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Molecules 277

prohibits two 1selectrons in an atom from having the same spins, which is manifested

in a repulsion between He atoms. Hence the He 2 molecule cannot exist.

A similar argument holds in the case of H 3. An H 2 molecule contains two 1selec-

trons whose spins are antiparallel (↑↓). Should another H atom approach whose elec-

tron spin is, say, up, the resulting molecule would have two spins parallel (↑↑↓), and

this is impossible if all three electrons are to be in 1sstates. Hence the existing H 2 mol-

ecule repels the additional H atom. The exclusion-principle argument does not apply

if one of the three electrons in H 3 is in an excited state. All such states are of higher

energy than the 1sstate, however, and the resulting configuration therefore has more

energy than H 2 H and so will decay rapidly to H 2 H.

Molecular Bonds

The interaction between two atoms that gives rise to a covalent bond between them

may involve probability-density distributions for the participating electrons that are

different from those of Fig. 6.12 for atoms alone in space. Figure 8.9 shows the

Orbital nlml

s 1,2,3, ... 00

px 2,3,4, ... 1 ± 1

py 2,3,4, ... 1 ± 1

pz 2,3,4, ... 10

z

+ y

x z y x z y x

+

z

y

x –

+

Figure 8.9Boundary surface diagrams for sand patomic orbitals. Each orbital can “contain” two elec-

trons. There is a high probability of finding an electron described by one of these orbitals in the shaded

regions. The sign of the wave function in each lobe is indicated.

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