energy than the 2π orbitals. This is attributed to interaction between the 2s MO and the 2pz MO. Distributing 8 electrons over 6 molecular orbitals leaves the final two electrons as a degenerate pair in the 2pπ* antibonding orbitals resulting in a bond order of 2. As in diboron, these two unpaired electrons have the same spin in the ground state, which is a paramagnetic diradical triplet oxygen. The first excited state has both HOMO electrons paired in one orbital with opposite spins, and is known as singlet oxygen. The bond order decreases and the bond length increases in the order (112.2 pm), (121 pm), (128 pm) and (149 pm). Difluorine and dineon In difluorine two additional electrons occupy the 2pπ* with a bond order of 1. In dineon (as with dihelium) the number of bonding electrons equals the number of antibonding electrons and this molecule does not exist. Dimolybdenum and ditungsten Dimolybdenum (Mo2) is notable for having a sextuple bond. This involves two sigma bonds (4dz2 and 5s), two pi bonds (using 4dxz and 4dyz), and two delta bonds (4dx2 − y2 and 4dxy). Ditungsten (W2) has a similar structure. MO energies overview Table 1 gives an overview of MO energies for first row diatomic molecules calculated by the Hartree-Fock-Roothaan method, together with atomic orbital energies. Heteronuclear diatomics In heteronuclear diatomic molecules, mixing of atomic orbitals only occurs when the electronegativity values are similar. In carbon monoxide (CO, isoelectronic with dinitrogen) the oxygen 2s orbital is much lower in energy than the carbon 2s orbital and therefore the degree of mixing is low. The electron configuration 1σ21σ*22σ22σ*21π43σ2 is identical to that of nitrogen. The g and u subscripts no longer apply because the molecule lacks a center of symmetry. In hydrogen fluoride (HF), the hydrogen 1s orbital can mix with fluorine 2pz orbital to form a sigma bond because experimentally the energy of 1s of hydrogen is comparable with 2p of fluorine. The HF electron configuration 1σ22σ23σ21π4 reflects that the other electrons remain in three lone pairs and that the bond order is 1. The more electronegative atom is the more energetically excited because it more similar in energy to its atomic orbital. This also accounts for the majority of the electron negativity residing around the more electronegative molecule. Applying the LCAO-MO method allows us to move away from a more static Lewis structure type approach and actually account