5860 
NATURE 
[JANUARY 22, 1914 
THE STRUCTURE OF THE ATOM. 
Bhs earliest developments of the electronic 
theory led necessarily to the conclusion that 
in every atom in its normal condition there were 
contained electrons which could be detached from 
it by suitable agencies; these electrons were the 
same in respect of the only two properties attri- 
buted to them, charge and mass, whatever the 
atom in which they were contained. This. con- 
ception of a constituent common to all atoms 
indicated for the first time the possibility of ex- 
plaining the relationships described by the periodic 
law between the properties of different atoms; if 
similar atomic properties represent similar numbers 
or arrangements of electrons, any theory which 
would make these numbers or. arrangements 
periodic functions of the atomic mass would 
explain in some measure those relationships. 
The first attempt to frame such a theory was 
made by Sir J. J. Thomson; the structure which 
he proposed for the atom is so generally known 
that it may be described here with great brevity. 
Since an atom in its normal condition is electrically 
neutral, it is necessary, if the principles of electro- 
statics be accepted, that it should contain a posi- 
tively charged portion, the total charge on which 
is equal and opposite to that of the electrons con- 
tained in the atom. Until recently there was no 
evidence whatsoever as to the form of this posi- 
tively charged portion; accordingly, Thomson 
adopted provisionally the form most convenient 
for his purpose; he supposed that the positive 
charge was distributed uniformly over through- 
out a sphere, the radius of which was taken to 
be the same for all atoms. In addition, he 
assumed that the number of electrons in an atom 
increases regularly with the atomic mass. 
The mathematical problem of determining the 
distribution of N electrons within such a uniformly 
charged positive sphere is capable of partial solu- 
tion whatever the magnitude of N. It can be 
shown that certain distributions are in equilibrium, 
but it cannot generally be shown that it is only 
these distributions that are in equilibrium, nor 
can it be shown generally that the equilibrium is 
stable. The problem of calculating from Thom- 
son’s assumption the structure of an atom is 
therefore not completely determinate; but if it 
be assumed that the distributions which can be | 
calculated are unique and that they are stable, 
certain conclusions can be reached. If almost any 
other assumption concerning the distribution of 
the positive charge on the atom is made, even 
this small amount of progress is impossible. 
Thomson showed that the distributions which 
could be calculated were those in which the elec- 
rons were arranged in circular rings, and that the 
number of electrons in any ring (e.g., the outer- 
most or the innermost) was a periodic function | 
of N, and therefore of the atomic mass. 
Before any theory of this kind can be regarded 
as complete, it must be shown that certain dis- 
tributions of electrons are connected with certain 
properties of the atoms containing them, and it ! 
NO. 2308, VOL. 92] 
must be shown that the same distribution of elec- 
trons is connected with the many different Pro 
perties which are found to be associated — 
similar elements. It must be shown, for aa | 
that a certain distribution (which is to be identified 
with an atom of the alkali metals) is necessarily 
connected with electro-positive chemical charac- 
teristics, metallic conductivity, a special type of 
spectral series, and so on. It is necessary that 
the theory should explain the relation between 
different properties of the same element as well as 
that between the same property of different 
elements. Thomson endeavoured . to” correlate 
certain chemical properties with certain electronic 
distributions by showing’ that some of these would 
be likely to lose electrons, leaving the atom posi- 
tively charged, while others would be likely to 
gain them; a difference in the tendency to lose 
electrons would probably lead to a difference in 
respect of metallic conductivity. But in no case 
could any observed atomic property be calculated 
with quantitative agreement from one of the sup- 
posed electronic distributions. The failure was 
especially important in the case of spectra, for the 
frequency of the vibration of the electrons could 
be definitely calculated in some cases, and it 
appeared that the relation between the frequencies 
of different vibrations in the same atom was not 
at all of the same form as that indicated by the 
known spectral series. 
However, there was no definite evidence for 
disbelieving the assumptions underlying Thom- 
son’s theory until investigations were made on 
the scattering of a and B rays. These rays con- 
sist of charged particles which can certainly pass 
through atoms, and it is to be expected that in 
their passage they should be deflected by forces 
exerted between them and the electrons or the 
positive charge in the atom; by examining these 
deflections some indications as to the number of 
the electrons and the nature of the positive charge 
may be obtained. Rutherford and Geiger showed 
that the experimental results were quite irrecon- 
cilable with Thomson’s theory, but that they were 
reconcilable with the view that the positive charge 
in the atom is concentrated on a single particle, 
like the electron of dimensions infinitesimal com- 
pared with the ‘‘radius of the atom’; the number 
of electrons in an atom must be taken as about — 
half the number representing the atomic weight, 
the total charge on the “positive nucleus” being, 
of course, equal and opposite to that on all the 
electrons.t 
The assumption that the whole positive charge 
on an atom is concentrated on a single positive 
particle had _ previously been suggested by 
Nagaoka, but it presents very great difficulties ; 
for it is quite certain that, if the principles of 
mechanics and electrostatics are true, no collec- 
tion of electrons round a positive nucleus can 
possibly be stable, unless all the electrons fall 
into the nucleus forming a single infinitesimal 
neutral particle. It has recently been proposed to 
1 Thomson had already advanced several lines of argument indicating that 
the number of electrons in an atom was not very different from its atomic 
weight, referred to that of hydrogen as unity. 
