The bond order of an O2 molecule can be determined using molecular orbital theory by calculating the difference between the number of electrons in bonding orbitals and the number of electrons in antibonding orbitals, divided by 2.For an O2 molecule, there are a total of 12 valence electrons 6 from each oxygen atom . According to molecular orbital theory, these electrons will fill the molecular orbitals in the following order: 2s, *2s, 2pz, 2px = 2py, *2px = *2py.Filling the orbitals with 12 electrons, we get:- 2 electrons in 2s- 2 electrons in *2s- 2 electrons in 2pz- 4 electrons in 2px and 2py 2 in each - 2 electrons in *2px none in *2py Now, we can calculate the bond order:Bond order = number of electrons in bonding orbitals - number of electrons in antibonding orbitals / 2Bond order = 2 + 2 + 4 - 2 / 2Bond order = 6 / 2Bond order = 3So, the bond order of an O2 molecule is 3.