Part 5 (1/2)

CHAPTER VI

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 relations.h.i.+p 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 relations.h.i.+p. 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 s.h.i.+ftings are noted in the cases of tellurium and iodine; cobalt and nickel; argon and pota.s.sium. 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 r.e.t.a.r.ded.

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 ma.s.s 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 quant.i.ty 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 ma.s.s of a helium atom. Thus from uranium with an atomic weight of 238 to radium there is a loss of three alpha particles.