M(VI)
M (vri)
ORIGIN OF THE ACTINIDE CONCEPT 21
UO 2 + is the most stable oxidation state of uranium. Neptunium, plutonium and americium form MO 2 + ions in solution with the stability ordering being U > Pu > Np > Am.
M(VII) species, in oxygenated form such as MO 0-, have been reported for neptunium, plutonium and americium but are unstable.
with the results of the individiual experiments being added together to produce a statistically significant result. The experiments must be very reproducible and involve sensitive detection techniques such as high-resolution «-particle spectroscopy and fission counting.
The chemistry of rutherfordium has been shown to be similar to the chemistry of hafnium rather than the chemistry of the heavier actinides, a clear demonstration of the expected end of the actinide series at lawrencium. This demonstration involved both aqueous and gas-phase chemistry. In the gas-phase experiments by Zvara et al.
(1972a,b) the 3-s isotope 259Rf, produced in the 242pu (22Ne, 5n) reaction was used.
Zvara and co-workers attempted to use thermochromatography to show a difference in volatility of RfC14 which seemed to condense at ~ 220°C as compared to the chlorides of the heavier actinides which have much higher condensation temperatures.
In aqueous solution experiments, the 1-min isotope 261Rf produced in the z48cm (180, 5n) reaction, was used by Silva et al. (1970). Atoms of rutherfordium recoiling from the target were caught in an NH4C1 layer sublimed onto platinum discs, dissolved with ammonium «-hydroxyisobutyrate solution and added to a heated Dowex-50 cation-exchange resin column, The neutral and anionic complexes of hafnium, zirco- nium and rutherfordium were not adsorbed on the cation-exchange column while actinides were strongly absorbed. Thus, the hafnium, zirconium (tracers) and the rutherfordium atoms eluted within a few column volumes while the actinides eluted after several hundred column volumes. The time from end of bombardment to start of sample counting was less than one half-life of 261Rf and after several hundred experi- ments Silva and co-workers were able to detect the decay of 17 atoms of 261Rf in the eluant.
This work was extended by Hulet et al. (1980) to the chloride complexes of rutherfordium. Computer automation was used to help perform the chemical opera- tions rapidly and reproducibly. An HC1 solution containing 261Rf was passed through an extraction chromatography column loaded with trioctylmethylammonium chloride which strongly extracts anionic chloride complexes. Such complexes are formed by the group IV elements such as rutherfordium while the actinides, and members of groups I and II, form weaker complexes and are not extracted. Thus, the actinide recoil products elute first and zirconium, hafnium and rutherfordium were shown to elute in a second fraction as expeeted for group IV elements.
However, more recent work shows that the chemical properties of rutherfordium differ in subtle ways from those of its homologs zirconium and hafnium, due to the complexities introduced by the presence of relativistic valence electrons in its larger electronic structure. For example, Czerwinski (1992), using the 6.5-s 261Rfisotope, has shown for the tributylphosphate-chloride system, at high chloride concentrations, that rutherfordium unlike zirconium and hafnium, forms anionic chloride species, similar to plutonium (IV). Furthermore, rutherfordium extraction increases with increasing hydrogen-ion concentration, while zirconium and hafnium do not, suggesting the formation of a neutral salt. These differences in chemistry show that chemical proper- ties exhibited by the lighter homologs cannot be extrapolated in a simple manner to prediet the properties of the transaetinide elements.
ORIGIN OF THE ACTINIDE CONCEPT 23 Similarly, Türler et al. (1992) showed that Rf Br 4 is unexpectedly more volatile than HfBr4, and RfC14 has a surprisingly high volatility. This may be due to relativistic effects leading to more covalent bonding in the rutherfordium halides.
Hahnium (Ha) is expected to have the valence-electron configuration 7s 2 6d 3 and thus to be a homolog of tantalum (with the valence electron configuration 6s / 5d3).
Zvara et al. (1976) have carried out a set of thermochromatography experiments similar to those done with rutherfordium. The isotope used for the study was the 1.8-s Z61Ha produced in the 243Am(22Ne, 4n) reaction, with detection by observation of its decay by spontaneous fission. The results of the experiments show the volatility of the chlorides and bromides of hahnium to be less than that of niobium (a 4d 3 element which has the same deposition temperature in the chromatographic apparatus as tantalum) but relatively similar to that of hafnium. Gäggeler et al. (1992) and Türler et al. (1992), using the isotopes 262.263Ha (half-lives 34 s and 27 s), similarly find that the bromide(s) of hahnium are less volatile than the bromides (NbBr 5 and Taßrs) of its lighter homologs niobium and tantalum. This is at variance with the theoretical, relativistic considerations of Pershina et al. (1992) for H a ß r » but the formation of an oxybromide or tribromide of hahnium, expected to have a lower volatility than the pentabromide, cannot be ruled out. In any case, the experiments show that hahnium behaves in a generally similar manner to niobium and tantalum in the volatifity of their bromides.
Gregorich et al. (1988) have investigated some aqueous solution chemistry of hahnium, using the 35-seconds 26/Ha, produced in the
249Bk (180, 5n)
reaction. With nearly a thousand batch experiments, hahnium was found to hydrolyze in strong HNO 3 solution and adhere to glass surfaces. Such hydrolysis is characteristic of group V elements and different from group IV elements as verified in experiments with tantalum and niobium, and zirconium and hafnium, tracers under the same conditions.In other experiments, hahnium did not form extractable anionic fluoride complexes in HNO3/HF solutions under conditions in which tantalum was extracted nearly quanti- tatively. This observation may be explained by an extrapolation of the properties of group V elements, in that the tendency to hydrolyze or to form polynegative fluoride- complexes may be stronger for hahnium than for tantalum, leading to a failure to observe extraction. In the pioneering work of Gregorich et al. (1988), the total study involved the identification of 47 atoms of 262Ha on the basis of observation of decay by spontaneous fission and «-particle emission, including the time correlation of «-particle decays from 262Ha and its 4-s daughter 258Lr.
To confirm the formation of such polynegative hahnium species, anion-exchange chromatographic separations using triisooctyl amine (TIOA) were performed by Kratz et al. (1989). They compared the halide complexation of hahnium (using :z62'263Ha) with the lighter group V elements niobium and tantalum, and the pseudo-group V element protactinium, and found that the halide complexes of hahnium are different from those of tantalum, and more like those of niobium and protactinium, indicating a reversal in the trend in going from niobium to tantalum to hahnium. It was suggested that the non-tantalum-like halide complexation of hahnium is indicative of the forma- tion of oxyhalide or hydroxyhalide complexes, in contrast to the pure halide complexes of tantalum.
: H
= 3
i : Li 11 Na
19 K 37 Rb 55 Cs 87 Fr
4 5 6 7 8 ~ 9 10
B e B C N 0 F I N ~ ß
Mg AI Si P S
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Ca Sc , Ti I V Cr Mn Fe C o Ni Cu Zn Ga Ge A s Se
38 39 I 40 41 42 43 44 45 46 47 48 49 50 51 52
Sr Y Zr Nb IVlo T c Ru Rh Pd A g Cd In Sn Sb Te
56 57 72 73 74 75 76 77 78 79 80 81 82 83 84
Ba La Hf Ta w Re Os Ir Pt A u Hg Tl Pb Bi Po
i
88 89 104 105 106 107 108 109 ( 1 1 0 ) ' ( 1 1 1 ) ' ( 1 1 2 ) ; ( 1 1 3 ) I [ ( 1 1 4 ) ' ( 1 1 5 ) ( 1 1 E : ( 1 1 7 ) ( 1 1 8 )
Ra A c Rf Ha Sg N s Hs Mt
i . . . i _ . . . t . . . ~ .
L A N T H A N I D E S ss 59 60 61 62 63 64 65 66 67 68 ; 9 70
Ce Pr Nd Pm Sm _Eu Gd Tb Dy Ho Er Yb .u ,
ACTINIDES 90 91 92 93 94 95 I E6 97 98 99 1 0 1 0 0 1 0 1 1 0 2 1 1 0 3 I
Th Pa U Np Pu A m I C m B k Cf Es FmlM«[ùo[ Lr L
Fig. 5. Modern periodic table of the elements (atomic numbers of undiscovered elements are shown in parentheses).
11 12 13 14 15 16
Na Mg AI Si P S
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
K Ca Sc Ti V Cr Mn Fe C o Ni Cu Zn Ga Ge A s Se
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
Rb Sr Y Z r Nb M o T c Ru Rh Pd A g Cd In Sn Sb Te
55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84
C s Ba La Hf Ta W Re O s Ir Pt A u Hg Tl Pb Bi Po
F~
He lO17 18 J Cl __Ar ]
35 36
Br Kr
- - 1 53 54 I
I X e j
85 86
A t Rn
i(111)! '
87 88 89 lo4 105 1o6 107 lO8 lO9 (110) (112) i ( 1 1 3 ) I ( 1 1 4 ) i ( 1 1 5 ) ( 1 1 6 ) i ( 1 1 7 ) (1181 i
Fr Ra A c Rf Ha Sg N s H s M t ~ i
ù , ! L ~ 4 . . .
i i i i i ' i , , ,
(119) ( 1 2 0 ) ' , ( 1 2 1 ) ' ( 1 5 4 ) ' i
i
(155), ( 1 5 6 ) , (1 57), (158), (1 5 9 ) ( 1 6 0 ) , 11 61 ) : (162) i (163), ( 1 6 4 ) , (165) I (166) i (167) : (168) ' ' ' ' ... . . . t ... ] ... ] . . . : . . .F 59 60 61 62 63 64 65 66 67 6 ~ E 8 r 69 70 7~ü~
LANTHANIDES icSe Pr Nd Pm Sm Eu Gd Tb Dy Ho Tm Yb
A C T I N I D E S 90 91 92 93 94 95 96 97 98 99 10o lOl 102 103
T h Pa U Np Pu A m C m B k Cf Es Fm Md No Lr
i i - - - T i - - - T - - i . . . . . . 7 r . . .. . . . . T - - -
S U P E R -
A C T I N I D E S : ( 1 2 2 ) I(123) (124)~~(125)I(126)I ~,
",
!(154)- i . . . t . . . • . . . . . . . . .
Fig. 6. Futuristic periodic table (atomic numbers of undiscovered elements are shown in parentheses).
ORIGIN OF THE ACTINIDE CONCEPT 25
The close similarity of the chemical behavior of hahnium to that ofprotactinium and niobium motivated Gober et al. (1992) to investigate this further using diisobutylcar- binol (DIBC) as an extractant. The extraction yield was greatest for protactinium, less for niobium, and least for hahnium. Since the extraction into DIBC is restricted to neutral or singly charged metal complexes, while polynegative species cannot be extracted, the results indicated that the tendency of hahnium to form polynegative complexes at high halide concentrations is stronger than for niobium and much stronger than for protactinium.
Thus, although rutherfordium and hahnium behave in the main in a manner highly consistent with that expected for members of group IV and group V, their chemical properties cannot be determined reliably in detail from trends exhibited by their lighter homologs because of the important, and probably understandable, role played by relativistic effects in these heavier elements.