Rediscovery of the Elements

Soddy and Isotopes

III~1

James L. Marshall, Beta Eta /971, andVirginia R. Marshall, Beta Eta 2003,Department of Chemistry University ofNorth Texas, Denton,TX 76203-5070,jimm@unt.edu

A Dinner Party in Glasgow, Scotland. Duringa social gathering in 1913 at the home of his in-laws, Frederick Soddy (1877-1956) (Figure 1)was musing what to call a new concept. For adecade a plethora of radioelements, each with aunique half-life, had been discovered at variousuniversities throughout Europe and NorthAmerica, and there was no room in the PeriodicTable to accommodate them. To solve this prob-lem, Soddy was proposing that radioelementswith different half-lives but with the samechemical properties should be confined in onebox in the Table. The scholarly Dr. MargaretTodd (1859-1918), an M.D. and novelist friendof his wife, Winifred, suggested' such elementsbe denoted by the Greek words "same place"("iso-topos") (Figure 2). Soddy shortlyannounced his idea of "isotopes" (December 4,1913) in the prestigious journal Nature.

The Life of Frederick Soddy. A native ofEastbourne in East Sussex, 85 km south ofLondon, Frederick Soddy was trained at theUniversity of Aberystwyth (Wales) and atOxford University, graduating in 1898. Afterthree years as researcher at Oxford, Soddyaccepted the position of Demonstrator atMcGill University, Montreal, Canada. After ayear he joined the group of Ernest Rutherford

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Figure 1. The home of Sir George and Lady Beilby,the parents of Frederick Soddy's wife, Winifred, at11 University Gardens, Glasgow, Scotland. Thehome is of Victorian architecture and was built in1884. Sir George was an industrial chemist whofurnished 50 kilograms of uranyl nitrate to Soddyfor his experiments. This home served as a popularmeeting place for politics, science, business, andleisure. In the drawing-room of this home, theword "isotope"was coined by Soddy at thesuggestion of Dr. Margaret Todd.

at that university, who was just beginning toinvestigate the strange emanation from thori-um.; For the next two years the two colleaguesestablished the elemental nature of thoron(radon-220), developed the understanding ofradioactive half-lives, and discovered the nat-ural transmutation of the elements.' It wasactually Soddy the chemist, well versed in thehistory of alchemy, who first understood thesignificance of transmutation, before Ruther-ford the physicist.'

In 1903, Soddy returned to England andjoined the research group of William Ramsay(1852-1916; the discoverer of the noble gases)at University College, London, to study thedecay by-products of radium. In Ramsay's lab-oratory, Soddy established that helium wasproduced during the decay of radium, thusfurnishing the first evidence of the transmuta-

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Figure 2. This plaque is mounted at 11 UniversityGardens. The building, now known as the GeorgeService House, presently houses the HumanitiesAdvanced Technology and Information Institute(IIATI!) of the Univcrsity ofGlasgow.

tion of a known element to another knownelement; this research was the basis of thefuture understanding of the alpha particle.'

In 1904, Soddy became Professor at theUniversity of Glasgow, where he formulatedthe concept of isotopes. After 10 years atGlasgow, Soddy moved to the University ofAberdeen (1914-1919), where he co-discoveredthe element protactinium ("eka-tantalum").World War I had a distressing effect on Soddy;the death of Henry Moseley"' elicited the fol-lowing from Soddy,' "When Moseley was killedat Gallipoli I felt enraged. Sometimes I thinkthat something snapped in my brain.... I feltthat governments and politicians, or man ingeneral, was not yet fitted to use science."Soddy moved in 1919 to Oxford University,where with a new mission he tried to establisheconomic policy based on scientific laws. Heretired in 1936, after a frustrating period in hislife, with a "sense of failure.... in the field ofeconomics." He was at Oxford when he accept-ed the Nobel Prize in 1921 for his research inradio-elements and isotopy at Glasgow, and forteaching us that atoms could have "identicaloutsides but different insides."'

The Discovery of the Radioelements (seeFigure 3). Radioactivity was discovered byAntoine Henri Becquerel when he observeduranium emitted invisible rays that fogged pho-tographic plates wrapped with paper or metal

238

236

234

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230

228

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U-238 = Klaproth's "uranium" U1

U-235 = AcU ("actinourani ") cuUX1 / UX2 (Fajans' "brevium") / U2

Th-232 = Berzelius' "thorium" TUY / Hahn's "protactiniu '

("ionium")

MsTh1 / MsTh2 / RdThDebierne's "actiniu RdAc

Curie's "radium"

AcK (Perey's"francium" AcX

Rn ("radon")

T ("thoron")An("actinon")

hAAcARaB / RaC / RaC'

ThB / ThC / ThC'c/AcC

RaD / RaE / RaF (Curie's "polonium")

'/ ThD ThD = stable Pb-208cC" / AcD AcD = stable Pb-207

RaG RaG = stable Pb 206

TI Pb Bi Po At Rn Fr Ra Ac Th Pa U81 82 83 84 85 86 87 88 89 90 91 92

Atomic number

foil.' His discovery prompted the search for

new radioactive elements by Marie Curie, whocanvassed all known elements and determinedthat of these only uranium and thorium exhib-ited activity." After noticing that ores of urani-um were much more radioactive than expected,she launched her classic research with her hus-band, Pierre Curie, to discover polonium andradium in 1898, and her colleague Andr6Debierne discovered actinium in 1899.4

The discovery of radioactive elements con-

tinued, which became known as the "radioele-ments." In 1900, William Crookes (1832-1919)- the discoverer of thallium-made an aston-ishing observation: he found that purified ura-nium nitrate (prepared either by precipitationwith ammonium carbonate or by extraction

with ether) exhibited no radioactivity at all!'

(This puzzling observation was shortlyexplained by Rutherford; vide infra). Instead,radioactivity resided in the separated residue,which Crookes named uranium X, or Ur-X(later named UX1). If Crookes had followed upthis work, he would have been the discoverer of

the natural transmutation of elements, becauseUX1 was actually thorium-234, produced by thea-decay of uranium-238.

The explanation of UX1 by Rutherford. Nowactive in his research at McGill University(where he discovered the element radon)'

Ernest Rutherford attempted to reproduce

Crookes' work, but was perplexed by his obser-vation that in his hands purified uranium wasstill radioactive. By following Crookes'procedureexactly, Rutherford realized the discrepancywas due to Crookes' using the older Becquerelmethod of radioactivity detection (fogging ofwrapped photograph plates) instead of the"electric method" which utilized an ionizationchamber pioneered by Pierre Curie'(Rutherford used the Dolezalek electrometer,7 arefinement of this method 4'). On the basis ofhis work at Cambridge where he defined a-and 13-rays, Rutherford realized that pure urani-um (and its compounds) was an a-emitter(whose rays were stopped by paper or foil) andthat UX1 was a 1-emitter (whose rays passed

through Becquerel's and Crookes' paper wrap-pings). Rutherford designed an ionization

chamber in which he could readily distinguisha-rays and 1-rays by alternatively using shieldsof 0.1 mm aluminum foil (impervious to a-rays)or 5 mm aluminum foil (impervious to P-rays).'(The third type of emission, y-emission wasrecently discovered by Paul Villard in 1900,could be inferred by its stoppage using by 50 cmof aluminum.) After a period of time, an undis-turbed sample of purified uranium nitrate pro-

duced a fresh supply of UX1.' Rutherford andSoddy determined the half-life of UX1 to be 22days.

Figure 4 (ABOVE). Map of Glasgow. Sites ofinterest in the University of Glasgow area:

Site where Soddy proposed the concept of"isotope,"11 University Gardens - N55 52.37 W04 17.45.

Joseph Black Building (modern chemistry building),University of Glasgow - N55 52.32 W04 17.57

Lord Kelvin's house, University of Glasgow - N5552.28 W04" 17.43.

Old Chemistry Building, Main Building, GilbertScott Building (now geography), where Soddyperformed his work on isotopes - N55 52.24W040 17.25.

Museum Stores, site of Soddy's equipment andmodels, Thurso Street - N55 52.17 W04* 17.84.

The thorium and uranium series ofradioelements (see Figure 3). Rutherford andSoddy turned to thorium and found it exhibit-ed similar behavior. Reacting thorium nitrate

with ammonia (to precipitate thorium hydrox-ide) gave a filtrate, which when concentratedgave a very active residue they called thorium-X (ThX). Careful study showed ThX in turntransmuted into emanation (thoron), whichitself changed to an entire sequence of newradioactive products!' After the Christmas hol-idays of 1901, Rutherford and Soddy returned

to study the original purified sample of thori-um, which was found to be replenished withthe -emitter ThX. Soon Rutherford and his

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Figure 3 (LEFT). The decay sequences of the natural radioelements (Fajans andSoddy "Displacement Laws"), patterned after Soddy's original list" andgraph."' Atoms decay by emitting an a-particle (moving left two units anddown four units) or a /3-particle (moving right one unit). The list of naturalradioelements includes some 40 members and organized into three main series:U-238, originally identified as U1 ("radium series," the black plot), U-235,originally identified as "actinouranium"("actinium series,"the red plot), andTh-232, originally identified as merely "thorium"("thorium series," the blueplot). The atomic number and mass for each radioelement can be determinedfrom the respective abscissa and ordinate. For each radioelement the originalname is given (in addition to the customary elemental abbreviations, Rd =radio, Ms = meso; for example RdAc = radioactinium). (For a more completedescription including half-lives of the radioelements, see reference 11).

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Figm 5. This plaque dcdicatcd to Soddy1

ismounted in the Joseph Black Building, the mainchemistry building of the University of Glasgow.

colleague Soddy had carefully outlined asequence of transmutations:'

Th (half-life =10yr) -+ ThX (4 days) +Emanation (54 sec) -+ThA (11 hr) -+

ThB (1 hr)Did other radioactive elements exhibit a cas-

cade of radioelements? Rutherford and Soddyturned to radium and found the followingsequence:2

Ra (1500 yr) -+ Emanation (4 days) --RaA (3 min) -+.- RaB (28 min) -+

RaC (21 min) -- RaD (40 yr) +RaE (6 days) -- RaF (143 days)

More researchers join the hunt. The researchof Rutherford and Soddy sparked the hunt anddiscovery of more radioelements by many sci-entists during the first decade of the 20th cen-tury (Figure 3). These researchers includedHerbert Newby McCoy (1870-1945; Universityof Chicago), Alexander Smith Russell(1888-1972; Oxford University), BertramBorden Boltwood (1870-1927; Yale University),Otto Hahn and Lise Meiter (1879-1968 and1878-1968, respectively; Kaiser WillhelmInstitute, Dahlem-Berlin), Kasimir Fajans(1887-1975; Karlsruhe University), andFrederick Soddy (and colleagues) now at theUniversity of Glasgow (Figures 4-7).' 'The listof radioelements ballooned during the first twodecades of the 20th century, reaching a total ofalmost 40 members, perplexing the scientificworld with a"radium series" (RaA through RaG,originating from uranium), a "thorium series"

(ThA through ThD, originating from thorium),and an actinium series (AcA through AcD, orig-

dk4

I igur' o. At the Unmversity of Glasgow, Soddyperformed his work in the southeast corner of theMain Building. His laboratory now houses theGeography Department. When he abandoned thisworking space, it had to be thoroughly scrubbedand reconstructed because of the intense radioac-tivity; even the doorknobs had to be replaced.Nearby is the historic home of William Thomson(Lord Kelvin, 1824-1907, named after the riverthat flows nearby).

inating from uranium). These three seriesincluded three different emanations of inertgases (radon, thoron, and actinon; all isotopesof radon), as well as later intermediate mem-bers named ionium (Th-230), mesothoriumland -2 (Ra-228 and Ac-228), radiothorium (Th-228), eka-tantalum (Pa-231), etc., and all end-ing with inert lead.' Because the quantitiesinvolved were generally unweighable traces,the major way to characterize and identify theseelements was by means of their half-lives.

It was unclear just how to organize this mot-ley collection, whose half-lives and randombehavior of a- or 13-emission exhibited noapparent pattern. A major additional problemwas the impossibility of fitting these new"ele-ments" into the Periodic Table - the situationwas bad enough with the rare earths, a dozenunusual elements that seemed to form an inde-pendent series (Bohr's atom with the f-subshellwas yet to be formulated). So how could oneaccommodate three times that many rogueradioelements?

The clue of chemical similarity. The key toorganization of the radioelements was foundthrough wet laboratory chemistry. As chemistsworked their analytical separations, theynoticed that there was difficulty in separatingsome of the members of the radioelements.Whereas the challenge of separating radium

from chemically similar barium (solved by the

Curies during their discovery of radium") hadbeen"merely formidable," the problem of sepa-rating elements of a certain grouping appearedto be practically impossible, suggesting group-ings of virtually indistinguishable chemical

behavior.

The Fajans and Soddy DisplacementLaws. Meanwhile, the roadmap (Figure 3) bywhich these radioelements were being trans-formed into one another was becomingunderstood. One could trace the zig-zag routeof an atom as it followed its predestined pathfrom its progenitor to stable lead by alterna-tively shedding a- and -particles. In early1913, two defining articles appeared almostsimultaneously by Fajans and Soddy,"

which stated that all a-decays resulted inmovement two units to the left and all p-decays in movement one unit to the right (theso-called Fajans and Soddy "DisplacementLaws"). Whereas Fajans' approach was moretheoretical, Soddy's conclusions were basedon extensive chemical analysis, with a heavydependence upon final analysis of his studentAlexander Fleck (1889-1968) at Glasgow whohad proven the chemical identicalness of anumber of 13-emitters."

In his 1913 articles,"' Fajans organized theradioelements in the Periodic Table, lumping

similar elements in a "pleiad,"''2 a name con-noting a tight cluster of chemical properties,alluding to the Pleiades, the well known starcluster in the constellation Taurus. Fajans thenboldly proposed that there should be an ele-ment "UX2" just to the right of Crookes'UX1 -and several months later he and his studentOswald Helmuth Gohring (1889-?) electroplat-ed out the element whose electrochemical

figure 7. Dr Jot' Conall/, our host at theUniversity of Glasgow, looks upon one of Soddy'smachines in the Historic Museum ("MuseumStores"). This is the "Statis machine,"which has acolumn with a series of chambers connected bydifferently sized orifices; as water flows throughthe column, one can visualize the steady-state dis-tribution of elements in a radioactive ore sample.

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Figure 8. This is the man building of theUniversity of Karlsruhe, 12 Kaisserstrascse; theoriginal building of Universitat Fridericiana(N49 00.56 E080 24.72), where Lothar Meyerrefined his Periodic Table. Behind lay the ChemicalInstitute (Kollegium) where Kasimir Fajans dis-covered "brevium""" (Pa-234m), the first isotope ofelement-91 in 1913 (now Kollegiengebdude derEhrenhof Englerstrasse 11 - N49 00.61 E080

24.72). Nearby is the Stdndehaus, where in 1860the international Chemical Congress was held,which inspired the conception of DimitriMendeleev's and Lothar Meyer's Periodic Tables.4"

potential placed it in this vacant space (Figure8)." This element could be detected because ofits high activity, with a short half-life of 1.17minutes, even though ponderable quantitiescould not be collected. Because"it was the onlyelement in the 5th group of the last row,"he feltjustified in giving it a name, "brevium"" (nowknown to be protactinium-234m) (Note 1).

Fajans in his studies was using an electro-plating technique whereby he could plate outtraces of specific radioelements and could char-acterize each by determining its half-life. By

contrast, Soddy was using wet laboratory pro-cedures and was impressed with the apparentimpossibility of chemical separation of variousradioelements. Soddy considered the elementsof a grouping not "a scattered pleiade," but aschemically identical. While Fajans had groupedthe radioelements loosely in a table,2 ' Soddydrew a precise two-axis graph." Soddy under-stood the implications of chemical indistin-guishability-for example, ionium (Th-230, theprecursor to radium in the uranium sequence)was identical to native thorium (Th-232) (Note2), even to the point of identical spectra,"despite their different half-lives (7.54x 104 and1.40x 10'" years, respectively). If one believedSoddy, then the disconcerting problem of stuff-ing too many elements into the Periodic Table

Figur 9. Otto H ahn Building ( Thielallee 63 - N52 26.85 E13 17.11), of th prsnt Inn Lniursitat ofDahlem, Germany, previously the Kaiser Wilhelm Institute, where Otto Hahn and Lise Meitner discoveredprotactinium-231." As has been done with so many buildings in Berlin, this edifice was extensively dam-aged in World War II but was reconstructed to closely replicate the original architecture. The building isnow used for biochemical studies. Much important history has occurred here, including the discovery ofnuclear fission (announced by the plaque attached to the turret, seen here to the right). On the second 6tage(third floor) is the Lise Meitner Horsaal (lecture hall).

was resolved: different radioelements could besimply lumped into preexisting boxes. The termSoddy coined-isotope-was fully intended toconvey the meaning that two species resided atthe identical location in the Periodic Table, evenif they had different atomic masses and half-lives. Soddy was proposing an astonishing con-cept: A specific element, which historically wasonce considered not fully characterized withouta specific atomic mass, now could be variable inits mass!

Corroboration of the predicted differentmasses for isotopes. It remained to prove thatchemically identical elements did in fact havedifferent masses. According to both Fajans andSoddy, lead (Pb) from different decaysequences should have different atomic mass-es. Fajans took the initiative and sent his stu-dent Max Ernest Lembert (1891-1925) to thelaboratory of Theodore William Richards(1868-1928) at Harvard University, the recog-nized expert in atomic mass determinations.Richards and Lembert repeated the atomicmass determination for ordinary lead andreproduced the published value of 207.15. Leadfrom uranium samples from around the worldgave a value ranging from 206.40-206.82," avalue soon confirmed in three more laborato-ries. There was no doubt about it-lead fromradioactive sources was lighter than ordinarylead, and there proved to be no spectroscopicdifferences. Later, the method of Moseley"'

"smoothed over the rough edges of isotopes"'when his method of X-ray spectroscopy wasexpanded by Karl Manne Siegbahn (1886-1978,Nobel Laureate in physics in 1924) to includethe atomic number of uranium. Siegbahnshowed that even though uranium was com-posed of several isotopes (Note 3), neverthelessthe element appeared to exhibit a clearlydefined atomic number." The method of massspectrometry, developed by Francis WilliamAston (1877-1945, Nobel Laureate in chemistryin 1922), soon allowed the direct observation ofisotopes and he wrote the definitive article onthe isotopes of uranium, lead, and otherradioactive elements." Even Fajans, who hadlong viewed each radioelement as a "new ele-ment," by 1923 had abandoned his concept ofthe scattered pleiads and had accepted the viewthat isotopes should be tightly boxed in indi-vidual cubicles of the Periodic Table."

Epilogue-the construction of the nucleus.What was the genesis of the different isotopesof an element? The prevalent view during thefirst quarter of the twentieth century was thatthe nucleus was composed of protons and alesser number of electrons that partially neu-tralized the positive charge, but it was notknown where these electrons resided. Fajansthought all electronic processes (either chemi-cal redox or nuclear decay) must occur in thesame region of the atom, since the particles

(continued on page 74)

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(electrons) were identical. However, a logicalargument of Soddy in 1913 underscored theclear difference between orbiting electrons andnuclear electrons.' Soddy compared the oxida-tion of tetravalent uranous to give hexavalenturanyl (U' + U") with the radioactivedecay of UX1, which loses two electrons to formU2 (Th-234 -- U-234). If electron loss (oxida-tion) and beta loss (radioactive decay) wereidentical processes, then UX1 and U2 should bechemically identical-but instead they couldbe cleanly separated. Clearly there was some-thing special about "nuclear electrons."

The answer to the riddle of the nuclear elec-trons was solved by James Chadwick(1891-1974, Nobel Laureate in physics in 1935),who discovered" the neutron in 1932. This neu-tral particle was not simply a combination of aproton and an electron, but an entirely newparticle that could eject an electron (to becomea proton) in special cases defined by a low pro-ton/neutron ratio. With Bohr's description ofelectronic structure," a complete pictureemerged of the atom and its behavior in theradioelement decay sequences. With thisunderstanding, the story of radiochemistry withits "suicidal success"' was completed, pavingthe way for the next chapter- "NuclearChemistry" and the birth of the artificial ele-ments. This shall be the topic of a future articlein The HEXAGON "Rediscovery" series. 0

Notes.Note 1. "Brevium" was Pa-234m.

"Protactinium" (Pa-231) was discovered andisolated in weighable quantities four years later,independently by Hahn and Meitner2

1 in Berlin(Figure 9) and Soddy and Cranston" inAberdeen (Figure 10), and named because itwas the precursor to actinium (Pa-231-* Ac-227). Fajans also predicted a pleiad just to theright of the emanation pleiad (radon)''-veri-fied when francium with a half-life of 21.8minutes was discovered in 1939 by MargueriteCatherine Perey (1909-1975).'' Fajans had acelebrated career but never received the NobelPrize, perhaps in part owing to political ene-mies he made in the science world by his brashbehavior.'

Note 2. Ironically, it was later found that iso-topes can have different chemical and physicalproperties, particularly where a small atomicmass can be a significant difference in the zero-point energy; deuterium was discovered byUrey in 1931 by distillation of liquid hydrogen'and pure heavy water was prepared in 1933 byG. N. Lewis using electrolysis.2'

Note 3. The possibility of more than one iso-tope of uranium giving rise to additional decaysequences was first proposed by Auguste

Piccard (1882-1962) of high-altitude (balloonflights) and deep sea (bathyscape) fame. In1917, he proposed" an element actinouranium,incorrectly suggesting a mass of 239; the correctmass of 235 was established by the mass spec-tral work of Aston," and Rutherford immedi-ately realized'' that protactinium (discovered byHahn and Soddy independently) would have amass of 231 (promptly verified in Hahn's labo-ratory). Piccard's concept of a naturally occur-ring isotope of uranium giving rise to the actini-um series was acknowledged by Soddy in hisNobel address.;

References.1. Personal communication, Professor Alan

Cooper, Department of Chemistry,University of Glasgow; who furnished amanuscript of a B.B.C. radio program,"Science Survey"where Soddy wasinterviewed, broadcast 10:30 p.m. 14 June,1951 (also described in less detail in ref 8).

2. F. Soddy, Nature, 1913, 92, 399-400.

3. M. Howorth, The Life Story of FrederickSoddy, 1958, New World Publications,London.

4. J. L. Marshall and V. R. Marshall,The HEXAGON of Alpha Chi Sigma, (a)2006, 97(4), 50-55; (b) 2010, 101(1), 6-11;(c) 2010, 101(2), 22-26; (d) 2010, 101(3),42-47.

5. F. Soddy,"The origins of the conception ofisotopes," Nobel lecture, Dec. 12, 1922.

6. W. Crookes, Proc. Roy. Soc. (Ser. A), 1900,

66, 409.

7. E. Rutherford, Radioactive Transformations,1906, University Press, Cambridge.

8. F. Soddy and J. A. Cranston, Isotopy.Lectures by Soddy and Cranston, 1954,[lectures delivered 1953-19541, New WorldPublications, Westminster.

9. L. Badash, Brit. J. His. Sci., 1979, 12,245-256.

10. F. Soddy, The chemistry of the radio-elements,(a) Part 1, 1911, and (b) Part II, 1914,Longsmans, Green & Co., NewYork.

Figure 10. Marischal Collegein Aberdeen, Scotland, where

-p- if Soddy independentlydiscovered "eka-tantalum"(protactinium-231) in 1917(Broad Street, N57 08.93W020 05.81). Although somepreliminary work oneka-tantalum had been donein Glasgow, his finishedobjective was not completeduntil he moved here in 1917.

11. J. L. Marshall and V. R. Marshall,Rediscovery of the Elements, [DVD], 2010,ISBN 978-0-615-30793-0,"HistoricalSketch of Discoveries," 101-113.

12. K. Fajans, Phys. Zeitschrift, 1913, 14, (a)131-136; (b)136-142.

13. F. Soddy, Chem News, 1913, 107, 97-99.

14. A. Fleck, J. Chem. Soc., 1913, 103, 381-399.

15. K. Fajans and O. H. G6hring, Phys.Zeitschrift, 1913, 14, 877-84.

16. K. Fajans, Ber d'utsch. chem. Gcs., 1913, 46,3486-3497.

17. T. W. Richards and M. Lembert, J. Amer.Chem. Soc., 1914, 36, 1329-1344.

18. K. Fajans, Radioaktivitat und die neuesteEntwicklung der Lehre von den chemischenElementen, 1920, Friedr.Vieweg & Sohn,Braunschweig.

19. F. W. Aston, Nature, 1929, 123.

20. F. Fajans, Radioactivity and the latestdevelopments in the study of the chemicalelements, 1923, Metheun & Co., London, xii.

21. F. Soddy, Nature, 1913, 92, 399-400.22. J. Chadwick, James, Nature, 1932, 129,

312; Proc.Roy. Soc., 1932, 136 (Ser A),692-708.

23. J. L. Heilbron, Historical Studies in theTheory of Atomic Structure, 1981, "TheGenesis of the Bohr Atom,"Arno Press,147-228.

24. 0. Hahn and L. Meitner, Phys. Zeitschrift,1918, 19, 208-218.

25. F. Soddy and J. A. Cranston, Proc. Roy. Soc.(London), 1918, 94, 384-404.

26. M. Perey, Comptes rendu., 1939, 208, 97-99.

27. L. S. Bartell, Bull. Hist. Chem., 2010, 35(1),62-63.

28. H. C. Urey, Ferdinand G. Brickwedde, andG. M. Murphy, Phys. Rev., 1932, 39,164-165.

29. G. N. Lewis and R. T. MacDonald, R. T.,J. Chem. Phys., 1933, 1, 341-344.

30. A. Piccard, Arch. sci. phys. nat., 1917, 44,161-164.

31. E. Rutherford, Nature, 1929, 123, 313-314.

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