A simple pictorial approach to the task of analyzing molecular shape in terms of delocalized molecular orbitals was devised by A.D. Walsh in a classic series of papers published in 1953. Walsh's approach to the discussion of the shape of an H2X triatomic molecule (such as BeH2 and H2O) is illustrated in Fig. 3.38. The illustration shows an example of a Walsh diagram, a graph showing the dependence of orbital energy on molecular geometry. The Walsh diagram for an H2X molecule is constructed by considering how the composition and energy of each molecular orbital changes as the bond angle is varied from 90° to 180°. The diagram is in fact just a more elaborate version of the correlation diagram that we illustrated for an H3 molecule for an H3 ion in Section 3.11 (Fig. 3.30).
The molecular orbitals to consider in the angular molecule are