THE STRUCTURE OF THE ATOM
Properties of Radium
A study of the properties of radium will aid in throwing light upon
the question as to the building up of the atom. First to be considered
are the usual properties which distinguish an elementary body.
Metallic radium has been prepared by a method similar to that used in
the preparation of barium. It is a pure white metal, melting at 700 deg.,
and far more volatile than barium. It rapidly alters on exposure to
the air, probably forming a nitride. It energetically decomposes water
and the product dissolves in the water. Its atomic weight is 226.
Radium forms a series of salts analogous in appearance and chemical
action to those of barium. In the course of time they become colored,
especially if mixed barium salts. The radiations from radium produce
marked chemical effects in a number of substances. Carbon dioxide is
changed into carbon, oxygen, and carbon monoxide, and the latter is
changed into carbon and oxygen. Ammonia is dissociated into nitrogen
and hydrogen; hydrochloric acid into chlorine and hydrogen. Oxygen is
condensed into ozone. In general, the action upon gases appears to be
similar to that of the silent electric discharge. Water is decomposed
into hydrogen and oxygen. If moist radium chloride or a salt of radium
containing water of crystallization is sealed in a glass tube, the
gradual accumulation of hydrogen and oxygen will burst the tube.
The radiations rapidly decompose organic matter with the evolution of
gases. Thus grease from stopcocks of apparatus used with radium or
paraffin will give off carbon dioxide. Under an intense alpha
radiation paraffin or vaseline become hard and infusible. White
phosphorus is changed into red.
The action upon living tissue is most noteworthy, as its possible use
as a remedial agent is dependent upon this. A small amount of a radium
salt enclosed in a glass tube will cause a serious burn on flesh
exposed to it. It therefore has to be handled with care and undue
exposure to the radiations must be avoided. Cancer sacs shrivel up and
practically disappear under its action. Whether the destruction of
whatever causes the cancer is complete is at least open to serious
doubt.
The coagulating effect upon globulin is interesting. When two
solutions of globulin from ox serum are taken and acetic acid added to
one while ammonia is added to the other, the opalescence in drops of
the former is rapidly diminished on exposure to radium, showing a more
complete solution, whereas the latter solution rapidly turns to a
jelly and becomes opaque, indicating a greatly decreased solubility.
Energy Evolved by Radium
The greater part of the tremendous energy evolved by radium is due to
the emission of the alpha particles, and in comparison the beta and
gamma rays together supply only a small fraction. This energy may be
measured as heat. It was first observed that a radium compound
maintained a temperature several degrees higher than that of the air
around it. The rate of heat production was later measured by means of
an ice calorimeter and also by noting the strength of the current
required to raise a comparison tube of barium salt to the same
temperature. Both methods showed that the heat produced was at the
rate of about 135 gram calories per hour. As the emission is
continuous, one gram of radium would therefore emit about 1,180,000
gram calories in the course of a year. At the end of 2000 years it
would still emit 590,000 gram calories per year. Such a production of
energy so far surpasses all experience that it becomes almost
inconceivable. It is futile to speak of it in terms of the heat
evolved by the combustion of hydrogen, which is the greatest that can
be produced by chemical means.
This effect is unaltered at low temperatures, as has been tested by
immersing a tube containing radium in liquid air. It should be stated
that these measurements were made after the radium had reached an
equilibrium with its products; that is, after waiting at least a month
after its preparation. The evolution of heat from radium and the
radio-active substances is, in a sense, a secondary effect, as it
measures the radiant energy transformed into heat energy by the
active matter itself and whatever surrounds it. Let us repeat,
therefore, that the total amount of energy pent up in a single atom of
radium almost passes our powers of conception.
Necessity for a Disintegration Theory
The facts gathered so far justify and necessitate a theory which shall
satisfactorily explain them, and since these phenomena are not caused
by nor subject to the influence of external agencies, they must refer
to changes taking place within the atom--in other words, a theory of
disintegration. In the main, these facts may be summed up as the
emission of certain radiations from known elemental matter: the
material alpha particles with positive charge, the beta particles or
negative electrons, and the gamma rays analogous to _X_ rays. The
emission of these rays results in the production of great heat. Then
there is the law of transformations by which whole series of new
elements are generated from the original element and maintain a
constant equilibrium of growth and decay in the series. Lastly, we
have the production of helium from the alpha particles.
Disintegration Theory
In explanation of these phenomena, Rutherford offered the hypothesis
that the atoms of certain elements were unstable and subject to
disintegration. The only elements definitely known to come under this
description are the two having atoms of the greatest known mass,
thorium (232) and uranium (238).
The atoms of uranium, for instance, are supposed to be not permanent
but unstable systems. According to the hypothesis, about 1 atom in
every 10^{18} becomes unstable each second and breaks up with a
violent explosion for so small a mass of matter. One, or possibly two
alpha particles are expelled with great velocity. This alpha particle
corresponds to an atom of helium with an atomic weight of 4, and its
loss reduces the original atomic weight to 234 with the formation of a
new element, having changed properties corresponding to the new atomic
weight. This new element is uranium X_{1}.
These new atoms are far more unstable than those of uranium, and the
decomposition proceeds at a new rate of 1 in 10^{7} per second. So at
a definite, measurable rate this stepwise disintegration proceeds. The
explosions are not in all cases equally violent in going from element
to element, nor are the results the same. Sometimes alpha particles
alone are expelled, sometimes beta, or two of them together, as alpha
and beta.
The new product may remain with the unchanged part of the original
matter. Thus there would be an accumulation of it until its own decay
balances its production, resulting eventually in a state of
equilibrium.
Constitution of the Atom
In order to explain the electrical and optical properties of matter,
the hypothesis was made that the atom consisted of positively and
negatively electrified particles. Later it was shown that negative
electrons exist in all kinds of matter. Various attempts were made to
work out a model of such an atom in which these particles were held in
equilibrium by electrical forces. The atom of Lord Kelvin consisted of
a uniform sphere of positive electrification throughout which a number
of negative electrons were distributed, and J. J. Thomson has
determined the properties of this type as to the number of particles,
their arrangement and stability.
Rutherford's Atom
According to Rutherford, the atom of uranium may be looked upon as
consisting of a central charge of positive electricity surrounded by a
number of concentric rings of negative electrons in rapid motion. The
positively charged centre is made up of a complicated system in
movement, consisting in part of charged helium and hydrogen atoms, and
practically the whole charge and mass of the atom is concentrated at
the centre. The central system of the atom is from some unknown cause
unstable, and one of the helium atoms escapes from the central mass as
an alpha particle.
There are, confessedly, difficulties connected with this conception of
the atom which need not, however, be discussed here. Much remains to
be learned as to the mechanics of the atom, and the hypothesis
outlined above will probably have to be materially altered as
knowledge grows. Perhaps it may have to be entirely abandoned in favor
of some more satisfactory solution. Until such time it at least
suffices as a mental picture around which the known facts group
themselves. In this picture energy and matter lose their old-time
distinctness of definition. Discrete subdivisions of energy are
recognized which may be called charged particles without losing their
significance. Some of these subdivisions charged in a certain way or
with neutralized charge exhibit the properties of so-called matter.
Scattering of Alpha Particles
This conception of the atom would doubtless fail of much support were
it not for certain experimental facts which lend great weight to it.
Certain suppositions can be based on this theory mathematically
reasoned out and tested by experiment. Predictions thus based on
mathematical reasoning and afterward confirmed by experiment give a
very convincing impression that truth lies at the bottom.
The first of these experimental proofs comes under the head of what is
known as the scattering of the alpha particles, a phenomenon which,
when first observed, proved hard to explain. If an alpha particle in
its escape from the parent atom should come within the influence of
the supposed outer electrical field of some other atom, it should be
deflected from its course and, the intensity of the two charges being
known, the angle of deflection could be calculated. For instance, if
it came to what might be called a head-on collision with the positive
central nucleus of another atom, it would recoil if it were itself of
lesser mass, or would propel the other forward if that were the
lighter.
The experiment is carried out by placing a thin metal foil over a
radio-active body, as radium _C_, which expels alpha particles with a
high velocity, and counting the number of alpha particles which are
scattered through an angle greater than 90 deg. and so recoil toward their
source. This has been done by a number of investigators and it has
been found that the angle of scattering and the number of recoil
particles depend upon the atomic weight of the metal used as foil. For
example, if gold is used, the number of recoil atoms is one in
something less than 8,000.
Taking the atomic weight of gold into consideration, Rutherford
calculated mathematically that this was about the number which should
be driven backward. But he went further and calculated also the number
which should be returned by aluminum, which has an atomic weight of
only about one-seventh that of gold. Two investigators determined
experimentally the number for aluminum and their results agreed with
Rutherford's calculations.
The metals from aluminum to gold have been examined in this way. The
number of recoil particles increases with the atomic weight of the
metal. Comparing experiment with theory, the central charge in an atom
corresponds to about one-half the atomic weight multiplied by the
charge on an electron, or, as it is expressed, 1/2 Ae.
There is only one lighter atom than helium, namely, hydrogen, which
has a mass only one-fourth as great. When alpha particles are
discharged into hydrogen, a few of the latter atoms are found to be
propelled to a distance four times as great as that reached by the
alpha particles.
Stopping Power of Substances
Parallel with the experiments mentioned, there is what is called
the stopping power of substances. This means the depth or thickness
of a substance necessary to put a stop to the course of the alpha
particles. This gives the range of the alpha particles in such
substances and is connected in a simple way with the atomic weight,
that is, it is again fixed by the mass of the opposing atom. This
stopping power of an atom for an alpha particle is approximately
proportional to the square root of its atomic weight.
Considering gases, for instance, if the range in hydrogen be 1,
then the range in oxygen, the atomic weight of which is 16, is only
(1/16)^{1/2} or 1/4. Generally in the case of metals the weight of
matter per unit area required to stop the alpha particle is found to
vary according to the square root of the atomic weight of the metal
taken.