Chapter 12

RADIO-ACTIVITY AND CHEMICAL THEORY

Influence upon Chemical Theory

It can easily be seen that the revelations of radio-activity must have a far-reaching effect upon chemical theory, throwing light upon, and so bringing nearer, the solution of some of the problems which have been long discussed without arriving at any satisfactory solution. The so-called electro-chemical nature of the elements will certainly be made much clearer. The changes in valence should become intelligible and valence itself should be explained. A fuller understanding of the ionization of electrolytes also becomes possible. As these matters are debatable and the details are still unsettled, it is scarcely appropriate to give here the hypotheses in detail or to enter into any discussion of them. But the promise of solution in accord with the facts is encouraging.

The Periodic System

Such progress has been made, however, in regard to a better understanding of the Periodic System that the new facts and their interpretation may well be given. No reliable clue to the meaning of this system and the true relationship between the elements had been found up to the time when new light was thrown upon it by the discoveries of radio-activity. The underlying principle was unknown and even the statement of what was sometimes erroneously called the Periodic Law was manifestly incorrect and its terms were ignored.

Basis of the Periodic System

The ordinary statement of the fundamental principle of the Periodic System has been that the properties of the elements were periodic functions of the atomic weights, and that when the elements were arranged in the order of their atomic weights they fell into a natural series, taking their places in the proper related groups.

In accepting this, the interpretation of function was both unmathematical and vague, and the order of the atomic weights was not strictly adhered to but unhesitatingly abandoned to force the group relationship. Wherever consideration of the atomic weight would have placed an element out of the grouping with other elements to which it was clearly related in physical and chemical properties, the guidance of these properties was accepted and that of the atomic weights disregarded. Such shiftings are noted in the cases of tellurium and iodine; cobalt and nickel; argon and potassium. It was most helpful that, following the order of atomic weights, the majority of the elements fell naturally into their places. Otherwise the generalization known as the Periodic System might have remained for a long time undiscovered and the progress of chemistry would have been greatly retarded.

Influence of Positive Nucleus

It is evident that the order of the elements is determined by something else than their atomic weights. From the known facts of radio-activity it would seem that this determining factor is the positive nucleus. And this nucleus also determines the mass or weight of the atom. Taking the elements in their order in the Periodic Series and numbering the positions held by them in this series as 1, 2, 3, etc., we get the position number or what is called the atomic number. This designates the order or position of the element in the series. We must learn that this number marks a position rather than a single element, a statement which will be explained later.

Determination of the Atomic Number

Since the atomic weight is unreliable as a means of settling the position of an element in the series and so fixing its atomic number, how is this number to be determined? Of course, one answer to this question is that we may rely upon a consideration of the general properties, as has been done in the past. Fortunately, other methods have been found by which this may be confirmed. For instance, the stopping and scattering power of the element for alpha particles has been suggested and successfully used.

Use of X-Ray Spectra

A most interesting method is due to Moseley's observations upon the _X_-ray spectra of the various elements. It has been found that crystals, such as those of quartz, have the power of reflecting and defining the _X_ rays. The spectra given by these rays can be photographed and the wave lengths measured. These _X_ rays are emitted by various substances under bombardment by the cathode rays (negative electrons) and have great intensity and very minute wave lengths. Moseley made use of various metals as anti-cathodes for the production of these rays. These metals ranged from calcium to zinc in the Periodic System. In each case he observed that two characteristic types of _X_ rays of definite intensity and different wave lengths were emitted. From the frequency of these waves there is deduced a simple relation connected with a fundamental quantity which increases in units from one element to the next. This is due to the charge of the positive central nucleus. The number found in this way is one less than the atomic number. Thus the number for calcium is 19 instead of 20 and that for zinc is 29 instead of 30. So, by adding 1 to the number found the atomic number is obtained.

The atomic weight can usually be followed in fixing the atomic number, but where doubt exists the method just given can be resorted to. Thus doubt arises in the case of iron and nickel and cobalt. This would be the order according to the atomic weights. The _X_-ray method gives the order as iron, cobalt, and nickel, and this is the accepted order in the Periodic System.

Changes Caused by Ray Emission

On studying the properties of the elements in a transformation series in connection with the ray emission which produced them, it was seen that these properties were determined in each case by the nature of the ray emitted from the preceding transformation product or parent element.

Atomic Weight Losses

Each alpha particle emitted means a loss of 4 in the atomic weight. This is the mass of a helium atom. Thus from uranium with an atomic weight of 238 to radium there is a loss of three alpha particles. Therefore, 12 must be subtracted from 238, leaving 226, which agrees closely with the atomic weight of radium as actually determined by the ordinary methods. Uranium X_{1}, then, would have an atomic weight of 234 and that of ionium would be 230. The other intermediate elements, whose formation is due to the loss of beta particles only, show no decrease in atomic weight.

Lead the End Product

From uranium to lead there is a loss of 8 alpha particles, or 32 units in atomic weight. This would give for the final product an atomic weight of 206. The atomic weight of lead is 207.17. It is not at all certain that the final product of this series is ordinary lead. The facts are such that they would lead one to think that it is not. It is known only that the end product would probably be some element closely resembling lead chemically and hence difficult or impossible to separate from it. Several accurate determinations of lead coming from uranium minerals, which always carry this element and in an approximately definite ratio to the amount of uranium present, show atomic weights of 206.40; 206.36; and 206.54. Even the most rigid methods of purification fail to change these results. The lead in these minerals might therefore be considered as coming in the main from the disintegration of the uranium atom and, though chemically resembling lead, as being in reality a different element with different atomic weight.

Furthermore, in the thorium series 6 alpha particles are lost before reaching the end product, which again is perhaps the chemical analogue of lead. The atomic weight here should be 232 less 24, or 208. Determinations of the atomic weight of lead from thorite, a thorium mineral nearly free from uranium, gave 208.4.

The end product of the actinium series is also an element resembling lead, but both the beginning and ending of this series are still in obscurity.

Changes of Position in the Periodic System

The loss of 4 units in the atomic weight of an element on the expulsion of an alpha particle is accompanied by a change of chemical properties which removes the new element two groups toward the positive side in the Periodic System.

Thus ionium is so closely related to thorium and so resembles it chemically that it is properly classed along with thorium as a quadrivalent element in the fourth group. Ionium expels an alpha particle and becomes radium, which is a bivalent element resembling barium belonging to the second group. Radium then expels an alpha particle and becomes the gas, radium emanation, which is an analogue of argon and belongs to the zero group. Other instances might be cited which go to show that in all cases the loss of an alpha particle makes a change of two places toward the left or positive side of the System.

Changes from Loss of Beta Particles

The loss of a beta particle causes no change in the atomic weight but does cause a shift for each beta particle of one group toward the right or negative side of the System. Two such losses, then, will counterbalance the loss of an alpha particle and bring the new element back to the group originally occupied by its progenitor. Thus uranium in the sixth group loses an alpha particle and the product UX_{1} falls in the fourth group. One beta particle is then lost and UX_{2} belonging to the fifth group is formed. With the loss of one more beta particle the new element returns to the sixth group from which the transformation began.

The table on page 48, as adapted from Soddy, affords a general view of these changes.

Isotopes

An examination of the table will show a number of different elements falling in the same position in a group of the Periodic System irrespective of their atomic weights. These are chemically inseparable so far as the present limitations of chemical analysis are concerned. Even the spectra of these elements seem to be identical so far as known. This identity extends to most of the physical properties, but this demands much further investigation. For this new phenomenon Soddy has suggested the word isotope for the element and isotopic for the property, and these names have come into general use.

Manifestly, we have come across a phenomenon here which quite eliminates the atomic weight as a determining factor as to position in the Periodic or Natural System or of the elemental properties in general. All of the properties of the bodies which we call elements, and consequently of their compounds and hence of matter in general, seem to depend upon the balance maintained between the charges of negative and positive electricity which, according to Rutherford's theory, go to make up the atom.

It is evident that any study of chemical phenomena and chemical theory is quite incomplete without a study of radio-activity and the transformations which it produces.

Radio-activity in Nature

In concluding this outline of the main facts of radio-activity, it is of interest to discuss briefly the presence of radio-active material on this planet and in the stars. Facts enough have been gathered to show the probable universality of this phenomenon of radio-activity. Whether this means solely the disintegration of the uranium and thorium atoms, or whether other elements are also transformed under the intensity of the agencies at work in the universe, is of course a question as yet unsolved.

Radio-active Products in the Earth's Crust

The presence of uranium and thorium widely distributed throughout the crust of the earth would lead to the conclusion that their disintegration products would be found there also. Various rocks of igneous origin have been examined revealing from 4.78 × 10^{-12} to 0.31 × 10^{-12} grams of radium per gram of the rock. Aqueous rocks have shown a lesser amount, ranging from 2.92 × 10^{-12} to 0.86 × 10^{-12} grams. As the soil is formed by the decomposition of these rocks, radium is present in varying amounts in all kinds of soil.

Presence in Air and Soil Waters

As radium is transformed into the gaseous emanation, this will escape wherever the soil is not enclosed. For instance, a larger amount of radio-activity is found in the soil of caves and cellars than in open soils. If an iron pipe is sunk into a soil and the air of the soil sucked up into a large electroscope, the latter instrument will show the effect of the rays emitted and will measure the degree of activity. Also the interior of the pipe will receive a deposit of the radio-active material and will show appreciable radio-activity after being removed from the soil.

This radium emanation is dissolved in the soil waters, wells, springs, and rivers, rendering them more or less radio-active, and sometimes the muddy deposit at the bottom of a spring shows decided radio-activity.

The emanation also escapes into the air so that many observations made in various places show that the radium emanation is everywhere present in the atmosphere. Neither summer nor winter seems to affect this emanation, and it extends certainly to a height of two or three miles. Rain, falling through the air, dissolves some of the emanation, so that it may be found in freshly-fallen rain water and also in freshly-fallen snow. Radio-active deposits are found upon electrically charged wires exposed near the earth's surface.

As helium is the resulting product of the alpha particles emitted by the emanation and other radio-active bodies, it is found in the soil air, soil waters, and atmosphere.

Average measurements of the radio-activity of the atmosphere have led to the calculation that about one gram of radium per square kilometer of the earth's surface is requisite to keep up the supply of the emanation.

A number of estimates have been given as to the heat produced by the radio-active transformations going on in the material of this planet. Actual data are scarce and mere assumptions unsatisfactory, so little that is worth while can be deduced. It is possible that this source of heat may have an appreciable effect upon or serve to balance the earth's rate of cooling.

Cosmical Radio-activity

Meteorites of iron coming from other celestial bodies have not shown the presence of radium. Aerolites or stone meteorites have been found to contain as much as similar terrestrial rock. Since the sun contains helium and some stars show its presence as predominating, this suggests the presence of radio-active matter in these bodies. In addition, the spectral lines of uranium, radium, and the radium emanation have been reported as being found in the sun's spectrum and also in the new star, _Nova Geminorum 2_. These observations await further investigation and confirmation. So far as the sun's chromosphere is concerned, the possible amount of radium present would seem to be very small. If this is true, radio-active processes could have little to do with the sun's heat. The statement is made by Rutherford that indirect evidence obtained from the study of the aurora suggests that the sun emits rays similar in type to the alpha and beta rays. Such rays would be absorbed, and the gamma rays likewise, in passing through the earth's atmosphere and so escape ordinary observation. All of this is but further evidence of the unity of matter and of forces in the universe.

INDEX

Actinium, discovery of, 6

Activity, induced, 17

Alpha particles, effect of loss on Atomic Weight, 45 electrical charge of, 26 form helium, 27 nature of, 25 penetrating power of, 39 position of element changed by its loss, 46 recoil, 39 scattering of, 38 solid, 26

Atom, constitution of, 36 Kelvin's, 37 models of, 37 Rutherford's, 37

Atomic number, determination of, 43

Becquerel's experiments, 2

Beta particles, change in position of element by loss of, 47

Chalcolite, natural and artificial, 4

Constants, table of, 31

Curie unit, 22

Disintegration of the element, 25

Disintegration series, 24

Disintegration theory, 35

Electroscope, 12

Equilibrium series, 22

Helium, characteristics of, 30 discovery of, 29

Ionium, discovery of, 6

Ionization, application of electric field to, 10 experimental confirmation, 9

Ionization of gases, 7 theory of, 8

Ions, size and nature of, 10

Isotopes, 47

Lead, atomic weight varies with source, 45 radio-active, 6 the end product, 45

Life-periods of radio-active bodies, 21

Periodic system, 41 basis of, 42

Polonium, discovery of, 4

Positive nucleus, influence of, 43

Potassium, radio-activity of, 3

Radiations, action on phosphorescent bodies, 13 action on photographic plates, 11 discharge electrified bodies, 12 magnetic deflection of, 14 measurements of, 15 penetrating power of, 13, 15

Radio-active bodies, elemental nature of, 20 examination of, 20 life periods of, 21

Radio-activity, an atomic property, 3 cosmical, 51 influence on chemical theory, 41 products in atmosphere, 51 products in earth's crust, 50 products in soil waters, 50

Radium, action on organic matter, etc., 33 amount in pitchblende, 5 discovery of, 5 emanation, 22 energy evolved by, 34 properties of, 5, 32

Rays, alpha, 15, 16, 26 beta, 15, 16 gamma, 15, 16 identification of, 16, 25 magnetic deflection of, 14 photographing track of, 10 types of, 14

Rubidium, radio-activity of, 3

Spinthariscope, 13

Stopping power of substances, 39

Thorium X, discovery of, 18, 21

Uranium atom, disintegration of, 36

Uranium minerals, radio-activity of, 3

Uranium X, discovery of, 17, 21, 23

X-ray spectra, 44

Zinc sulphide screen, 13

TRANSCRIBER'S NOTES

1. Passages in italics are surrounded by _underscores_.

2. Images have been moved from the middle of a paragraph to the closest paragraph break.

3. The original text includes certain Greek alphabets. For this text version [alpha], [beta], [gamma] indicate first three letters of Greek alphabet respectively.

4. In this version, the number following carat character ^ is to be interpreted as follows. The expression 10^{-2} means multiplying by 0.01; 10^{10} means multiplying by 10,000,000,000.

5. In this version, the subscripted text has been replaced by an underline character _ followed by the same with curly braces { and }. For example, X_{1} indicates X with subscript 1.

6. The fractions are indicated with the help of forward character /. For example, 1/4 indicates one-fourth.

7. Other than the changes listed above, the original text has been reproduced as such.