ATOMIC STRUCTURE 
v. 3] 
75 
that whilst in Langmuir’s theory the number of electrons in 
the outer shell increases up to 32, in Bohr’s theory the outermost 
electrons never exceed eight in number. 
If we think, for example, of the element neon with atomic 
number 10, this must have ten electrons arranged in some 
regular configuration of great stability. Fluorine, which has 
only nine electrons, must owe its electro-negative character to 
the fact that it has a tendency to capture an additional electron 
to make up the number ten, corresponding to maximum stability. 
The electro-positive character of sodium may be explained by 
supposing that one of the eleven electrons of sodium in the 
neutral atom is readily lost, a fact illustrated in the photo-electric 
activity of such an element. 
Bohr attempted to solve the problem of finding the dis 
tribution of the electrons among various possible types of orbit. 
This he did by methods partly theoretical, partly empirical. In 
his theory of atomic structure emphasis is laid on the capture 
and binding of successive electrons by the positively charged 
nucleus. For an element of atomic number Z the process of 
formation of the neutral atom is supposed to occur in Z 
stages. 
It has been found possible to divide the planetary electrons 
surrounding the nucleus into groups characterized by the total 
quantum number of the orbit. In the hydrogen atom, as we 
have seen, a large number of orbits are possible, some being 
circular, some elliptic. The most stable arrangement correspond 
ing to the hydrogen atom in its normal state is that in which 
the single electron is bound in a circular orbit. The structure 
of the neutral helium atom is still somewhat uncertain, but the 
two electrons form a definite group called the K group. Formerly 
the two electrons were supposed to move in i x orbits in planes 
inclined to each other at an angle of 120°. Sommerfeld in 1923 
suggested that they revolve in opposite senses in co-planar 
orbits. 
Whatever may be the correct model, it must constitute a 
“ closed ” or “ completed ” (abgeschlossenes) configuration, char 
acterized by j = o. As the charge of the nucleus increases, this 
K group retains its characteristics, but the electrons move more 
rapidly and approach closer to the nucleus because of the more 
powerful electrostatic attraction. In lithium, the next element 
in the periodic table, when Z = 3, the third electron is bound 
in a 2i orbit. As we pass from helium to neon a new group of 
electrons is added which we may call the L group, characterized 
by the quantum number 2. The complete L group contains 
eight electrons as in neon. In all atoms of atomic number 
greater than 18 this L group appears. When it has been com