Detailed calculation shows that there are some small differences in the order
of energy levels from molecule to molecule (BOX 4.3). FIGURE 4.31 shows the
order for the Period 2 elements, with the exception of O2 and F2, which lie in the
order shown in FIG. 4.32. The order of energy levels is easy to explain for these
two molecules. First, because each O and F atom has many electrons that contribute
to shielding, the 2s-orbitals lie well below the 2p-orbitals and we can think of
building -orbitals from the two sets of atomic orbitals separately. However,
because the atoms of elements earlier in the period have fewer electrons, their 2sand
2p-orbitals have more similar energies than in O and F. As a result, it is no
longer possible to think of a -orbital as being formed from either the 2s-orbitals
or the 2pz-orbitals separately, and all four of these orbitals must be used to build
the four -orbitals. It is then hard to predict without detailed calculation where
these four orbitals will lie, and it turns out that they in fact lie where we show
them in Fig. 4.31.