Electronic Structure of the Heaviest Elements

UCRE /a2 6 p// c.2 1

NIVERSITY OF CALIFORNIA

AIJC 2 1 1987

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ELECTRONIC STRUCTURE OF TIE HEAVIEST ELSNm

By G, T. Seaborg

July 10, 1948

~e&le~,California UCRL 102 Chemistry-Transuranic Elements

Standard Distribution --No. of Copies Argonne National Laboratory Atomic Energy Commission, Washington Brookhaven National Laboratory General Eleatrio Company Hanford Directed Operations Iowa State College Kellex Corporation Los Alamos Naval Radiological Defense Laboratory NEPA New York Directed Operations Oak Ridge National Lotboratory Patent Advisor, Washington Technical Information Division, ORDO University of California, Information Division Chemistry Department kitrersity of Rochester Office of.Chicago Directed Operations UCRL-I02 July 19, 1948

ELECT??@PICSTRUCTURE OF TITE HEAVIEST ELENElTTS FY O. T. Seaborg

Tcblc of Contents

I. Kistorical Rackground ...... r . . . . . 5 A'. Rcforo the Dfscoverg of thc Transuranium 7lcmcnts . .. 5 Re kftcr the Discovcrg of thc Transurnnium r';lcmcnts. . . 8 11. ~ctinLdcConcrpt ...... 9 A,. Goncral...... 9 R. Chemical vidcncc ...... 10 C'. Absorption Spcctra in Aqucous Solution and Crystals . 18 n-b Cr$stallogrc~l~icTE-t:! ...... r . . 20 E; Mc%nctic Susccptibf lity Datz ...... 22 Pa Spootroocopic Data ...... 24 111. Correlations and Deductions ...... 25 A. Electronic Configurations ...... 25 3. Possible Deductions without Data on Transuranium 7i;llerncnts ...... 28 C. Position in Fcrioclic Tnblc 2nd Nomcnclaturc . . . . . 29 Dm Prc?dictcd! Properties of Transcurium xlcmcnt s . . . . 31 Refcrcnccs ...... 33 Page 4 ELECTRONIC STRUC TITBE Or! TTTE HEAYLEST EX:Z.?TFTS By Glenn T. Seaborg

Abstract-- All of the available evidence leads to the view that the 5f electron shell i.s bein: filled in the heaviest elements givin,? rise to a transition series which begins with actinium in the same sense that the rare earth or 1' lanthanide tt series begins a-ith

lanthanum. Such ar? 1' actini6e 11 series is suggested on the basis of evidence in the following lines: (1) chemical pro?erties,

(2) absorption spectra in aqueous solution and crystals, (3) cry- ~tallo~zraphicstructure data, (4) rnaenetf c sus ceptibilitg data and (5) spectroscopic data, The salient point is that the characteristic &idation state (i.e ., the oxidation state exhibited by the member cont;ainin& seven 5f and presumably also by the mom- ber coztaining fourteen 5f electrons, curium and element 103) is the 111 state, and the group is placed in the priodic table on this basis, The data also make it possible to ~ivea suggested table of electronic configurations of the ground state of the gaseous atom for each of the elements from actinium to curium inclus ive . UC7L-102 July 19, 1945 Page 5

ELECTROFIC STRTC TUPE 0" TV4: FZAVIEST ELEPIEKC S By Glenn T. Seaborg

I. Historical Eackground P,. Before the Discovery of the Transuranium xlements. The intensive study of the heavy elements during the last few years has given information and data which now enable us to make some definite statements as to their electronic structure. The fnfor- mation obtained about the recently discovered synthetic transuranium elements has been particularly useful in this connection, and it is largely on the basis of these new elements that this question is now well understood. The neaviest natural elements, thorium, protactinium and

uranium, of atomic numbers 90, 91 and 92, respectively, lie in

corresponding positions just below the 6th period 11 trans it ion ft elements, hafnium, tantalum and tunzsten, in which the 5d elec-

tron shell i s being filled. The elements, hafnium, tantalum and tungsten are similar in their chemical properties to the corresponding transition elements in the 5th period., zirconium, columbium, and molybdenum, in which the 4d shell is beins filled. It has long been known that the chemical properties of thorium, protactinium and uranium resemble those of these 4d and 5d elements and for this reason most of the textbooks and standard works on chemistry and physics in which the electron stru-cture is discussed

have accepted the view that it is the 6d shell which is being filled. Thus the structure of the elements above radon (element P 86) through uranium is written to show the addition of the next

two electrons in the 7s shell tor element 87 (francium) an6 element UC9L-102 Page 6 # 88 (radium) and addition in the 6d shell for tho following four rr*l . elements, actinium, thorium, protactinium and uranium (1). Many of the early papers which appeared after Y. Rohrts clas- sical work(') on the qantj-zer? n~clearatm: discuss the electronic structure of the heaviest elements. Therc has bcen general agree- ment that some type of transition group should begin in the neighborliood of the se e lernent s, although there has been difference of opinion as to %here it begins and as to which electron shells

are involvsc7. A nuinbor of the earliest publications even have aug.;ested that this transition series involves the filling of the 5f shell, thus possibly giving rise to a rare earth" group in

a manner analogous to the filling of the 4f shell. This filling

of thc: 4f shell rcsults in the well known grou~of 14 rare earth elements of atomic numbers 53-71 inclusive, following lanthanum.

It is of interest here to note 2 few of thcse early and also latep suggestions in order to rovicw thc general. previous status of this question. Most of these emly investigators were of tho opinion that the filling of the 5f shell should begin at some point beyond uranium, that is, beyond the then known e lem~nts. In an early paper Bohr (3) suigested that the addition of the

5f electrons might begin in this region, and in a Rolir-Thomsen type of periodic table he pictured the first entry at the element

with atomic number 94, Y. Suguira and He C. ~ro~(~)gave calcula-

tions Indicating that the first entry of the electron into th~

5f shell should occur at element 95, J. C, McL~nnan, A. R.

McLag ad Y. F, Smith ()suggested as an alternative to the fill-

ing of the 6d shell the possibility that thr 5f shell begins to

be occupied in thorium. In a review article, S. Dushman (6) stated it is doubtful that the added electrons enter the 6d le vel (thus k* implying an analosg with. cerium, etc.). V. Karapetoff and 0 Ta-You-Wu and S. ~ouc?smit(') suggested that thc element with

atomic number 93 might be the first in which the 5f shell besins

to be filled, while A. von C-rossc(9) suggested, zs n possible a1-

ternative to filling of the 6d shell, the entry of the first electron in the 5f shell with uranium. More recently L. L. Qurll (10), largely for the purpose of' illustration, presented periodic table

arranccrncnts in vvhich thc first 5f electron appears in element

number 95 in one case and in elenent nunbcr 99 in another. Tb

later calculations of PI. C-oeppert yayPr(I1) ind-icate that the

filling of the 5f shell might begin at protacthiurn or uranium. ,wid TZ. Rudy ( l2a) J. Prrr3.n(l2)/ on zemrsl conside~ationa, proposed as a possibility the theory that the first 5f electror, a;.;poars in thorium (13) and Ci. 7. Trillar suggested that some ot tho chemical evidence

supports this viewpoint.

On the basis of his crysta1logrs;~hicxork, V. M. Goldschfdt (14) favors the view that the first 5f electron entoys at protactinium,

thc first element beyond thorium, although he points out thc possibility that this mag occur either earlier, in thorium, or later, in uranium or in. the (at the time unlmowr?) transuranium clments. Ry analogy with the namc 11 IantharildeI' serLes which he had alrcadg

proposed for the rare earth elements because these 14 clemnts followj-ng lanthanum have lanthanum as their prot otypc, he pro-

posed the name " thoridc 11 scrios for the ld elornents folloxing kS thorium. On tho basts of his much morc complete crystallo~r~phic evidence, -including especially observations on thc t ransuranium F. After the I7iscovorg of the Transuranj.um---- -1ement- s. The ' C"* recent discovery of the 'translnanium elcmcnt s and thc study of 0 their properties, e speci~llgthe chcmical proprt fcs, have given us a tremendous amount of additional ovidcncc of just thc type

needed to clarify this problem. AS it turns out, it is Ln the transuranium elements that thc really definitive chcmical properties, from thc standpoint of olacing the heaviest cl-emnts in the >criodic table, first appear. Thc f trst bcst definite cvidcnce

that the 5f shell undergoes filling in this hcavg region came from the trmcr chemical observations of 3. M. McMillan and P. He

Abelson (I6)on element 93 (nontnnium); upon their discovery of this,

thc first transuranium *-lemcnt, thcg nerc able to show definitely that it rrsembles uranium in its chemical properties and. hems no resemblance to rhenium, thc element immcclintclg above it in

the periodic tablc. This exccllcnt; cxpci.incntn1 cviri enco wns

interpreted by them to idicatc that thLs ncw 1' rare earth" group of similar elemcnts starts with uranium. K. ~tnrke(~~)and

C. C.. E3edrcag(18) also intorprctcd tho traccr ezporimcnts with

elencnt 93 as indicati~?~that the first 5f clcctron comes at

element 93, while F. Strassmann and 0. Hahn (19)felt on tho basis of thcir tracer cxpwimcnts with this elcmcnt, that 5t was diffi-

cult to make any deduction. As a result oi" thcir first tracer cxporiments with elcncnt 94 to ,cthcr with their consideration of

the traccr investi:

A. S .1-'nh1(20) in 1942 m& the sug~estionthat this transition group might begin as early as with thorium or ttstiniwn. 4- Since this first traccr work on th~transuranium clcmcnts,

thc olcment s neptunium (atomic numbcr 93 ) md 3lut onium (ntomic VC9L-102 Page 9

number 94) now have bccn extensively invest igntcd with substantial, bA wcighablc quantities, Americium (atomic nu~bcr95) and curium

(atomic number 96) nlso have becn avnil.-.blc for investigation, on

a more limited sccle, in wcS.zhzble quantities. The recent cxten- sivc investl.gations ol thorium, protactinium and uranium also

havc co~tributedto tho cvidencc which is now uscful to intcrprct this question.

A. ---Gcnr:ral, The cvidcnce now available leads to thc definite view that it is thc 5F electron shell which is being filled in these

heavicst clemcnts. Thc: ovidoncc scems suff icient to go further (21) than this and to suggest that this rare-earth-like sc~fcs -I begins with actinium in the samr scnsc that thc rare carth or .. f11anthnnide8f scries begin? with lanthanum. On this basis it

might be tcrmcd thc tf actinidelf sc:iiies ('I) and tho first Sf

electron might zppc ar, although not necessarily, in thorium. The salient point is tlnct thc characteristic oxidation state (ic. the oxidation stctc cxhibitcd by thc mcmbcr containing

seven 5f' and presumably also by the rncmnbcr covtainin; f ourtccn

5f clcctrons, curium and clomcnt 103) is thc ITI s tatc.

There is much cvidcncc, 211 ~ointi~itovrard this vicw. :?re

will d5.scuss this cvidcnce which Is in thr followfnz lincs:

(1) chcmic~lproprrtics, (2)absorption spcctra in aqueous solution an6 crystals, (3) crystel.lo~yaphicstructure data, (4) magnctic susceptibility data 2nd (5) s~ ctroscopic data. It should be emph~.sizcdthat thp cl.iscu~sionso far has been

carricd on in 2 so~c1,vh~.tovcrsfm~lificfi. mnnr.cr, bccausc the dctails

concerning thc possible pliysical or shtniczl. forms in which thesc elements might exist have a bearing on thc electronic structure, F". as is the case for all thc othcr transition groups fncluding the

rnrc earth clemonts. Thus thc numbcr of 5f clcctrons in the atom

In thc gaseous state dght c7f ffcr from l;lmt, in the mztnllic statc

(which ir turn cnn differ from onr ;3h~:sc to another), ~ndin turn

nctthrr one of those ntructurrs might correspond ? ircctly to thc

chemical propcrtics. It is thc chcmica'l pro~c~tfes( includirg absor?tion spcctrn, crgstallo&rn~hicdata, ctc. on thc compou-ds) which shoulfl be dcterminstivr in placin; thcsc elemnnts in thc

periodic table, just as wrts thr cnsc f0.r tii~rarc: cartl~clcmcnt s which would be plxed c?iffc--ontlg considering their elc ctro~ic str~ct~~~c*s alone . P,. Chemical Evidence. On thc basis or" m actinide scrics the chclracteristic oxiclotion n:mbcr for thc scrics is IIT, ar-d

datfon states with increasing ctomic number. In going from uranium plutonium it becone s incrcasin~l~;C-ifficult to cffect

thc oxir'etion from thc ITT to thc ~TTstnStc and, in fnct, with amcri-

cium it is imposr:ible in nyucous solutfon to cf'fcct an oxidation to thc VI statc. Similnrlg, it bccorncs ir,crczsin,l;l.y difficult to effect oxidation fron thc IT1 to the ITTstate in goirz Srom urenium

to plutonium, and arith americium thc cvidcncc fndfcatcs that it probably is not possiblo to cffect thfs oxidation acid solut Lon at all. If this oxid ation shorld be proved possiblc thc indl.c a-

tfons nro that the potential is so grcnt th?.t thc highor oxicin-

tion statc of arnrricfurn is rcduc ecl rapidly by cater and cmnot rb4 be maintained in aqueous solution for any groat lcngth of t Lm. Thcse consid.erations crc illustrated by thc ra in which tklc standard 0x1 dat ion-reduc tion potentials, rcfcrred to

@ tho hidroeen-hydrogen ion couple as zero (see, for cxcm~lc,V. M.

Somc Oxfr1ntion Potentials of the Actinides

limitccl to thc trccer scale and therefore it has been impossible to xakc corresponding qu~iltitat ivc r'educt ions . This work, how - ever, hcs lcci to the dcfinitr qualitntivo conclusion that it is fmpossiblo in aqucous-acid solution to oxiflizc curium to thc VI statc: r.nd that it is also lmpossiblc to oxidizc it from the I11

to the 117 statc. In fact, thc: cxpcrimcnts of S. G. Thompson,

L. 0. Tiorgan, 7. A. Jamcs and I. Pcslinan (24 ), in which trnccr

amounts of curium ~mlamericium wers subjected to strong oxidation under alkulfnc fusion, indicate that it is more difficult

to oxicqizc curiwn from thc IJT to an uppqr stntc than is thc case

for amcricium, an? it mzg be impossible: to cffcct this oxfd at ion at 211. These cxpcrimcnts indicate that lmcricium mag bc oxfdizcd I" in alknlinc mcdin, and can in this monncr bc scpnrated from

curium. In fact, L. B. Wcpncr 2nd I. Pcrlman (25' wcrr nblc to e oxidizc arncrician(111) in 405 potassivm carbon2.t e solution by tho use of the strong oxidizinz aycnt hv~ochloritcto an insolilble compound, probably of amcricium(tr), and hence effect an cfficicnt scparat ion from curium(111) which a?parcntl:q is not oxidized under the se severe condf tions. Thc microchcmical cxporimcnts of Werncr and ~erlrnan(~~)with macroscopic conccntrntions of curium nlso point towni?d thc cxistcnce of curium solely in

the 111 oxi6ation s tatc. This tcndcncg t ovrard increasing stabiliza'tion of thc lower

oxidation statos, cspccially thc TI1 st~tc,with increasing atomic number nlso manifcsts itself not2bly in thc stability of the solid compounds of the various oxidstion states of these clcments. The best illustration criscs from a consideration of the solid non- oxygcnated halides of these c1.cments. Thc first possibility of

the production of' a trifluoridc oppcars with uranium trifluoride whfch can bc prepared only under drastic rcducfn~conditions and

thc: stability and case of rcduc tiorr to the trifluoridc 'increases in going to neptunium 2nd thcn to plutonium. Mopcovcr, while ft is rolr,tlvelg casg to Yg?oduce uranium hcxafluoridc, it is more difficult to produce neptunium hcxfluoridc and. vcry difficult or impossible to produce and to kecp the plutonium hcxaf luoride. In the ccse of americium it has not beer* ~ossiblrto produce any

highcr fluoride than thc t rif luor id.€. %th rcspoct to thc other halides, it has, in fzct, been impossible to prepare any plut onim or amrf cium chloride , bromide or iodide of oxidation statc higher than 111, anti. it tsbeen 6.a posslblr: to prepare only thc ch1orid.c and brornidc of neptunium of oxidation state IV (in addition to thc chlor5.de, bromide and UC RL -1 02 PL?GC 13 e iodide of oxid.~.tionstate ITI). In thc cosc of uranium it hcs . bccn known for some timc th.t there arc ch3.oric7.e~ of oxidatton stato higkrcr than IV and c. chloric!c, hromidc 2nd iodittc: of oxic

dation str?,te IV. Thcsc: consider~?tionsmc well illustrnted in the tollowing table (~zblc11) which lists zll of thc halides of urmi~lrn, neptunium, plut onium, 2nd ,mcr icium which have bccn prc - pzrod 2nd maintc5ncd 2s stable in the solid state. So fcr thc chemical cvtdcncc indicates thct it will bc difffcult, probchlg

impossiblc, to propQrc clnp of thc non-oxygenated halfdcs of mzri- ciwn (2nd .:uriurn) of oxic1ation statc higher than 111. In fact,

S. Fricd nnd A. T. lori in'^^) h?vc trcatcd ArnF with fluorine 3 at clcvctcd ternpcraturcs xi0 havc obtained no evidence for thc formation of a higher fluoride.

;"his chemfcel evidence fndtcatcs thzt tl~5f clcctrons >re

less tightly bound thm thc 4f clcctrons tmd. thcrcforc can be

morc casi lg removcd by oxidation, 2s shoulc! bc cxpcctcd on thc

basis of tha prcdictcd lower binding cnrrgy of 5f 2s comp~~cdto 4f clcctrons. Tlius thc 1x1 stztc of thorium c2nnot o~ist5.n

aqucous solution, and until thc prcscnt the IV 2nd 111 statcs of

protxtinium havc not bccn obscrvcd in nqucous solut ion, although it soems rczsonable that with further study one or both of thcsc: stztcs will bc obscrvcd. In thc case of solid compounds it sccms not impossible th2.t tripositive thorium compounds with one or more of thc hczvier hzlogcnr; will be pcparcd unCcr rathcr scverc reducing conditions, 2nd that tetra- and tri2ositfvc compounds

bL of protactinium will bc prepared as soon Z?.S efforts in this dircc-

~?lrcadghave somc crystallo~rfiphicevfc'cncc for 2. dioxide of UCRb-102 Page 14 P?gc 15 protnctiniclm with thc f 1~034t c struc turc althou~hthis mtght 4- 4- bc a case of solid solut?o-r? of' two ~::ic~~tionstates similm to L thc solutions studPcd by J. K, Mrrsh (30, ~5)in thc Fro2 -Pd2C3,

TbO2 -7~d~0~and hZ03-DrCZ sgstoms 2nd siinil?r to tlic well known

solid solutions in thc irou: oxide systems. ) Thc charzctcrf stic

IV oxiclat mn stntc dcmonstratcd by thorium is thcn 371-locous to

the TIT oxiclation st?tc of ccrfwn and thc V statc of potactinium

is analogous to thc somcwhr,t unccrta~nV statc of praseodymiu~(50,311 From the bchr vioy o f -urmium, mptunium 2nd plutonium, i t mus t bc dcduccd that 2.s mny as throe of thc 5f clcctrons zrc givcn up

fcirlg readily. In thf s connection it is ir?tcrcstin(;: to note that in thc case of the lanthanide clcmcnts thxx are in gcncral only two clcctrons (beyond thc xrnon structure) outsfde of thc 4f

shell (see Tablc T71 bclow), nlthough thc ocrsistcnt oxid?.tion statc is cortalnly tho IrL state. Americium should posscss ?n oxidation st?tc of II vdiich

it would attc,in through the prc=;cnce of scvcn clcctrons in thc Sf

shcll in 5 mnnncr quitc an:?lo~o~nsto the I1 str?te of europium, the clement immcdia tcly prlcccdi.in, g~.dolinium, with its scvcn 4f

clcctrons. Bccnusc of thc grc-.tcr ccsc in thc 7-cmovnl of thc 5f

electrons it shoulcJ rcquirc 2. consic!cr2bly strongc~?rcducin~ -.gent

to rcduce Ln~rlciumfrom thc ItT to thc I1 state thzn is the case

for europium. It is not impossible, howcvcr, but in fact probable,

that it will be found th~.tc.mcricium cm be rcducsd maintained

in the I1 s tatc in nyucous solution. Thompson and co-workers (24)

have made partial scpnrct ions of amcriciixm (presumably as A~(II) ) F4 from curium in trcccr amounts in aqucous solutlon by usix soc1fum amalcam as tho rcd.ucing agcnt and ci?rrgi;ag nmcriciwn sclcctiv~lg UTPL-102 P2:c 16

v~ith s?.mzriurn(t~)sulf at c, ?.nd in ~ddit ion by usins b2.rium 7.s 6. reducing ?.~;cntand cmrging comcricium sr lectfvclg with barium chloride from conccntr~~tedhyd.rochloric acid solution. It is

fntcrcstfng to note as well th[?.t Am0 h2s bccn ~eparcd(32) by

trcn ting amcricim oxic7c with hyr'ro:;en at 900'~ (although some irnpui-it,~~mF.7 hzvc been rcsponsiblc for thn reduction here). Curium ln!ith its seven 5f electrons should cxhibit the ITI stztc cxclusivcly and all of the cvidencc, obtxincd both from trccer 2nd macroscopic qumti.t3.es, indicates th2.t this is tlx casc.

Tlx followin; tablc (TC-blc111) summarizes thc known oxidz-

tion st-tcs of th? l?.nthmidc mi! cctinldc clcrncnts in such cl

vcy as to bring out thc nn~loggbctwccn thc two groups 2nd to shanr thc grictcr ease of oxic?~.tionfor tho mambcrs of the lzttcr group.

Thc uncertain states wc dcsl&mtcc1wi-th pnrcnthc ocs.

TASL7 I11

oxidction States of T,nr,th.nidc md Actinide Slancnts ... - P-_I ------kt. Pro. 57 55 59 60 61 62 63 64 65

"lcmc nt L3. CC Pr PTd 9n Sm Tu Gd Tb ...--- Oxid. States- 2 2

333333333

At.-- uo. 89 90 91 92 93 91 95 96 Zlemnt Ac Th Pa U Np Pu Am Cm Oxid. Statcs -_I--- 2 3 33333

4 (4) 4 4 4 5 5 5 5 (5)

-. - -. 666..-._IC.I_____C_I__ Thc mctals of the elomc.nts thorium to mer%ciuminclusive have been prcpmcd and thcir pro;xrtics studicd and thesc bear a stri.king rcsernbloncc to thc mctcls of tbc rarc earth elerrcnts.

All of them nrc highly olcctropositivc, and to a2out. the same dc- grcc, which is 2 similarity to the rorc cnrth mctcls nnd 3. difference from thc corrcspondin~5d clcmcnts,, hafnium (clcmcnt 72) to iridium (clcmcnt 77) inclusive, in which the clcctropositivc chmac- tor of h.afniwn is lost as 5d clectrona are cdded in sing tovard iridium. Another rcmnrlcablc roscmblancc to thc rare carth clcmnts

lics fn thc dcnsit ics of thc metals. Both ?.mcricium(33) and- tho cnologous 4f clcmcnt, europium( 34), hlvc donsi tics much lovw r thm thcir nclghboring clcm nts. Thus thc sc rnctcis sccin to have radium- likc or bnrium-likc struc turcs, with abnormz.lly high rzdii and cnnlogous elcctronfc structurcs. This nnzlogy in thc d-cnsitics is shorn in Tcblc IV where thc dcnsitics (of tlic forms of the metals which me stcblc zt room tcmpcraturo) arc li~ted(~~,~~).

Densities of Soom Tcmpcraturc Forms of Somc -- Lanthmfde and Actinide Mctcls Ate NO* 60 61 62 63 64 Tleme nt Nd Pm ~rn Eu Gd Dcnsi ty 7.00 6.93 5.24 7.95

--.-- ' , - - At, NO. 92 93 94 95 96

Ylmcnt U ITp Pu Am Cm Density 19.0 19,7 19.8 p.11

A compnrison with tungsten, rhcni~m,osmium and iri.d.ium brings forth no such annlogg . PJZC 18 C Absorpti on Spcctra innAqucou~ Solution and Crystals. One )14 . of thc charnctcristic propcrtics of tho clcmcnts of the lanthmids sories, a propcrtg which dcpcnds upon thc 4f clcctrons, is thcir

sharp cbsorption bands, to I. l?.r;c cxtcnt in thc visiblc spcctrwn.

Thi s absorption duo trmsitf ons thc clc ctrons and thc shmpncss is n conscqucncc of the shielding of thcsc,

both in tho ground -nd cxcitcd states, by clcctrons in the outcP shells. ~hc:invcstig~~tions of this type with thc clcmcnts urmium, neptunium, plutonium, shown striking

sinlilzrity in this propcrtg to thc rare earths, which is furthcr cvfdcncc that vrc s.rc d~)r.lln~xith 5f clcctrons. The cnnlogg bc-

twecn thc rmc c3rths c,y:cl urnnim(~~~)in this ~ropcrtgvas noticed

by C~oldschmidt(36) rn6 bctwocn thc ram earths ~rdurnnium(111) and (1V) by n.~?hrairn ~.nd Eczcncr (37) mny ycrs ago. This

similarity bctwccn thc xtinidc cnC thc lnnthanidc o lcmnt s f s more thcn quzlitativc in tlmt thc gcncral complcrity of thc 2b-

sorption picture undcrgocs an~.lo;ous sirnplific?tion 2.s wc approach thc middlc of th~two scrics, th?.t is, cs vc ap3roach thc elements gad-olinium i?nd curium, with thcir scvcr! 41 or 5f clcctronsc Un- fo-rtunatcly, up to thc prcscnt the rrost cxtcnsivc -i-ork on thc absorption of urznium and thc transuranium clcmcnts has becn danc

in solution, vhcrc much of thr sharpness is lost, and with instru- ments of not vcry high rcsolvfng powcr. Thc aqueous solution nbsorption spoctra of thc tri~ositivcactinides, t21.ccn from various sources on thc Plutonium Pr~jcct('~)arc sho~?n in nig. 1, whcrc P' thc corrcspording spcctra of ths rzrc carth clcm-rits -.la0 arc shown for purposcs of comp?r lson, Alt:!ough thc i?bsor$ ion curvcs

of Tyro Prandtl and K, Scl~cincr(39) .re avail-hlc for compsrison, UCRL-102 Page 19 this work vms not done unc!er corditions comp-r~.blcto thosc unc?er F. which the work on the actknidcs rrzs dcnc. Thercf'orc, D. C. Stctv'art(4 0)

has mcasured the ebsorption spcctra of t3c. rcYc emth clcmcnts

unccr com r.rzblc conclftions 2nd his rrsults :rc 2.1~0 givcn in

F.1. Thc prclfmincry results of P. TI. Lmtz and G. \Va Fcpkcr (41)

on thc nbsorption spectrw~of promcthium (clzmcnt 61) arc P-lso included. It a7pcrzrs th2.t 2s wc approach" the middlc. of the two scrios (i.c ., ?.s v~c ~9proc.chthc clcmcnt s ~?~dolinium2nd curium) thc ground statcs invclvi-% thc scven f electrons lfc vcry dc-

finitely bclovr thc ncxt Ii'.@cr stztcs, lccclin' to -1-1crgy diff~mnccs

of a sufficicnt rnco.snftv.dc to c~.uscthe majn absorption to fzll out- side of thc viaiblc in thc ultr?.violct region. The bcst method for thc compwison of the cbsorption spcctra

of the two groups of clcmcnts is to cornporc thc spcctra obtained ~~5thcrystals, where thc nbsorption linrs nrc lrrrov~nto bc very

shzrp for tho rarc earth clcrnents on the b2.si.s of r. 3.n--gc nimbcr of maasurcmcnts rrith mng of thcsc clcmcnts. Such m~asu~crncnts arc undcrnzy for a numbsr of transur2niu.m clemcnts and thc results (42) so f21- indicctc striking ~nzlogics. S. Frccd 2nd F. Lcitz

havc rncasurcd the cbsoi~ptionspectrum of solid mcrlcium tri- ch1orid.c and 'chcg find sharp lincs, of thc ordcr of onc to ffvc

Angstrom:: widcp which i s ?. width comparable with thc sharpcst rarc

cnrth spcctra. In fact thc shnrpncss in thc ~~ctra(~~)of

mcriciuq chloPidc and nmcricfum bromide is so cxtrcm, at room tcmpcraturc 2nd at 77°~., thzt only thc tripositivc auropium ion

is compn.rablcf sfnco thc absorption spcctrum of thz latter ori- f4 ginates from c ground st~~tcinvolvm~ six 41 clcctrons, it sccms vcry likely tm.t tho basic st~.t;cof trizositivc ~mcricfumcontains UCYL-102 Pngc 20 six 5f clcctrons. They (42) also mc?surcd thc absorption spectra P 0 at room ternperaturo 2nd at 77 I< of urmium tctrrichloridc, neptunium tctrzchloridc 2nd p1utoni.m trichloridc and the shzrpncss of the lines indfcntcs thct thc lccst stcblc clcctrons of thcsc

ions 2xc ic thc inner 5f shell in thc activated as well as in the basic clcctronic s tatos .

grnphic cvidcncc in nddi.i;?^.onpoints to tlic filljmg of the 5f shcl.1 in this ncichborhood of hr~.vyclcmmts. Somc years z~o,cold- sch~nidt(l*~43) had clroni!y noticed the isomorphism of ThoZ 2nd

?rd thc dccrc~scin sizc in going to TT02, 2nd :~?..c! interprctcd

thfs to indicate thc prcscncc of 5f clcctrons in uranium. Tllc

obscrv~~ttonof Zxharinscn(28 ) of thc Isomorphism of tho compounds

thcsc oxidca, has becn intorp??otccl by him to bc cxccllcnt cvid.encc

th2.t thc c1.cctrons arc ~oin~into the 51 shcll. (1n this scrics,

hov~cvcr, thr lzttice constants of the pzo2 (28,29) and. &no2 (45) do not fit pcrfcctlg into the rcgulnr pr.ttcrn for decrcasc in ctomic

radius of the metallic ions; this might bc: due tc mixrd. oxidation

states sirnilzr to thc: situation found for thc highcr prascodymfum 0xid.o by ~3rsh(~').)Eoth of thcsc invcstigctors have advancod the hypothesis that ft is o, tboridc"scrics, i.ce, that the first

51 clcctron ,o..ppcars In the first clcmcnt bryond thorium, namcly

Zccharfascn has used thc x-ray diffrxtion rncthod to detcrmino P the rnolcculw structurcs of z grczt number of compounds of thorium, uranium 2nd thc trmsurznium elcmcnts. All. of thcsr; mcasurcmonts p0in.t toa?..rd thc fill.in;; of tlx. 5f shell slnco -.nc?.lo~ouscompounds FC

allow thc ~.nc?.logouscompounds of su.cccssivc , clcmcnt s to h~.vcic?..c.n- ticc1. nolc culzr s tructurc s . Thus, ?:chn~i.-scn('~) rhds tlx t

I..:.ch othcr, mcl th~smmc is trilc. for thc group IXl3-F'yC1 3 -WC13-i~mC13-

therc, is ? chmjr in structure typo in p~~~~ctl.f~up the srouq,

hut this is to hi. cxp.ctcc! or tho basis of tho contr-ction li~hich

in 2 mnncr quite nn logoun to thc ivcll known 91 lrnthmid.: con-

traction" obscrvcd i:jitl?, t;hc rn.yc czrth clcmcnts, thc conzpound3

of v;lz< ch ?rc in turn isomorphous vith thry c orrr spo~clinp; coa~pounds

of thc nctlnidc clcmx~ts. Ir or?ey to fl~i?tlicrillusti~?tc thcsc

considcr,n.tions thc folloain,, tcblc (T-.l>lc T') givr s thc ioni c rxlii

of n numbcr of thr i"ctinicic -?nd lrntl?~ridc:rlcmcl-ts (47) Lanthnnf do Scrics

yz~ncticsusccptibility

mc:surcment s on compounds of thr hc-.vies t elcmcnt s iclc,?llg should

lcad to thc resultant rn-.::~c tic mcznnts in fumd~r.ic,ntr,lunits md

in this 17~3.7 givc fni'orx.tfon 3s to the qucntum stntes of the: rcsponsiblc clrctrons . Act~~lly,as cvidcnccd by thc rnrc aarth clomcnts, thc situ2tion is rr.thcr complcx :,nd thc exact bchnvior expcctod for the hcavicst olcmnts, on thr basis of th~nrrscncc

of cfthcr 5f or 6d. clactrono, cannot bc, or nt k2st h~.snot bzcn, predictct! . Feverthe lcss, such mcnsarcmcnt s sho~ldyf vc, ctnd indccd hzvz given, in'Cormr".t ion on thf n point.

The cmlicst m?.;nctic susccptibility mt-gsurcrncnts thzt wcrcl

mrdc on compounds of urcniu~("* 499 5C9 51) mP plutonium ( 52 showed th~tthose zrc pmamagnctic, -yet thc results :rc difficult to intcrprct; qunntitatf vcl-g . ,4 simplc qu,n,lit?tiv~ cxplarration of thc maGnctic susccptibilitics of PU(ITT), ~(TTT)2nd licu 4- 4- ~(vI) %;,,- 2 3

which arc successively rcmoved ,?.s o:ir ;ocs to thc higher oxidztion bL stctcs. Thcso mcasurcmc~ts, horrcvcr, do not lcod to tMs intcr-

prctntion as the solc and unzmbi~uousO~C, 2nd zs n rcslrlt must bc rcgmdcd only as being consistent with cnd lcndi% u:clght to

tlif s Tii C-, but not ~ivingproof of it.

T,ctcr C. A. Fiutchison and IT.rlliott made mc.pctic nts cop-

tihi15 tg rnc~~u~cmcnt::ovcr n wide rmgc of tempcrr,tupcs on 2

nurnbcr of' compoun.d.s of thc hc?.vfcst clcmcnts. In thc case or ( .3,54 ) urmiwn(11~) compounds thcy found thzt a nurnbcr bchcvc in

=i in:nPcr simil:.~ to asco?..gniuui(~~~) compounds, indicat ing thct

tho::.: tvo groups of c om -,ounds h,nvc i soclcctronic s tructurcs vrith

respcjct to f clcctrons 2nd thus hnnc two such clcctrons. Thcg disclosed in t=?clition thc.t tlic tcmpcr ?.t;xrc cr?opcndcncc of thc

mcgnctic susccptibilitg 0.f thcsc. ur?niu1n(l'rr) coin.?ounds obeys tbi:

Curi~-~'icissl2.w ovcr 2. rm(e of tcmyrz.turcs, -.nd throu;ll extra-

pohtion with thc usc of thls l2vi thcg Pcducc(! 1 rcsult?rlt IT.$- netic momcnt vcrg closc to thct cx2ccted for two :? clsctrons . They also concludccl that thc crystal ficlds produco mop; pronounced

perturbing cffccts in th5s ccsc th?-n in thr corrcspond5-r:~casc

involvi:~:: 4f c lc ctrons. Thcir mcnauromcnts ( Eui ) on ncptunium(r~), tr~rhfchis isoclcctronf c with urmiurn(11~), zlso fnd:i.cc?teA th~

susccpt ib5 li tj.os of the c ?.t ions of urmium, neptunium, plutonium, and ~mcricfumin most of their st-blo oxidation otatcs. In

order to locount consistcrtl:~fop thc obsprvcd v-lucs of the rnlg- Ir" nctic susceptibility, thc cclqtrnl ?tom must bc cssui~ci!to ?vve clcctronic conf~urntio~s(beyond. the r-.dorl struct~nc)of thc typn ~~(IIz)to 5f5 and .QI~(ITT) to sfE. The cxg~rirncntnlcffcctivc rna~ncticmormnts ?rc in thc cxamplcs of t~o,thrcr, 2nd four olcc-

trozzs lovcr than vrould bc expcctod for thhc lowest clrctronic

st?.tos ?rr~~.ictedby II;undlo rules for L-S cou7ling; bccausc thc

spin-oi-bitcl intcrz.ction is vrry s trow in hclvy atoms, othcr

stotcs vrhich bclo~gto thc loncst gpoup in j -j coup1j.n;; c c?.n lfc

deep rnouy;h to be s5:;nifianntl-J populntcd ?t room tonpcr2turr:. 6 7 For thc confi~urntions5f1, sf5, 5f , 2nd 5f , horrcvcr, no such romplic~tionexists: ci thoy ~torniccouplin~g schcnr lacids to only

om, thc s,cmc, lor; 1-?!'.1.~gst:l.tz-. i". sp~ctroscopicpntz. Spcotroscoaic cvidcncc also lends

support to the actinide intcrpr;trtion. C. C. Xicss, C. J. Humphrcgs and. D . ". Laun (56, 573 invcstigotod thr spoctmm of uranium ?toms and thcg znvc thc intcrprctation that thr, clcctron 2 oonfigur:ition of thc lomst stntc of ncutr.1.l urnnium is 5r36d7s (beyond rodon), n confi.guration which fEts in very ~1ol3.since

uranium is thc thirfl clcmcnt in thc scrics. Othor I-OT!~ by

D, ~chuurmansz.nd co -workers ("9,53) 017 mr spectrum or gcsnnus

TJ(II) cnd TT(J) l-2~given rcsults r:hich r.rr consi~tcntvith this struoture for thr noutr~~lurani1m ntorn. Siyctroscopic ovidcncc

thzt thorium poss~ss~sn ground st::tc analogous to thlt of cerium

also j.nd-j.cati:s that thc; ch~.rcctorlsticconfi,urztic;n for urr.niwn (60,61,62 ) could. involvc thrcc 5f clcctrons. ~th.?robscrrvnt ions F"" m-dc on tho gnscoils ~h(1~)spcctrum frdicatr, th2t th*i 5f 23 acrg closc: to thc Ed clcctron in bintilng rxcrgg ir?. t'!l:Ln ,?.tom.

H. 7?usscll (63) has mzdc a complctc nn~1yoi.s of thc x-rz:? d-to for p radium, thorium and. uranium md- h?s conclud.-d that thc 5i' lics

lovr thi".n thc 6d lc vcl md thct tlic 5f shcll bcgir s to fill 2.t thorium.

v. 2. Toripkin s ?.nd Pi. Prcd ( 6L') h2vc rnr.dc ? qu.15. tct ivc c om-

p?.rison oS thc ccmissian sgrctr,: of thc nctinic'c cnd thc 1-ln-

thy.ni6c -1cmcnts. T~CJTfound such a strorqq -nrlo~':bct.i.rro~ tlu,

cvcrngc fntcnsitg of thr: lircs in thc cc,ar, of n.;lcricium :.nC iuyo-

pil~mthn,t it snc,ms szfc to conclildc thzt thesc 'nrv.2 sTxil?+~ckc- tronic, structuycs in thc (-rounrl s t2t c s . mhi. s then 5 nd5.c-t c s thc confi-up-.t5on 5f7'7s2 iirr the g?scous >ton of mcricium. -- - -- It should be pointed out th-t it woul.2 bc cnt irclg ccnsis- tent from thc point ol" vkov th?.t wc c.rc dpclfnz hcrc witlz r. scrics of ,n.ctinide clcmcnts if it should. cvcntuallg be found tlxt thcpc arc no 5f clcctrons prcscnt In thorium for protzct5.nium). / It is quitc: possiblc on thc b,zsis of prcscnt cvidcncc that protx- tfniwn, or cvon ur?ni.un, might bc thc ffr:;t to h3vz 5f eloctrcns.

It sccms quite lilwly, h~~-~cvcr,thzt clcctrons vrill bc nl-ccd en the 5f skcll o~rlfc,~in thr SC~~OSthm wmium and tkt protx- tinim vill h7.vc 7.t lc-st om. An csscnti?.l point is i;ht cui-ium dcfinf tcly srcms to lx.~rc scvon 5f clcctrons 2nd clmirnt 103 pilo- bably would hn-trc: fourtccn .5f clcctrors.

In the czse of some of tlic clmr-nt:; in the sc~fcsit mag bc difficult to cssfgn clectrcrns to thc 5f oy 6d shclls sincc: the thc .* p cncygy ncccssnry for tho shift from onc shc11 to otlicr my bo withfn thc rmge of chcmic-".l bindin~f;cn:.rgics. Tho clcctron con-

fi,-~~r?-tion m-g df ffcr from compound to corilpound or cvcn ~iththc

pliysic~.lstate of 2 given compoun6. Morcovcr onc ccrt?.inlg cannot

bo supc thnt thc conl"f,~ur~tion of the c.scous ?.tom, for cx-m2lc,

will corr,:spond to th-t of the compounds or of the hgdrntc?~?ions

5.n aolution. I11 the; ccsc of thc hnthznidns, in f'xt, tho confi=;uiic:-

tion of thc gnscous :.tom ;P.S in gonc.rry.lonl-1 two clrctrons (beyond

tkc ;;rnon s'cructurc) out s~.c!r, of thc 4P shcll, ,?lthough thc ::PC- cloxil-1-:nt oxidzt9.o~st::Cc is ccrtainlg thc 111 st,..tc, Sinco thc

2ncrgy dffrcrzncc bctvccn such far outlgin~lc~ls ns the 5f md

6d. shclls is r:;"c;?.cr smzll 2nd sincc pcson,n,ncc rf'fccts should be T

cne7?gy lcvcl lics lo"cst, Thus somc of Ylcsc olcmnts could

possibly vka t might morc Fropc --ly bc:

group . Thc cvidcncc th2.t hn; xcumul?tcd so fr.r sccms ncvcrthclcss

to point to loncr cnc-rgics for thc 5f cmp~~.rcclto thc 6d lcvcls

for t?ir c~m':\ounclsof t'nt. clcx-nt, .:s cclr I$ 2.s uranium in this scrios, Tt Is in thr- c-sc of thc clLcmcnts thorium zrd potzctinium

most uncci-t~in, As 5.n thc: otlicr trnrisitfon scpion, thin rclltivc

cncrgy lcvcl of thc shell vhic h is undcrgoinp, thr filliq; procc ss

bccorncs loY;crC.E the SUCCCSS~VC: clcctrons arc ,?Cdcd, and by thc

t 3'.111c amcrici-~m2nd curium, ?.nd yeeswnr b1.y thc subscc,urnt rncmbcr s F

of lo1.r~~cncrgy than th~Ed shcll. Tt is not possiblc to plccc a . (Ir thc electrons in ncptuniluln ?ild plutonil~mwith con~7idcncc2.nd hcnce 6 2 in Tablc V thc altern~ti-mstructures 5f57s2 2nd 5f 7s , respec-

tivolg, for gcseous neptunium 2nd plutonium wc su~~estedin recognition of the possibility thzt ~lrozdywith thcsc clcmcnts the 6d

shell is not occupicd. F . Possible 9cduc tions without Data 2 TpcnsurnniumumTlcirsnts. P,lthou,l? it is the infom?.tion on the trnnsuranium clcmsnts thiot has bccn dccfsfvc in cnablfn~us to como to thc prcscnt vlcn con- ccrning the elc ctronic atructurc, or more prop~~lyslqak-ng, the

position in thr: pcriodic table of the heavicst clcmcnts, it is intcrcsting t o con jccturr, in rrtrospect, :.bout thc ?oss5-bi lity

of having rpi.:ivrcl nt 2 similnP c onclusion arithout thls inform?tion.

Actually thcr- hns bccr, much inrorrnstion cbout -.ctinim, thorium,

protcctinium and urmium, espcci: 117 :.bout thr lnttcr, ivhich

pointcA in this Circction. As mcntioncd "hove, thcrc is thc

simik.Fity amon2 thc mctals of tbcsc clcmrnts vjith ycspect to clectropositivo chnracter. In cadition, thc mclting point of uranium metal srcms to rclntc Lt more to t!;c im:lcdiT";ly prcceclfng

clcmcnts than to tunzstcn md molybdrnum. Tho r..nalozg of urn.?iwn

to neodymium with respect to lizht ?l-)sorpt5on by tht- triposit ivc

ions c.nd thc ~p~ctroscopiccvidcncc for r. trounrl strctc of th.:

g=.scouo urcnim -.tom j.nvololng thrcc Sf c lcctrons hcc, 2 lr cadg bccn mcnt ioncd.

Urznium df ffcrs consldrrnbly from tungstqn (and riolgbd~cnum)

in tho cYL(m5stry of thc lovrcr oxid3tion stntcs ,.nd, in fact, P urmium(~~?)hzis ,rpcn,t s imilrrity to thc t,riqositivc rcre carth clcmrnts 2nd cctiniuni, ~11~1~urmium(~V) ~c~~rnk~l.cs thorium 2nd.

ccrium(1V) . T~US~rr~nium(111) cnc! urmf'~rn{~~r)^re not acidic in . ch~.~actcr,do not tcnd to form stiqon: complox 5 ons in solution, have fluoric?~s whf ch i?~cinsolublr. (ind isomorphous witli the

cryst2.l ~tructurnswhich 2.r~in gonc;rcl ri.somorpl:~ous aith thc

corrc ::pond ing rio,-cr cc~rthhz1icl.c~. on thc othcz7 h?

coi?~:)lox ions,' for ~:x?~mplc,tungsten( 111) hi?s a strong chloride

complcx ion and. tungstcn(1~)forms stron:; fluoride md. cyq7.rzido

complcx ions. In thf s connection Thompson (63) h~spointrd out -6 th-t tungstcn:~!~)forms thc vcry stzblo complcx j on "r(~*~),-,

ur?.nFum(l~)pos:;csscs no ~i~-~lfic~nt;tcndcncg to form r.n unalogous

complcx c-,;?,niclc ion cl.s noul-d sus~~lgbc ~xp~ctcdif urenium pos-

scss~ilthc snmc outnr clr ctronic stiquctlrrc " s tu.ngstcne

~lthoughnolybclonum die::;.? - an6. tuq;~tcn d.ioxid.c h?vc i so- mor~,1-1ouscrgst:.l ~t~uctures,tunzsten d lojri.de 2nd wmium dioxide

do not, i-%j 10 urqnium 6 foxidc, thorium clf.oxiclc -.nd cerium d ioxido cll do posacss isomorphous structurr:~. It is Fntc-~cstiry:to note

thc.-i. -1though ur,n.nium is not -ssocL?tzd rvi th tun~stcnin minc?rals,

uranitzr; ?.nd thorium minr r,o 1s pr ec ti'callg sl.nngs lmvc tlw r FLrc

earth clcmcnts ISSOC~.?.~~~with them 2nd ti?c r?.rc earth mincrzls practlc-11-g c.lv:.gs co~tcin urr7.nl.um or thorium.

Argurncnts on thc basis of the sczntg cvidcncc from thc chcmi-

cal propcr tic s of thorium :.nlrl l:.rani~mnlonc havc b-zn given by

othcr:;, lnc1ud:l.r.g ~illnr('~)2nd. rnorc rrccntlg D. ?. Stoclm-n (69)3

n 5f typc transit Lor, :: ci7Ei?s in thc hcnvi:: st c lcmcnt s, bsglnning 4- r;2 wit, 11 t ho:& urn. .' p, of prcscntinr; the cctinidc clcrncnks in thc ycri.cdic tjblc is

thr common liatilx of thc fourtecn rmc c-rth clcnicnts of ~toric

thc pFo%oty;x.. It is not proposed th~.tthis pmticul~rfo~m of

tho pcriodic t~.blch2.s r.ny more mrrit than any of F. number of

otlwrs ~hScl?pl~,cc thcsc clcmnnts in positions hornologo~:; to the

~'3i-c~fiiqt!? clernnts, sincc it is obvious th'lt thcg c7.n be x-r.10-

Thc clcm,:nts 90 to 96 Znclusi~~ccolllc?. bc 1.istr:C scprc",l.y b~low

thc 5cl ~lcmcntsin recognition of thc rcscnblnncc of thc fipst

howover sincc thr l9st mnbcrs of this group bcar no such rcscm-

blcncc a~dit is prcbab3.g impossible to d.:?avr P. line as to just whcrc the rcscmblancc ends. As mcotioncd zbovc, thc mportant point is tkc. prczcmc of

scvcn sf clcctrons in s%.tbl.c, triposf tivc curium (c~c~zPI.;~96),

makiw; this clcrncnt very zctlnium-like. A scrics of thoridc It elarcnts, for cx$mplc, ~oultlimply stcblr IV oxid_:tion stcltcs in

elcmcnts 95 21x7 96, nith thc prcscncc of SCVC~5f ~1c~tror-i~md.

ths IV st?.to cxclusfvcly in elcmcnt 97. A snrics of th:~s typc .' + 1' IV oxiPztion stntc tr scrics.

The group probably covlcl hrvc '~.)cpn just as 7;:-11 dcscrihcd by

some otlirr tcrm such cs curidc sr:,riisu , r:c.thcr th-n '1 actii~ilic'~,

which is d-crivcd Pror:! nnalogg 8.f th thc tcrm lm.thznic'.c, Another

A 3% ~cstionlwrc would bo It sgnthc tic cmths in vicw of thc:

syntllctir: nourcc of 2x1 cxccpt thc first thrcc mombcrs. (‘;van mo::,.; t hc s c , thc syntl2.r t i.c proc'u ction of pc4231 zccorclin~;to

233 7$~ ---)?T~~', ~ccountafor thc bcst sourcis of important %so- topr?~of protnctiniwn and urmiwn; the bcst aourco of actinf~zr,l 226 nlso comcs from pilc nc:itrons, by thr rTn.ctions 9a (n,

Irrcspcctivc of thc n:mc which usage rvill finally nssi~nto

t*is Cro?lp of clc~wnts,hovicvcr, it sccms thnt the outstanding

olemint 1.03), togctiicr with thr rc~ullrlyincri:~sin; trilnd ton-rd

cct'rnium-like chnrnctcr in going Crom thorium to curium, zrc bcst

of thc clerncnts h.vc nppnnrcd rccmtly in u~hichthcsc clcrrcnt s,

stnrtiny vith thori~~m2s tk.: homolopc-: or ccrium, 2,rc listed "S

I thc chcnlcrl homo1oy;ucs of t!ic rnro cwth clcmnts, but thc rrnson

in thcsc c?.srs ~31~31-9 to DC iw.ln1y conncctcd v?itli the zgnmctr"j p - f, and the ccsc of mnking, such an crinnli~cmi>nt.) p undiscovcrcd elements beyond. curium (atomic number 96). The

immcdiatclg following scvcn elcmcnt s, th?t is, c lcrncnts 97 to 103

inclusive, should constf tutc thc second h::lf of this rarc cclrth- likc transition group. It ?.ppcars likcly that tho addcd clc ctrons

in proceeding up this scrics will bc placed in a 5f shcl!.. of c!c-

f initcly loasr cncrgy ttxn thc Ect shcll. Tlcmcnt 97 will probably

h?vc a ITr r.s lwll 2s c IJT oxid?.tion statc znd in view of thc

lowcr binding cnergg of the Sf as compnrcd to thc LLf clc ctrons

it should bc smcwhnt casior to oxidizc clcrncnt 97 (ckn-tcrbfwn) to this IV statc thsn is thc ccsc for tcrbium. Corrcspord.ingly, it might bc sorncwb,:.t cnsirr to oxidize clcrncnt 98 (clcn-dgsprosiurn)

to the IV 2nd IT oxidation st:?tcs than is the case for dysprosium, for which, in fact, oxidntion zbovc thc IT1 stcl.tc is practically

impossible. Toward thc: end of tbc sryics, clcm;.nts 102 2nd. 101

should bc cspcblc of bcin; rcduccd to thc IT oxieation stntc,

annlo~ouslyto yt tcrbium and thulium, whi lc c lcmnt 103 should

bc nolo logo us to lutccium with rcspcct to thc complcte stability

Tlcmcnt 104 should cmtinuc wLth thc filliw of thc 6d. shcll

and be 2 true cka-hafnium. After thc fillin:- of the Ed shcll

in thc! following clcmcnts th.crc might ho zd6ition to thn 7p shcll with thc nttainmcnt of the rnrc gcs structurc nt clcmnt 118. 1. aor cxmplc, (a) Lp.t'im-r, '1. TI., ~nclJ. TJ. 3ildchrmd, 'icfcrcncc Fook of Inorganic Chcmfs try, pa 519. M2cTlil13.n C ompmy, Fsv~ York, 1940,' (b) Ta- lor, Y. S., and S.Plasstonc, Trcn.tf sc on Physicc.1 Chemistry, Volumc Ono, pa 29Y. 17. Van 170strznd corn- pcny, Inc ., Pow Yorlr, 1942. (c) randbook of Chcmis try and Physics. Chcrnrical Yubbcr Publishing Com~ang, Clcvclnnd, Ohio. (6)7fchtmrgcr, Fa I<., 2nd ". Ha ~ennnrd; Introcluction to Morlcim Phjs-cs, pa 707. McWan Hill Book Company, Inc., lTcvr Uork, 194.2.

13ohr; N., Fcturc, llg, 29 1923); see also T\'. Bohr and C. Coster, 7. fa ~bysikl2, (1923\ Sugufra, Y ., (?.nd K.. C Urcy, Kzl. Danskc Vidcnskb, Srlsknb, ?%?.tha-fysoM~ddo, & NO, 13, 3 (1926).

11. Kagcr, Ma Gocpxrt, Phgso ?cv. 60, 194 ( 1941); see also 9. Daudpl, Compt. rend. 217, g96 (E13). 12. Pcyrm, J., Grzins cic Maticyc ct dc: Lumierc,, 11-30, Hcrmcn and Cic, P7ris (1935). 12a. Yudg, R., Fiev. genera Sci. pur. appl. 38, 671 (1927) 13. T:iill~.~, 2'., J. of Chm. Yd. -19, 329 (1942); Ann. Acad, Z~-~.35.1Sci . 51 ( 194 0) .

-5. Zizci?2ri~scn, Ha, Report CK-1518, pa 3, plzrch, 1944, 2nd Z Scport SF-1807, Junc, 1944. 16. McMillan, ?. M., 2nd PO V. Abclson, Phys. Rcv. -57, 1135 (1940). 17. Starkc, K., Z. Anor~l;.nllgcm. Chcm. 251, 251 (1943)r . .,

19. Strtlssmann, Pa, and 0. H?.hn, W.turwf ss. 30, 256 (1.9.1-2). Lctirncr, -:I. Ma, Tlic Oxid,?.tion Stztcs of thc Slcmrnt s nnd Thcir Pot cnt 521s in Aqucous Solutions 1' , Pronti-cc-W.ll, Irlc ., I.?( vi York ( 1938 ) . Scc, c .go, for urmium, Su-rvcy Volumc on 3r~rdumCh-mistrg, Volume 1; for neptunium, p'PD TTol.' 14A, Chzptcr YVf for ;,lu- tonfum, DP'I Vol. 14A, Ch2ptcr 111; for mr 9.ci1un pP YTol. 3.4F, ?Tc)~ 19.2.

Verncr, L. B., 2nd I. Prslmqn, rrportccl 5-11 IJnivsrsitg of Cclifornin ri:.~'ktfon Lq.borntorg (~h~mistr7 C-roup) Progrsss ?cport FC-90 (~xust,1947).

Mnrsh, J. K., J. Chcm. c 1346, 15,

33,. 'Tcstrlxn, F., rcportcd in TTriivcrsi t:? of Calf fornic P(?CfI,?- tiolz Lr.l~orntory (~hrmistrg~roup) nroSrrss -?cport PL-4.5.49 (~cptcmbr,1946 ); TTq?i-46 (~?n., 1943 ) .

35. Tor urmium dcnsfty, scc c.;l;., C. ''r, J::cob c.n6. Fa --.Y:?rren, J. Am. Chcm, Soc. 59, 2585 (1937) or nzttc11c P?~morI?,l Golc?r;chrnidt, V. rl ., Fortschrittc dcr Mincralogic, -:ristnllo- zpqhic md Petrographic l.5, 73 (1931).

"phrafm, P., and M. Nczcncr, Hclv. Chime Act3 -316 1257 (1333) 2ncl J. Indian Chcm. 3oc ., Tag-Mcmorinl Vol., p. 243 (1933);

%r ur:.nium, Yovrl-?vd, J. J., 11 Absorption Spcctrz of ~~rc?.ni~i(TI?) and ~Tran-ii.m(I??) in MOl2r PSdrochlori c Acid 1' , M*SwTwSw, VO~W.11~; far ncptuni.m, Mr:gnusson, L., Tw Jw L?- Ch-apcllc mc? J C, . Sindman, CPT-3053, ;Tune 1, 194 5; for pix- tonl.unl, FTin6.nc.n, J. C ., cnd. P. P. Amcn, CP-3053, Junc, 1% 5s for mcric imn, Cunnin~;k?zm,B. 3. , P?? VOI. 143, Yo. 19 .2 (1946); for curium, lJ!orr~cr, I,. ?\., 2nd I. Porlmr.n, Priv~tcCommmic,~=tfon (~stobcr, 1947).

Fyccc1, S., 2nd F. Jw Lcitz, Jr., Rcport CPT,-6, p. 46 (sop- tcmber -TJovcmbcp, 1947 ) ; 2cport at Sgmposfum on Chcmistry of Tp2.nsurznium Tlmcnts, 1948 Spring Mi cting m. Clicm. Soc ., Chi.cago, Illinois . Goldschmidt; V. PI., Frc Fysikkcns Vardlon (-;'rorsk Fgsisk ~ldsslcrift)2, 179 (1941-42).

Tmpl~ton,37 . ii., rcportcd 5.n TJnivor sit7 of Czlif orni?. ?a$ i~.tionLrLbor.t org (~hcmistrgcroup) "rogrcss Snpo~t 17c1L-15 (Fovcmbcr, 1947 ) . Har?.k?sen, H., c?nd Fie E?kkcn, Vnturwiss. 28,- 127 (1940).

r;omilcr, I{., 2,. morg. ~lI.;;crn. Chcm. 247, 24 9 :1941). PC-

Sucksmith, Try., Phil. mg. 14, 11.15 (1932).

C?.lvin, ?I.,Rcport CT:-2411 (~ctohc-21, 1344).

TTutchfson, C. A*, Jr., ~.r,dPo'711.iott, Rulletin of the, 4-mcrf - crn nhysfc?.l 30cicty, Vo1. 22, Fo. 6, 1-bstrnct T)9 (1947).

Putchison, C. ii., Jr., and TT. Tlllotit, Tcportcg nt Sy-ri"pociua on Thcni str - of Trznsurmium Ilcmcnts, 1945 Spring ZIcot ?"_T-ZLm. chcx. Soc ., Chic zgo, 7'1 li~~ois.

Troy-lnnd, Jm J., rnd M. Czlvin, Rcportcd at Symposium on Chemistry of Tr;nsur:mfum Tlemrnt s, 1948 Spr i-ng Mcct in: Chm. Soc ., Ciifcc~o, Illiriof s .

TT1c9~,C C ., C J. Fiumphrcgs ~nd7. D. L?.un, J. Scs. ?Tqtion7.1 Rurc2.u of StnnCI~~rds,37, -IC sr~rch'p- pcr NO. 1729 (1946 ); J. Optfc2.l Soc. Am. 36, 357 (1946). Schuurmans, Pap P~-:sS.CC -ll., A19 (19.16 ) . Schdxrmc?.ns, P., J. C. Van 3cn Bosch 2nd P. Di jkwl, P-iysicr. -13, 117 (1947).

Toxp'.-ins, F. Smr z.nd Yo Frcd, Privntc Coiniiunication arch 3, 1943 ) ,

67. I%g:crs, I?. Fer Rev. Mo?~ Physics I& 996 (194%) .lr--vio~ .+ . P p?.por, with rcfcrenccs to 'individual papers, on thc known spcctra of' the rmc cc0.rtlL1 c1eri;cnt;s.

70. 7jounkovsBy-, G. la, and S. Knvos, J. phys. rcd.ium (4), 2, 53 ( 194-4) . ABSORPTION SPECTRA of the AQUEOUS TRiPOSlTlVE

ELEMENT a ATOMIC ACTINIDES NO I LANTHANIDES I

ANGSTROM UNITS [A*)

FIG. I