A Brief Account of Radio-activity Part 4

Characteristics of Helium

Helium, on account of its chemical inactivity and physical properties, is cla.s.sed along with argon, neon, krypton, and xenon in the zero group of the Periodic System, and forms with them the monatomic, inert gases. In this cla.s.s are now placed also the three radio-active gases, emanating respectively from radium, thorium, and actinium. These are generally known as radium emanation, thorium emanation, and actinium emanation. The first mentioned was once called niton. Emanium was the name originally proposed by Giesel for the body now known as actinium.

The calculated rate of production of helium in the series in equilibrium with one gram of radium is 158 cubic millimeters per year.

This corresponds quite well with the experimental results.

Table of Constants

Some of the more important atomic and radio-active constants are given in the following table. They are recorded here to show how helpful the study of radio-activity has been in working out the composition of matter, and to give some idea of the magnitude of the numbers and the minuteness of the quant.i.ties dealt with.

Electric charge carried by each H atom in electrolysis 4.65 10^{-10} e.s.[1]

Electric charge carried by each [alpha]

particle 9.3 10^{-10} e.s.

Number of atoms in 1 gram of H 6.2 10^{23} Ma.s.s of 1 atom of H 1.6 10^{-24} gram Number of molecules per cc. of any gas at standard pressure and temperature 2.72 10^{19} Number of [alpha] particles expelled per second per gram of radium itself 3.6 10^{10} Number of [alpha] particles expelled per second per gram of radium in equilibrium with its products 14.3 10^{10}

[1] The expression 10^{-10} means multiplying by .000,000,000,1; 10^{10} means multiplying by 10,000,000,000.

CHAPTER V

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, 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 a.n.a.logous 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 gla.s.s tube, the gradual acc.u.mulation of hydrogen and oxygen will burst the tube.

The radiations rapidly decompose organic matter with the evolution of gases. Thus grease from stopc.o.c.ks 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 gla.s.s 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 surpa.s.ses 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 pa.s.ses 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 a.n.a.logous 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 ma.s.s, 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 ma.s.s 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 acc.u.mulation of it until its own decay balances its production, resulting eventually in a state of equilibrium.

Const.i.tution 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 ma.s.s 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 ma.s.s 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 ma.s.s, 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 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 ma.s.s 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 ma.s.s 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.