CHEMISTRY: W. D. HARKINS 
219 
must express something inherent in the structure of the atoms. How- 
ever, what it is desired to emphasize here is that nearly all of the physical 
properties of the elements vary in periods which are either the same or 
nearly the same as these. The chemical properties also vary in rather 
long periods, which in the case of the short periods 1 and 2, are identical 
with those given. 
From this it is seen that both the chemical and the physical prop- 
erties of the elements vary in periods which are long in comparison with 
the change in periods of 2 elements as indicated by the division of the 
elements into the odd and even series. If now neither the physical nor 
the chemical properties vary according to these extremely short periods, 
what, it may be asked, is left which can so vary? 
Now it might easily be shown that the hydrogen-heHimi system of the 
structure of the elements, which divides them into the odd and even 
series, is in reaUty more directly applicable to the structure of the 
nuclei of the atom than to the atom as a whole. If then the Ruther- 
ford theory that the nuclei of atoms are extremely minute, is used as 
a basis for reasoning, it would be expected that the variations in the 
structure of the nuclei should not cause variations in the properties of 
the elements except in so far as they influence the nuclear charge. This 
nuclear charge has been assumed to be equal to the atomic number, 
and therefore rises with perfect regularity from odd to even or from 
even to odd. It seems probable that the number of electrons external to 
the nucleus is equal to the nuclear charge, and that it is the change in 
their number and arrangement which causes the physical properties to 
vary according to the periods Hsted above. This question has been 
discussed in a previous paper.^ 
It might be expected, however, that the composition of the nucleus 
should affect its own stabiHty, which from radioactive evidence means 
the stabiHty of the atom. From this standpoint it might be reasonable 
to suppose that the atoms of one of the series, the even or the odd, should 
be more stable than those of the other. Now unfortunately there is iio 
known method of testing the stabiHty of the Hghter atoms, but it 
might seem, at least at first thought, that the more stable atoms should 
be the more abundantly formed, and to a certain extent this is undoubt- 
edly true. If then, at the stage of evolution represented by the solar 
system, or by the earth, it is found that the even mmibered elements are 
more abundant than the odd, as seems to be the case, then it might be 
assumed that the even numbered elements are on the whole the more 
stable. However, there is at least one other factor than stabiHty which 
must be considered in this connection. The formula of the even num- 
