ATOMIC STRUCTURE 
v. 3] 
73 
Z electrons surrounding the positive nucleus may be divided 
into several groups, each group being characterized by the work 
which must be done to extract an electron of that group from 
the atom. This “ work of extraction ” determines the “ level ” 
of the group. The critical absorption frequencies correspond 
to the extraction of an electron from a particular level. 
The law as stated by Moseley must now be regarded as 
superseded by an exact selection rule, based on quantum theory. 
One very remarkable feature in the graphs obtained is the 
occurrence of breaks in the curves due to the completion of 
inner shells. 
The absorption co-efficient of an element varies with the 
wave-length of the X-radiation employed, and in general the 
co-efficient increases rapidly with an increase of wave-length. 
At certain particular wave-lengths, known as the critical ab 
sorption wave-lengths, sharp discontinuities appear, a sudden 
decrease in absorption taking place for a slight increase in the 
wave-length. In 1913 de Broglie obtained spectral photographs 
showing bands with sharp edges indicating a sudden change 
in absorption for a particular wave-length. The absorption dis 
continuity is directly explained by Bohr’s theory. 
It is found that the K series gives one absorption edge, the 
L series three, and the M series no less than five. We may 
conclude that there are three different energy levels within the 
L level, and five energy levels within the M level. Fig. 11 shows 
a scheme of energy levels based on these considerations, and 
some of the transitions corresponding to observed lines. 
There is a striking analogy between optical spectra and 
X-ray spectra, the classification of the terms and the distribution 
of the lines being similar in the two cases. This resemblance 
has led to very interesting new ideas which will be taken up 
later. 
For detailed discussions of X-ray spectra reference may be 
made to X-Rays by Maurice de Broglie (Methuen) or to X-Rays 
and Electrons, by A. H. Compton (Macmillan). 
3. The Arrangement of the Electrons in Space 
When we attempt to construct a model showing the spatial 
distribution of the electrons, it is necessary to make some hypo 
thesis with regard to the distribution of the positive electricity 
of the atom. Lord Kelvin suggested that the atom might be 
regarded as a sphere of positive electricity in which negative 
electrons were distributed in definite configuration. Following 
this suggestion, J. J. Thomson made a famous attempt to explain