur, Rose, Ruhina, Thomas on 19 Jun 2015.
    • Photo: Ruhina Miller

      Ruhina Miller answered on 19 Jun 2015:


      A sigma bond is the head-on overlap of two orbitals from two different atoms (one from each) to form a covalent bond. A pi bond is the sideways overlap of four orbitals from two different atoms (two from each) to form two covalent bonds.

    • Photo: Thomas Farrugia

      Thomas Farrugia answered on 24 Jun 2015:


      Ahoy – Ruhina has done this pretty well, so thought best to add a drawing – hopefully it is clear enough.

      I’ve used carbon as a model system- but you can get sigma and pi bonds in other compounds as well. Overarching idea is that your sigma bonds are caused by linear overlap of orbitals – kind of like a handshake – the hands grasp each other, whilst the orbitals overlap really nice and tight next to one another too.

      Pi orbitals on the other hand are like having two hands waving at one another. They can be a distance apart, but they have to be facing each other, and in the same plane. If you were to look at one hand straight on you shouldn’t be able to see the other hand. Pi bonds are usually formed using the p orbitals in Carbon – so alkenes have one pi bond (since two p orbitals are used in sp2 hybridization), whereas alkynes have two pi bonds (only one p orbital used in sp hybridization). So in terms of electron density around the double or triple bond you can expect to see two rings (one above and one below the sigma bond) for alkenes, whereas for alkynes it’s a cylinder.

      If your lateral overlap is disrupted, i.e the hands don’t cover one another you tend to lose pi bonding – this tends to happen if the system is non-planar, which can sometimes be caused by bulky groups disrupting the carbon skeleton.

      It gets a bit more complex when you get to University level chemistry since a new theory steps in, but I won’t give away the game yet (there’s something really fun about chem during which your previous concepts are shattered and rebuilt).

      edit – image here, although I think the hands explanation might work without it.

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