The order of filling of molecular orbitals $\ce{O2}$ by MOT is this :
But from it, how can I deduce that there is one sigma and one pi bond in an oxygen molecule ?
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$\begingroup$ If I am not mistaken, the $2\sigma_g$ should be higher in energy than $\pi_u^x,\pi_u^y$. $\endgroup$– Martin - マーチン ♦Nov 18 '14 at 13:06
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1$\begingroup$ @martin That's for molecules having electrons less than or equal to 14 , like nitrogen $\endgroup$– biogirlNov 18 '14 at 16:26
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Overall bond order equals number of electrons that occupy bonding orbitals minus numer of electrons that occupy antibonding orbitals divided by two.
Now you can apply the same concept indivudually:
For s molecular orbitals, both electron pairs "cancel out".
For p molecular orbitals, you have 2 electrons in sigma bonding orbitals and none in sigma antibinding orbitals, which means (2-0)/2 = 1 sigma bond. Also, you have 4 electrons within pi bonding orbitals and only 2 electrons within pi antibonding orbitals, which means (4-2)/2 = 1 pi bond. This yields the result that you expected.
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$\begingroup$ Actually, there is not a sigma and a pi bond. The MO scheme clearly depicts that - the bonding situation is a little bit more complicated. $\endgroup$– Martin - マーチン ♦Nov 18 '14 at 12:37
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$\begingroup$ Yeah, right, but that just makes the OP question nonsense. In MO theory there are no individual bonds, just that, molecular orbitals filled with electrons. $\endgroup$ Nov 18 '14 at 12:47
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$\begingroup$ @martin I am studying MOT as a part of high school chemistry and this was a question in one of the tests. But actually, are MOT and VBT mutually exclusive theories ? Should I not try to find anything in common ? $\endgroup$– biogirlNov 18 '14 at 16:35
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$\begingroup$ @biogirl They are not mutually exclusive, quite the opposite is the case. They are complementary. So they are just views from different angles. If you want to know more about this matter I encourage you to ask a new question, it is too extensive to be dealt with in the comments. $\endgroup$– Martin - マーチン ♦Nov 18 '14 at 16:39