Frederick Soddy Nobel Lecture
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O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S 393
This suggestion of Van den Broek was adopted by Bohr 16
in his theoretical researches on the structure of the atom. Bohr’s views required that the elec- tronic system is stable, so that to remove an electron involves the expenditure of energy. Hence it followed that the B-particles expelled in radioactive change must come from the nucleus and not from the external electronic system.
I had arrived at the same conclusion from totally different evidence 17 . If, for example, the two electrons that are expelled as B-rays, when uranium X, changes into uranium II, come from the same region of the atom as the two that are lost when U IV in uranous salts is oxidized to U VI in uranic salts, then the latter ought to be chemically non-separable from thorium, just as ura- nium X
1 is. Fleck 18 , trying this, found that, whereas there is a very close resemblance between uranium in the uranous salts and thorium, yet the two may be separated chemically without difficulty. The expulsion of two + charges as an α -particle and of two electrons as β− particles from the nucleus causes the element to come back to the original place that it occupied in the Periodic Table. It followed therefore that the place in the Periodic Table is an expression of the nett nuclear charge, i.e. of the difference between the numbers of positive and negative charges in the nucleus. Thus the chemically identical elements - or isotopes, as I called them for the first time in this letter to Nature, because they occupy the same
nuclear charge, but with different numbers of + and - charges in the nuc- leus. On the view that the concentrated positive charge is the massive particle in the atomic structure, since positive electricity has never been observed free possessing less than the mass of an atom, the atomic weight of the iso- tope is a function of the total number of positive charges in the nucleus and the chemical character a function of the nett number. Though the nucleus possesses electrons there can be no in- or out-going of electrons between the nucleus and the external electronic system. Thus Rutherford’s atom affords for the first time a clear picture of the difference between a transmutational (radioactive) change and chemical one. Changes of the number of electrons in the external system are chemical in character and produce changes in the valency of the element. These are reversible and have no effect at all on the central nucleus. Whereas changes of the nucleus are transmutational and irreversible, and they instantly impress changes upon the external electronic system to make it conform to the new nucleus. So far as I was concerned, this interpretation of isotopes, in the light of Van den
394 1921 F.SODDY Broek’s conception and Rutherford’s nuclear atom, resulted in a great clari- fication of my own ideas. It was completely independent of Moseley’s researches into the spectra of X-rays, which then had not been published. Moseley
19 determined by the crystal reflection method the wavelengths of the homogeneous secondary X-rays, characteristic of the chemical ele- ments, discovered by Barkla. He found that the squares of these wavelengths are a function of a number, now called atomic number, which alters by one unit in passing from the X-ray spectrum of any element to that of the next to it in the Periodic Table. He was thus able to extend the definite determina- tion of the number of places in the Periodic Table, which had been accom- plished for the elements from uranium to thallium by radioactive change, to the rest of the Periodic Table as far as aluminium. Moseley’s atomic number is the same as Van den Broek’s intra-atomic charge and represents(I) the nett positive charge on the nucleus, (2) the number of electrons in the outer electronic system of the atom, (3) the number of the place in the Period- ic Table occupied by the element, counting from hydrogen as unity to uranium as 92, as was first definitely determined later by his method.
In conclusion, I may very briefly deal with the verification of the two chief predictions from the α− and β− change generalization. The possibility that actinium resulted in a β− ray change necessitates that its parent should be an isotope of radium.This I disproved directly by an exam- ination of a specimen of Giesel’s radium bromide that had been kept ten years without any chemical treatment 2 0
. There was a total absence of radio- actinium, after that time, in the preparation. The remaining alternative that actinium was produced in an α -ray change from "eka-tantalum", a missing element occupying the place between uranium and thorium, and isotopic therefore with uranium X 2 , was finally established by the independent work of Hahn and Meitner 21 , and of J. A. Cranston and myself 22 , in 1918. As regards our work, we found that uranium X 2 could be readily vola- tilized from uranium X 1 in a stream of carbon tetrachloride and air at an incipient red-heat. Thus we used uranium X 2 as a radioactive indicator of the chemical properties of the unknown substance, something in the same manner first employed by Paneth and von Hevesy. When pitchblende was treated in the same way, a sublimate was obtained free initially from all the
O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S 395
known pm-emanation members, but continuously generating actinium, as shown by its characteristic emanation, with the lapse of time. Hahn and Meitner found the missing element in the insoluble residues from the treatment of uranium minerals for radium, and separated it by methods designed to remove tantalum, which was added in minute quantity as required to assist the separation. From their work, the new element, pro- toactinium as they call it, promises to be of more than ordinary interest. They have not succeeded in separating it from the tantalum present or added during the operations. It gives α−
ray of low range, generates actinium at a rate which has enabled the average life of the latter substance at length to be definitely established, as 28.8 years. The production of protoactinium from uranium has been demonstrated by the examination of old uranium salts. From the quantity present, its average life is 17,000 years. (Mr. Mennie working in my laboratory, with a large quantity of uranium, which I had very carefully purified in 1909, has confirmed this.) This period corresponds with a quantity of 72 mg of protoactinium per 1,000 kilograms of uranium, or 333 mg of radium, in minerals. So that it appears likely that it will be the second new radio-element to have its atomic weight, spectrum and chemical nature determined, in the same way as Mme. Curie accomplished for radium. Protoactinium appears to be produced as a branch product, claiming 3% of the total number of atoms disintegrating, from either uranium I or ura- nium II through Antonoff’s uranium Y, which is its direct parent. But it has also been suggested by Piccard that it may be derived from a totally inde- pendent isotope of uranium (actinouranium). The series runs: VI Iv v III IV II 0 The atomic weight of actinium is thus still indefinite. The atomic weight and spectrum of lead and ionium The ultimate product:. Immediately after the prediction that the main end products of uranium and thorium must be different isotopes of lead with atomic weights 206 and 208, determinations were undertaken of the atomic weight of lead separated from thorium minerals as free as possible from ura- nium, and of that from uranium minerals as free as possible from thorium. 396 1921 F.SODDY These resulted in the complete verification of the prediction. The highest value yet found for "thorium-lead" is 207.9 (Fajans and Hönigschmid) and the lowest value for "uranium-lead" 206.05, the accepted value for common lead being 207.20. In my own work, 30 kilograms of Ceylon thorite, containing 55% of thorium, about 1% of uranium and 0.4% of lead, was hand-sorted, and, from 20 kilograms of selected pieces, 80 grams of metallic lead were sep- arated.
The view that isotopes have identical external electronic systems naturally leads to the deduction that their atomic volumes must be identical and, there- fore, that the atomic weights must be proportional to their density. The thorite lead was cast in vacuo and its density compared with that of a simi- larly purified and treated specimen of common lead. Its density was found to be 0.246% greater, leading to a calculated value of 207.7 for the atomic weight. By fractional distillation in vacuo three fractions were obtained and the atomic weight of the middle one found, from the ratio Pb : PbCl 2 to be
207.69, as compared with 207.20, found in a parallel estimation of ordinary lead. The war interrupted these researches, but it is of interest to record that, for another specimen of my thorite lead, Hönigschmid found, by the silver titration method, 207.77 ± 0.014 23 .
and his students at Harvard, and by Hönigschmid and Mlle. Horovitz, gave values all below the international figure. In the case of the two most carefully selected minerals, Morogoro uraninite and Norwegian bröggerite, values 206.46 and 206.063 were found 24 . For these also the atomic volume was found to be the same as for ordinary lead, the density being as much less as the atomic weight is less than for ordinary lead. For a similar reason, that the molar solubilities of isotopes must be equal, the actual solubilities of salts of different isotopes must be proportional to their molecular weights. Two independent determinations of the melting point of the lead of radioactive origin failed to reveal any difference from that of ordinary lead. To ordinary methods the spectrum of the lead prepared from radioactive minerals is identical with that of common lead. Hönigschmid and Horovitz have determined the atomic weight of thori- um and compared it with that found by the same methods for the ionium- thorium preparation prepared by Auer von Welsbach. For the first they obtained the value 232.12, and for the second 231.51, a difference of atomic weight of 0.61, corresponding with a percentage of ionium of about 30, if
O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S 397
230 be the value for the latter. This is in conformity with the period of ionium and the a-radiation of the preparation. But the spectrum of the iomum-thorium preparation was in every respect identical with that of thorium.
Although no difference in spectrum is observable by ordinary methods, Harkins and Aronberg 25 , photographing the strongest line of the lead spec- trum, 4058, in the sixth order of a 25-cm Rowland grating, found a minute difference. The wavelength of the line for uranium lead of atomic weight 206.34 is 0.0043 Å greater than that for common lead. This infinitesimal difference has been confirmed by Merton 26 , who found also for my thorite lead a wavelength 0.0022 Å less than that of common lead. Summary We may now sum up the various distinct steps in this long and tangled story of the origins of the conception and discovery of isotopes. I . Experimental methods are available, uniquely for the radio-elements, which enable isotopes to be severally recognized, by a suitable combination of chemical analyses at appropriate intervals of time, whereby, owing to the successive changes of the constituents, they may be separated, although chemical analysis alone is quite unable to effect this separation. This dates from 1905. 2. The complete chemical identity of isotopes, as distinct from close chem- ical similarity, came gradually to be recognized. McCoy and Ross were the first to express a definite opinion in this sense (1907). 3. The existence of chemical identities among the radio-elements led to the deduction that they might exist among the common elements and be responsible for the exceptions in the Periodic classification, and for the fact that the atomic weights in some cases depart widely from integral values. Strömholm and Svedberg first made this deduction (1909). 4. The recognition of the effect of the expulsion of, first, the a- and then, the β
gration series from end to end in the Periodic Table. On the experimental side the names of A. Fleck, and on the theoretical side that of G. von Hevesy and A. S. Russell, but pre-eminently that of Kasimir Fajans, are associated with this advance. 5. The identity of isotopes was extended to include their electrochemistry
398 1921 F.SODDY (Paneth and Hevesy) and their spectra (Russell and Rossi), though here in- finitesimal differences were subsequently found (Harkins and Aronberg, Merton). 6. Isotopes, on Rutherford’s theory of atomic structure, are elements with identical external electronic systems, with identical nett positive charge on the nucleus, but with nuclei in which the total number of positive and nega- tive charges and therefore the mass is different. The originator of the view that the places in the Periodic Table correspond with unit difference of intra-atomic charge is Van den Broek. 7. Moseley extended this view to the non-radioactive elements, and ulti- mately for the whole Periodic Table, and the definite determination of the number and sequence of the places in it became possible. 8. The chemistry of the radioactive elements and the lacunae previously existing in the radioactive series, especially in connection with the origin of actinium, have been cleared up, and this led to the discovery of a new ele- ment, proto-actinium (eka-tantalum) in uranium minerals, occupying the place between uranium and thorium and existing in sufficient quantity for the compounds of the element to be prepared in a pure state, and its spec- trum and atomic weight ultimately to be determined, as Mme. Curie did for radium. Cranston and I share with Hahn and Meitner the original discovery, but the subsequent developments are due to the latter. 9. The preparation from radioactive minerals of different isotopes of lead followed and the determination of their atomic weight, spectrum, density and other properties, established that the same chemical character and atomic volume can coexist with differences of atomic weight. The work on the ionium-thorium mixture from pitchblende is a second example. 10. The last result, and perhaps the most important of all, is the subject of the award of the Nobel Prize for Chemistry for 1922. 1. H. N. McCoy and W. H. Ross, J. Am. Chem. Soc., 29 (1907) 1709. 2. B. Keetman, Jahrb. Radioaktiv., 6 (1909) 268. 3. A. Schuster, Nature, 91
(1913) 30, 57. 4. A. Fleck, Chem. News, 106 (1912) 128. 5.
Chem. Soc, 103
(1913)
381.
6.
G.v. Hevesy, Physik.Z., 14 (Jan.15, 1913) 49.
7.
G.v. Hevesy, Phys.Z. 14 (Jan. 15, 1913) 60. 8.
A.S. Russell, Chem.News, 107(Jan.31,1913)49. O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S 399
9. K. Fajans, Physik.Z., 14 (Feb. 15, 1913) 131. 10. F. Soddy, Chem. News, 107 (Feb. 28, 1913) 97; Jahrb. Radioaktiv., 10(1913) 188. 11. A. Fleck, Trans. Chem. Soc., 103 (1913) 1052, compare also W. Metzener. Ber., 46 (1913) 979. 12. K. Fajans and O. Göhring, Physik.Z., 14 (1913) 877. 13. E. Rutherford, Phil. Mag., [6] 21 (1911) 669. 14. C. G. Barkla, Phil. Mag., [6] 21 (1911) 648. 15. A. van den Broek, Nature, 87 (1911) 78; 92 (1913) 372. 16. N. Bohr, Phil Mag., [6] 26 (1913) 476. 17. F. Soddy, Nature, 92 (1913) 400. 18. A. Fleck, Trans. Chem. Soc., 105 (1914) 247. 19. H. G. J. Moseley, Phil. Mag., [6] 26 (1913) 1024; 27 (1914) 703. 20. F. Soddy, Nature, 91
(1913) 634. 21. O. Hahn and L. Meitner, Physik.Z., 19 (1918) 208; 20 (1919) 529; Ber., 52 (1919) (B) 1812; 54 (1921) (B) 69. 22. F. Soddy and J. A. Cranston, Proc. Roy. Soc, 94A (1918) 384. 23. F. Soddy and H. Hyman, Trans. Chem. Soc, 105 (1914) 1402; F. Soddy, Nature, 94(1915)615;Eng.,(May28,1915);(Oct,1,1915); Ann.Rept.Progr.Chem.,(1916) 247; O. Hönigschmid, Z.Elektrochem., 25 (1919) 91. 24. T. W. Richards and M. E. Lembert,J. Am,. Chem. Soc, 36 (1914) 1329; Compt. Rend., 159 (1914) 248; T. W. Richards and C. Wadsworth, J. Am. Chem. Soc., 38 (1916) 221, 1658,2613; T. W. Richards and W. C. Schlumb, J. Am. Chem. Soc., 40 (1918) 1403; T. W. Richards and N. F. Hall, J. Am. Chem. Soc, 42 (1920) 1550; O.Hönigschmid and S.Horovitz, Compt.Rend., 158 (1914) 1676; Monatsh., 36 (1915) 353; O. Hönigschmid, Z.Elektrochem., 25 (1919) 91; M. Lembert, Z. Elek-
25. W.D. Harkins and L. Aronberg, Proc. Natl. Acad. Sci., 3 (1917) 710; J. Am. Chem. Soc., 42 (1920) 1328. 26. T. R. Merton, Proc. Roy. Soc., 96A (1920) 338. Yüklə 161,11 Kb. Dostları ilə paylaş:
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