72 
QUANTUM THEORY 
In one sense the ionisation energy W is not particularly associated 
with the electron which is being removed; it is the difference of energy 
of the whole atom before and after the removal. We might suppose that 
a considerable part of this difference would be due to the other electrons 
requantising their orbits in the modified field of force. There is, however, 
an important principle in the quantum theory—the adiabatic principle— 
which shows that this requantisation is brought about automatically 
during the removal of the electron and is not an after-adjustment. If, 
for example, a field of uniform magnetic force is established slowly, the 
classical electromagnetic forces acting during its establishment will trans 
form the orbits of the electrons into the new orbits required by the rules 
of quantisation for the new condition; the action of the magnetic field 
is not divided into two effects, a classical perturbation + a re quantisation. 
Similarly the effect of removal of an electron is not to be divided into a 
progressive disturbance + a final adjustment. 
Finally, it must be remembered that the Bohr atom is only a model 
and is not a literal description of the atom, although we accept it as such 
for most purposes. No one has insisted on this limitation more strongly 
than Prof. Bohr himself. Some progress has been made in the attempt to 
free the theory from its geometrical bondage but as yet we cannot afford 
to dispense with the model. 
Optical Spectra. 
51. The inert gases helium, neon, argon, etc. mark the completion 
(temporary or permanent) of the K, L, M, ... groups of electrons. The 
elements immediately succeeding the inert gases have one rather loose 
electron starting the new group; this is called a valency electron and is 
responsible for their chemical behaviour as monovalent elements. They 
are succeeded by divalent elements with two loose electrons, and 
so on. 
Usually the lines constituting the optical spectrum of an element are 
absorbed and emitted by transitions of a valency electron from one 
quantised orbit to another. The complexity of the spectrum increases with 
the number of valency electrons, and the account here given refers especially 
to elements or ions with one or two valency electrons. 
As in § 42 we describe the orbit of an electron by two quantum numbers 
n and n' with n' < n. The third number n" is neglected for the present. 
The orbit in the normal atom is assigned the numbers n, n' — ( 1 , 1 ), ( 2 , 1 ), 
(3, 1 ) ... according as the valency electron is starting the K, L, M, ... group. 
The possible orbits fall into a number of series. Taking, for example, 
Sodium in which the normal orbit of the valency electron is a (3, 1 ) orbit, 
the series with their conventional nomenclature are—