Part 5 (2/2)
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 a.n.a.logue 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 cla.s.sed 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 a.n.a.logue 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 s.h.i.+ft 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 a.n.a.lysis are concerned.
Even the spectra of these elements seem to be identical so far as known. This ident.i.ty 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.
[Ill.u.s.tration: RADIO-ACTIVE ELEMENTS FROM URANIUM AND THORIUM PLACED IN THE PERIODIC SYSTEMS Adapted from Soddy]
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
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