# How does MO theory explain bond length trends for O2 and its ions?

Are these bond lengths consistent with the bond order determined from a molecular orbital diagram?

O2+ = 112 pm
O2 = 121 pm
O2- = 128 pm
O22- = 149 pm

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Bond strength depends on how stretchy or stiff the bonds are (with stiffer bonds being stronger). Stretchier bonds tend to be longer bonds. Since an increase in bond order leads to stronger bonds, there is an expectation that higher bond order means shorter bonds. It's a reasonable expectation when you're comparing bonds between a particular pair of atoms, as you are.

In the table below, the bond orders are predicted from electron configurations by subtracting the number of electrons in bonding orbitals from the number of electrons in antibonding (*) orbitals, and dividing by two.

MoleculeElectron configuration Bond order Bond length (pm)
O2+
 1s 1s* 2s 2s* 2p 2p 2p*
2.5 112
O2
 1s 1s* 2s 2s* 2p 2p 2p*
2 121
O2-
 1s 1s* 2s 2s* 2p 2p 2p*
1.5 128
O22-
 1s 1s* 2s 2s* 2p 2p 2p*
1 149