i029Fig. 29—Hypothetical atomic structures
Fig. 29—Hypothetical atomic structures
Fig. 29—Hypothetical atomic structures
Precisely the same procedure is repeated in the fifth period of eighteen elements between krypton and xenon, the rare-earth group which intervenes between strontium (Sr) and silver (Ag) corresponding to the elements in which, with increasing atomic number, the added electrons are filling up the empty orbits in the fourth shell instead of going into what is now the outer or fifth shell (seeTable XV).
Now in considering the sixth period of thirty-two elements from xenon (Xe) to niton (Nt), a glance atTable XVshows that the fourth shell in xenon contained only eighteen electrons, whereas in niton there are thirty-two, i.e., there are fourteen unfilled orbits in xenon in the fourth shell; and a similar glance at the fifth shell showsvacant orbits there. The first two elements in this group, viz., caesium (Cs) and barium (Ba), take the added electrons inorbits, then the electrons begin to go inside until gold is reached, when the fourth and fifth shells become full and from gold (Au) to niton (Nt), as the added electrons go to the outer shell, the chemical properties again progress as from sodium to argon, or from copper to krypton.
It will be noticed that inFig. 30element 72 is hafnium, the element discovered in 1923 by Coster and Hevesy[164]by means of X-ray analysis. It is because its chemical properties resemble so closely those of zirconium that it had not been found earlier by chemical means. Hevesy estimates that it represents one one hundred-thousandth of the earth’s crust, which makes it more plentiful than lead or tin.
i030Fig. 30—Bohr’s form of the periodic table, the most illuminating thus far devised. The elements which are in process of orbital reconstruction, because of the passage of electrons into thus far unfilled inner quantum orbits, are inclosed in frames. Lines connect elements which have similar properties.
Fig. 30—Bohr’s form of the periodic table, the most illuminating thus far devised. The elements which are in process of orbital reconstruction, because of the passage of electrons into thus far unfilled inner quantum orbits, are inclosed in frames. Lines connect elements which have similar properties.
Fig. 30—Bohr’s form of the periodic table, the most illuminating thus far devised. The elements which are in process of orbital reconstruction, because of the passage of electrons into thus far unfilled inner quantum orbits, are inclosed in frames. Lines connect elements which have similar properties.
NUMBER OF ELECTRONS IN DIFFERENTORBITS
The seventh period begins (Fig. 30) with an unknown element of atomic number 87, which, with its singleorbit, should have a valency of 1, then passes to radium with its twoorbits (seeFig. 31) and valency 2, and breaks off suddenly with uranium because thenucleushas here become unstable.
It should be clearly understood that the detailed theory as here presented, and above all the models of complicated atoms, are to a very considerable degree hypothetical and speculative. But it is highly probable that they give a more or less correctgeneralpicture of the way electrons behave in atoms. So far as the general conception of orbits which behave in the main, especially in the simpler atoms, in accordance with the Bohr assumptions, is concerned, if the test of truth in a physical theory is large success both in the prediction of new relationships and in correctly and exactly accounting for old ones, the theory of non-radiating orbits is one of the well-established truths of modern physics. For the present at least it is truth, and no other theory of atomic structure need be considered until it has shown itself able to approach it in fertility. I know of no competitor which is as yet even in sight.
I am well aware that the facts of organic chemistry seem to demand that the valence electrons be grouped in certain definite equilibrium positions about the periphery of the atom, and that at first sight this demand appears difficult to reconcile with the theory of electronic orbits. But a little reflection shows that there is here no necessary clash. With a suitableorientationof orbits, these localized valencies of chemistry are about as easy to reconcile with an orbit theory as with a fixed electron theory.
i031Fig. 31—Hypothetical structure of the radium atom
Fig. 31—Hypothetical structure of the radium atom
Fig. 31—Hypothetical structure of the radium atom
It is only forfree atomsthat spectroscopic evidence has forced us to build up orbit pictures of the foregoing sort. When atoms unite into molecules, or into solid bodies, these orbits will undoubtedly be very largely readjusted under the mutual influence of the two or more nuclei which are now acting simultaneously upon them.
It has been objected, too, that the Bohr theory is not a radiation theory because it gives us no picture of the mechanism of the production of the frequency.This is true, and therein lies its strength, just as the strength of the first and second laws of thermodynamics lies in the fact that they are true irrespective of a mechanism. The Bohr theory is a theory of atomic structure; it is not a theory of radiation, for it merely states what energy relations must exist when radiation, whatever its mechanism, takes place. It is the first attempt to determine in the light of well-established experimental facts what the electrons inside the atom are doing, and as such a first attempt it must be regarded as, thus far, a success, though it has by no means got beyond the hypothetical stage. Its chief difficulty arises from the apparent contradiction involved in a non-radiating electronic orbit, and there appears to be no solution to this difficulty save in the denial of theuniversalapplicability of the classical electromagnetic laws. But why assume the universal applicability of these laws, even in the hearts of atoms, when this is the first opportunity which we have had to test them out in the region of the infinitely small?
There is one other very important relation predicted by the Bohr theory and beautifully verified by experiment, but not involving at all its orbital feature. The frequency value of the inmost, orlevel,can be exactly determined by measuring theabsorption edge so beautifully shown on the De Broglie photographs oppositep. 200. Let us call this frequency.Similarly, to each orbit in the second orquantum state, there corresponds a definite absorption edge.Two of these are shown clearly inFig. 23. The difference between theabsorption frequency and eachabsorption frequency should obviously, according to Bohr, correspond exactly to the frequencyof an emission line in theX-ray spectrum, i.e.,This so-called Kossel relation is of course applicable to all X-ray and optical spectra. Indeed, in the latter field it appeared before the Bohr theory under the name of the “Ritz combination principle.” It has been one of the most important keys to the unlocking of the meaning of spectra and the revealing ofatomic structure.