The molecular orbitals in the water molecule are classified as a, b or b orbitals as determined by their symmetry properties. This labelling of the orbitals is analogous to the use of the o-T and g-u classification in linear molecules. In addition to the symmetry properties of the atomic orbitals we must consider their relative energies to determine which orbitals will overlap significantly and form delocalized molecular orbitals The 1s atomic orbital on oxygen possesses a much lower energy than any of the other orbitals of a, symmetry and should not interact significantly with them. The molecular orbital of lowest energy in H2O should therefore correspond to an inner shell 1s atomic like orbital centred on the oxygen. This is the first orbital of a, symmetry and it is labelled la,. Reference to the forms of the charge density contours for the la, molecular orbital substantiates the above remarks regarding the properties of this orbital Notice that the orbital energy of the la, molecular orbital is very similar to that for the 1s atomic orbital on oXygen. The 1a, orbital in H2o is, therefore, similar to the lo inner shell molecular orbitals of the diatomic hydrides The atomic orbital of next lowest energy in this system is the 2s orbital of a, symmetry on oxygen. We might anticipate that the extent to which this orbital will overlap with the (1s,+ 1s,)combination of orbitals on the hydrogen atoms to form the 2a, molecular orbital will be intermediate between that found for the 2o molecular orbitals in the diatomic hydrides Ch and H. the 2o orbital in Ch results from a strong mixing of the 2s orbital on carbon and the hydrogen 1 s orbital In Hf the participation of the hydrogen orbital in the 2o orbital is greatly reduced, a result of the lower energy of the 2s atomic orbital on fluorine as compared to that of the 2s orbital on carbonThe molecular orbitals in the water molecule are classified as a1 , b1 or b2 orbitals, as determined by their symmetry properties. This labelling of the orbitals is analogous to the use of the s- and g-u classification in linear molecules. In addition to the symmetry properties of the atomic orbitals we must consider their relative energies to determine which orbitals will overlap significantly and form delocalized molecular orbitals. The 1s atomic orbital on oxygen possesses a much lower energy than any of the other orbitals of a1 symmetry and should not interact significantly with them. The molecular orbital of lowest energy in H2O should therefore correspond to an inner shell 1s atomic￾like orbital centred on the oxygen. This is the first orbital of a1 symmetry and it is labelled la1 . Reference to the forms of the charge density contours for the la, molecular orbital substantiates the above remarks regarding the properties of this orbital. Notice that the orbital energy of the la1 molecular orbital is very similar to that for the 1s atomic orbital on oxygen. The 1a1 orbital in H2O is, therefore, similar to the ls inner shell molecular orbitals of the diatomic hydrides. The atomic orbital of next lowest energy in this system is the 2s orbital of a1 symmetry on oxygen. We might anticipate that the extent to which this orbital will overlap with the (1s1 + 1s2 ) combination of orbitals on the hydrogen atoms to form the 2a1 molecular orbital will be intermediate between that found for the 2s molecular orbitals in the diatomic hydrides CH and HF. The 2s orbital in CH results from a strong mixing of the 2s orbital on carbon and the hydrogen 1s orbital. In HF the participation of the hydrogen orbital in the 2s orbital is greatly reduced, a result of the lower energy of the 2s atomic orbital on fluorine as compared to that of the 2s orbital on carbon