Multielectron atoms spectra

The three quantum numbers n, l, and wi/ discussed in Section 5.7 define the energy, shape, and spatial orientation of orbitals, but they don t quite tell the whole story. When the line spectra of many multielectron atoms are studied in detail, it turns out that some lines actually occur as very closely spaced pairs. (You can see this pairing if you look closely at the visible spectrum of sodium in Figure 5.6.) Thus, there are more energy levels than simple quantum mechanics predicts, and a fourth quantum number is required. Denoted ms, this fourth quantum number is related to a property called electron spin. 

But this theory was limited in its application in that it gave an exact account only of the spectrum of hydrogen, the simplest case. Atoms with more than one electron are much more complicated, since the various electrons exert influences on each other. Nevertheless, Bohr had sufficient confidence in his quantum theory of the atom to try to apply it to multielectron atoms in an approximate manner. 

This is rarely applied to organometallic species in part because laser irradiation can cause complexes to decompose, but the method is in principle useful for detecting nonpolar bonds, which do not absorb, or absorb only weakly in the IR. Tlie intensity of the Raman spectrum depends on the change of polarizability of the bond during the vibration. One of the earliest uses was to detect the Hg-Hg bond in the mercurous ion [y(Hg-Hg) = 570 cm ], a case where the polarizability change is large as a result of multielectron atoms being involved. Unlike IR spectroscopy, the method is compatible with measurements in aqueous solution. 

In the previous section, we pointed out that the interpretation of atomic line spectra posed a difficult problem for classical physics and that Bohr had some success in explaining the emission spectrum for the hydrogen atom. However, because his model was not correct, he was unable to explain all features of the hydrogen emission spectrum and could not explain the spectra of multielectron atoms at all. A decade or so after Bohr s work on hydrogen, two landmark ideas stimulated a new approach to quantum mechanics. Those ideas are considered in this section and the new quantum mechanics— wave mechanics—in the next. 

---