QUANTUM THEORY 
71 
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emission with emission edges as well as continuous absorption with 
absorption edges. 
I he energy required to remove an L electron from an atom is slightly 
different according as it is taken from a circular or elliptic orbit. Con 
sequently there will be two L levels. Observation shows that there are 
actually three L levels which are denoted by L x , L 2 , L 3 — L 3 being the 
lowest, i.e. nearest to K. There has for some time been considerable doubt 
as to the proper classification of the L levels; but at the time of writing 
the difficulty seems to have been cleared up. It appears that L 3 corre 
sponds to the elliptic orbits; and L x , L 2 both belong to the circular 2 2 
orbits, being discriminated from one another by a third quantum number. 
Formerly the classification was L x circular and L 2 elliptic, whilst L 3 was 
a redundant elliptic orbit of obscure significance. 
A detail which may be of importance in astronomical applications 
should be noted. Electrons falling to the K level from the L x and L 2 
levels produce a pair of emission lines, the line due to L x being the more 
intense. If we adhere to the old classification this means that an electron 
in a circular orbit is quicker to seize an opportunity of falling than an 
electron in an elliptic orbit. With the new classification we reach the same 
conclusion for a different reason. There is no emission line corresponding 
to L z , and falls from the elliptic orbit seldom if ever occur. This is in accord 
ance with the “selection principle” which governs optical spectra (§51) 
and molecular spectra (§ 244), viz. that in all transitions the second 
quantum number must change by ± 1 , so that a fall from 2 X to lj orbits 
is excluded. Use is made of this result in § 166. 
The following example will give an idea of the amount of perturbation 
exercised by the electrons in an atom on one another. Consider an iron 
nucleus attended by one K electron, the other electrons being absent. 
The ionisation energy is obtained by setting n — 1 , Z = 26 in (42-62). 
The corresponding wave-length is found to be 1-35 A. But the K absorp 
tion-limit for iron is 1-74 A. The difference is due to our dealing in the 
first case with the iron nucleus alone and in the second case with the 
complete iron atom. Now for copper (Z = 29) the observed limit is 1-38 A, 
i.e. practically the same as the theoretical limit for the iron nucleus; hence 
we can regard the satellite electrons in copper as shielding the positive 
charge of the nucleus to the extent of approximately 3 units so far as the 
motion of a A electron is concerned. Again, consider a platinum nucleus 
[Z = 78 ) w p;h two K electrons and one L electron. The K electrons being 
comparatively close to the nucleus may be treated as effectively reducing 
its charge by 2 units and the ionisation energy of the L electron is then 
found approximately by setting n = 2 , Z = 76 in (42-62). The corre 
sponding wave-length is 0-63 A; the observed value for the complete 
platinum atom is 1-07 A, so that the shielding is considerable.