My Chemistry textbook says this in regard to a C2 molecule:

It is important to note that double bond in C2 consists of both pi bonds because of the presence of four electrons in two pi molecular orbitals.

Is it possible to account for the double bond in an Oxygen atom consisting of a sigma bond and a pi bond in the same way, based on MOT?


1 Answer 1


$\ce{O2}$ MO diagram

From the MO diagram, we can see that it has 2 electrons in a $\sigma$ orbital, 4 in $\pi$ orbitals, and 2 in $\pi$* orbitals. Taking this together gives a bond order 2, with 1 coming from the sigma electrons and another 1 being the net value for the $\pi$ and $\pi$* electrons.

  • $\begingroup$ Wow! Thanks for the brilliant answer! So we have to use this method for checking type of bonds are present between 2 atoms in a molecule? Didn't find it anywhere else... $\endgroup$ Commented Sep 14, 2019 at 4:50
  • $\begingroup$ It becomes a bit more complicated for anything beyond diatomic molecules and there are cases where it fails (e.g. CO) but this is how the concept of bond order is generally introduced at a general chemistry level. $\endgroup$
    – Tyberius
    Commented Sep 15, 2019 at 3:30

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