bk-1980-0131.ch012

Sep 23, 1980 - From data gathered in a rather small number of experiments and limited by working with scarcely more than a few atoms, we can now disce...
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12 Chemistry of the Heaviest Actinides: Fermium, Mendelevium, Nobelium, and Lawrencium

Downloaded by CORNELL UNIV on September 8, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0131.ch012

E . K.

HULET

Lawrence Livermore Laboratory, University of California, P.O. Box Livermore, C A 94550

808,

From data gathered i n a rather small number of experiments and l i m i t e d by working with s c a r c e l y more than a few atoms, we can now d i s c e r n that the chemical p r o p e r t i e s of the heavy actinides s y s t e m a t i c a l l y deviate from those of t h e i r lanthanide counterparts. The d i f f e r e n c e s between the l a t e r elements of the 4f and 5f s e r i e s can be g e n e r a l l y i n t e r p r e t e d on the b a s i s of s u b t l e changes i n e l e c t r o n i c s t r u c t u r e . The most important change i s a lowering of the 5f energy l e v e l s with respect to the Fermi l e v e l and a wider separation between the 5f ground s t a t e s and the first e x c i t e d s t a t e s i n the 6d or 7p l e v e l s . Thus, i n comp a r i s o n with analogous 4f e l e c t r o n s , the l a t e r 5f e l e c t r o n s appear more tightly bound to the atom. Our conclusions regarding these s h i f t s toward greater s t a b i l i z a t i o n of 5f o r b i t a l s with increasing atomic number are mainly supported by the appearance of the d i v a l e n t o x i d a t i o n s t a t e w e l l before the end of the a c t i n i d e s e r i e s and the predominance of the d i v a l e n t s t a t e i n the next to l a s t element i n the s e r i e s . It i s these conclusions and the underlying experimental evidence that will be the main subject of t h i s review. Because of the uniqueness of d i v a l e n c y w i t h i n a s e r i e s of elements that are commonly t r i v a l e n t , most of the chemical research concerning the heaviest a c t i n i d e s has been concentrated on s t u d i e s of lower o x i d a t i o n s t a t e s . The chemical p r o p e r t i e s of the t r i v a l e n t ions of the lathanides and a c t i n i d e s are v i r t u a l l y the same throughout both s e r i e s and, f o r t h i s reason, there has been l i t t l e i n c e n t i v e to s p e c i f i c a l l y study t h i s o x i d a t i o n s t a t e i n Md, No, and Lr. This c l o s e r e l a t i o n s h i p between the s c i e n t i f i c s i g n i f i c a n c e and the research completed up to now i s s t r o n g l y c o r r e l a t e d with the e x t r a o r d i n a r y e f f o r t r e q u i r e d to produce experimental information concerning these elements. In a s c i e n t i f i c sense, the p r i n c i p l e of c o s t - e f f e c t i v e n e s s has governed the s e l e c t i o n of research t o p i c s . Beside the s c i e n t i f i c e f f o r t r e q u i r e d , there are a d d i t i o n a l r e s t r i c t i o n s to o b t a i n i n g extensive experimental data.

0-8412-0568-X/80/47-131-239$06.25/0 © 1980 American C h e m i c a l Society Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

240

LANTHANIDE

A N D ACTINIDE

CHEMISTRY

A N D SPECTROSCOPY

Chemical s t u d i e s of these elements must be performed with isotopes having not only a f l e e t i n g existence but producible only i n atom q u a n t i t i e s . In Table 1 we l i s t the most f r e q u e n t l y made i s o t o p e s , t h e i r h a l f l i v e s , and the atoms that have been synthes i z e d f o r each data p o i n t . Except f o r 2 5 5 p i i d e s listed can be created only by nuclear r e a c t i o n s between a c c e l e r a t e d charged p a r t i c l e s and transplutonium t a r g e t n u c l e i . For t h i s reason and the short l i f e t i m e s of the i s o t o p e s , a l l chemical s t u d i e s are c a r r i e d out a t l a r g e heavy-ion a c c e l e r a t o r s . Such research c a l l s upon nuclear physics f o r the methods of element synthesis and d e t e c t i o n while the research goals are aimed toward atomic and chemical p r o p e r t i e s . Therefore, t h i s f i e l d of r e search most e a s i l y f a l l s i n t o the domain of the nuclear chemist. Downloaded by CORNELL UNIV on September 8, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0131.ch012

m>

Element

Half L i f e

255

F m

20.1

256

M d

77 min

255

N o

256

L r

Table 1.

h

3.1 min 31 s

t

n

e

n u c

Average At oms/Exper iment 1012 10

6

103 10

The isotopes commonly produced f o r chemical studies of Fm, Md, No, and L r . Their h a l f l i v e s and numbers o f atoms a v a i l a b l e s e r i o u s l y l i m i t the information o b t a i n able by experiment.

To i n s u r e that a s t a t i s t i c a l average behavior i s observed i n the chemical experiments with No and L r , i t has been necessary to make repeated measurements f o r each data p o i n t . Indeed, the determination of the d i s t r i b u t i o n c o e f f i c i e n t s f o r L r i n a s o l v ent e x t r a c t i o n experiment required over 200 experiments to define the behavior of about 150 atoms of L r (1). Experiments of t h i s kind are e x c e p t i o n a l l y d i f f i c u l t and computer-controlled equipment has been devised to perform e i t h e r a p o r t i o n or a l l of operations needed f o r the chemical t e s t s and the a n a l y s i s of samples. Computer automation, although r e q u i r i n g a l a r g e r e f f o r t to implement, permits an experiment to be repeated many times i n r a p i d sequence with the added advantage o f doing each q u i c k l y before the complete decay o f the r a d i o a c t i v e atoms o f a s h o r t l i v e d isotope. It i s c l e a r that many fundamental and important p h y s i c a l constants, e l e c t r o n i c and molecular s t r u c t u r e s , and magnetic and thermodynamic p r o p e r t i e s cannot be determined when only a few atoms of these elements a r e a v a i l a b l e . As an example, the energies o f l o w - l y i n g e l e c t r o n i c l e v e l s , obtainable from o p t i c a l

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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12.

HULET

Chemistry

of the

Heaviest

241

Actinides

emission spectroscopy, would provide i n f o r m a t i o n e s s e n t i a l to understanding i o n i c and bonding p r o p e r t i e s and would a l l o w the c a l c u l a t i o n of some thermodynamic constants. Yet, with a small number of atoms, we are p r e s e n t l y unable to o b t a i n t h i s kind of b a s i c knowledge. Nevertheless, many other measurements are f e a s i b l e and these can provide q u a l i t a t i v e and a d e t a i l e d knowledge from the behavior of only a few atoms. Among the demonstrated p o s s i b i l i t i e s are the study of i o n i c p r o p e r t i e s i n aqueous and nonaqueous s o l u t i o n s , measurement of the magnetic moment of f r e e atoms, and the v o l a t i l i t y of the h a l i d e compounds. The l i s t of f e a s i b l e experiments w i l l undoubtedly expand i n time as advancements are made i n technology and as we s t r e t c h our i n g e n u i t y . Undoubtedly, there i s some s k e p t i c i s m with regard to deduct i o n s and c o n c l u s i o n s about the true chemical p r o p e r t i e s of an element when they are based upon observing the behavior of l e s s than one hundred atoms. This question has never been f u l l y addressed by any u n d e r l y i n g t h e o r e t i c a l treatment using thermodynamic and k i n e t i c arguments. In some i n s t a n c e s , a s e r i o u s case c o u l d be made f o r a cautious view, and one that we can imagine, i s the vapor pressure of a metal. Since the v o l a t i l i t y of a metal i s dependent on the s t r e n g t h of bonds between l i k e atoms, i t seems l i k e l y that vapor pressures would be perturbed when there are too few atoms present to c o n s t i t u t e a m a j o r i t y that are interbonded. However, there are a vast number of cases where the t r a c e r chemistry of an element i s i d e n t i c a l to i t s bulk behavior. New a c t i n i d e elements have been i d e n t i f i e d on the basis of the e l u t i o n p o s i t i o n of 17 atoms from an ion-exchange column (2). At l e a s t f o r a c t i n i d e ions i n aqueous s o l u t i o n s , we would not a n t i c i p a t e any unusual behavior dependent on t h e i r conc e n t r a t i o n u n t i l they become one of the major c o n s t i t u e n t s . The p r i n c i p a l j u s t i f i c a t i o n f o r t h i s view i s that i n any given s o l u t i o n , every element on e a r t h i s l i k e l y to be present at the l e v e l of one to a m i l l i o n atoms together with major concentrations of added reagents. The few atoms of heavy a c t i n i d e s introduced i n t o the s o l u t i o n are not l i k e l y to be s i n g l e d out f o r extraneous s i d e r e a c t i o n s because of the presence of l a r g e r numbers of other metal c a t i o n s with s i m i l a r chemical p r o p e r t i e s . Thus, the behavi o r of a s i n g l e a c t i n i d e i o n should be c l o s e to average because of the d i l u t i o n with g r e a t e r numbers of c h e m i c a l l y - s i m i l a r ones. These i n t r i g u i n g aspects of "one-atom chemistry" are now being explored from a t h e o r e t i c a l viewpoint and should be on f i r m e r ground i n the f u t u r e ( 3 ) . Fermium Several Fm isotopes with h a l f l i v e s of n e a r l y a day to 100 days are a v a i l a b l e i n amounts of at l e a s t 109 atoms. The n u c l i d e s 255p j 257p conveniently used f o r chemical i n v e s t i g a t i o n of Fm and they are obtainable as products from long neutron i r r a d i a t i o n s of ^ P u and ^^Cm. The 20-h Fm is m

a

n

(

m

a

2

r

e

2

2

2 5 5

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

242

LANTHANIDE

AND

ACTINIDE

CHEMISTRY

AND

SPECTROSCOPY

generated by the beta decay of 40-d 255g produced i n the neutron i r r a d i a t i o n s . By c h e m i c a l l y i s o l a t i n g the Es and p e r i o d i c a l l y r e s e p a r a t i n g Fm from i t s parent, one can secure a f a i r l y long-term source of 255p adequate f o r a l l t r a c e r experiments. The ground-state e l e c t r o n i c c o n f i g u r a t i o n of Fm i s 5f_^ 7s_ or an l e v e l ( 4 ) . This was e s t a b l i s h e d by an atomic-beam measurement of the magnetic moment g^ of 3.24-h Fm. In t h i s elegant measurement, F111F3 reduced with ZrC2 i n an atomic-beam apparatus to produce a beam of n e u t r a l Fm atoms. Three magnetic resonances were detected and the best value f o r g j was c a l c u l a t e d . To o b t a i n the l e v e l term, i t was necessary to e x t r a p o l a t e the mixing due to intermediate coupling i n the e l e c t r o n s p i n - o r b i t i n t e r a c t i o n s ( j - j and L-S). These e x t r a p o l a t i o n s were made from lower a c t i n i d e s and supplemented by Hartree-Fock c a l c u l a t i o n s f o r f r e e atoms. From s i m i l a r c a l c u l a t i o n s , the next higher l e v e l i s p r e d i c t e d to be Gy s t a r t i n g about 20,000 cm" above the ground s t a t e and having the c o n f i g u r a t i o n 5_f^^6d7s^ However, the f ^ s p and fH-s^j) c o n f i g u r a t i o n s are very c l o s e i n energy (_5) to the f^-^-ds^so that i t i s impossible to unambiguo u s l y estimate the next l e v e l above the ground s t a t e . The e l e c t r o n b i n d i n g energies of Fm have been measured f o r the K, L 1 - 3 , N j - 5 , N , 03-3, 0 , and P , 3 s h e l l s ( 6 ) . These were determined to an accuracy of ~10 eV by c o n v e r s i o n e l e c t r o n spectroscopy i n the beta decay of ^^ Es to 254p ^ s u r p r i s i n g l y low b i n d i n g energy f o r the P 3 (&21/2 3/2^ s h e l l of 2 4 + 9 eV was found. P r e d i c t e d values derived e i t h e r from e x t r a p o l a t i o n s of those measured i n lower a c t i n i d e s or c a l c u l a t e d by Hartree-Fock methods are about 20 to 60 eV higher i n energy. As the authors suggested, a b i n d i n g energy of 24 eV might provide a p o s s i b i l i t y f o r 6j? involvement i n chemical and s p e c t r o s c o p i c interactions. The p r o p e r t i e s of Fm metal and of i t s s o l i d compounds are f o r the most part unknown because there are i n s u f f i c i e n t q u a n t i t i e s to prepare even microsamples. In the numerous thermochromatographic s t u d i e s by Zvara and coworkers, the evaporation of Fm and Md t r a c e r from molten La at 1150°C was compared with the behavior of other s e l e c t e d l a n t h a n i d e s and a c t i n i d e s ( 7 ) . The v o l a t i l i t y of Md and Fm was found to be g r e a t e r than that of Cf and Cf was about e q u i v a l e n t to Yb and Eu, and a l l were much more v o l a t i l e than Am. The v o l a t i l i t i e s are c o r r e l a t e d by the number and energy of the valence bonds minus the energy needed to promote e l e c t r o n s to the valence bands i n the metals. Therefore, w i t h i n the normally t r i v a l e n t l a n t h a n i d e s and a c t i n i d e s , the more v o l a t i l e elements are a s s o c i a t e d with the d i v a l e n t metals. The unusual v o l a t i l i t y of Fm and Md was then construed by Zvara as evidence f o r d i v a l e n c y i n the m e t a l l i c s t a t e . s

m

2

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w

a

2

s

2

#

6 > 7

2

4 j 5

2

m

m#

2

The s e p a r a t i o n methods f o r Fm are the same as those used f o r separating other t r i v a l e n t lanthanides and a c t i n i d e s . For separat i n g the adjacent elements, Es and Md, a h i g h - r e s o l u t i o n chromatographic method i s necessary. E i t h e r i o n exchange, using s t r o n g l y

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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12.

Chemistry

HULET

of the Heaviest

Actinides

243

a c i d i c r e s i n s ( 9 ) , o r e x t r a c t i o n chromatography employing a l k y l phosphoric a c i d s (8) i s s t r o n g l y p r e f e r r e d . A complexing agent (a-hydroxyisobutyric a c i d ) i s required to s e l e c t i v e l y e l u t e the a c t i n i d e s from cation-exchange r e s i n s . The separation f a c t o r s , defined as the r a t i o of the d i s t r i b u t i o n c o e f f i c i e n t s of two metal i o n s , are small f o r both c a t i o n exchange and e x t r a c t i o n chromatography. These f a c t o r s range from 1.7 to 2.04 f o r Es-Fm separations using a Dowex-50 c a t i o n exchanger (9) or e x t r a c t i o n chromatography with HC1 as the eluant and b i s ( 2 - e t h y l h e x y l ) phosphoric a c i d d i l u t e d with heptane as the extractant (10). The Fm-Md separation f a c t o r s obtained by these two methods were 1.4 and 4.0, r e s p e c t i v e l y (9,10). The major d i f f e r e n c e between these methods of chromatographic separation l i e s i n the e l u t i o n sequence. With alkylphosphoric a c i d e x t r a c t a n t s , the elements are eluted i n order o f atomic number while i n c a t i o n exchange, the order i s reversed. The s o l u t i o n chemistry o f Fm deals l a r g e l y with the h i g h l y s t a b l e t r i p o s i t i v e o x i d a t i o n s t a t e although the d i p o s i t i v e s t a t e i s a l s o known. Formation constants f o r c i t r a t e complexes (11) and the f i r s t h y d r o l y s i s constant have been a c c u r a t e l y determined for Fm (12,13). Since the formation and h y d r o l y s i s constants for Am, Cm, Cf, and Es were measured simultaneously with those for Fm, the complex strengths of many of the t r i v a l e n t a c t i n i d e s can be compared (13). A l l constants were determined a t an i o n i c strength of y = 0.1 i n a p e r c h l o r a t e medium by measuring the p a r t i t i o n i n g of the r a d i o a c t i v e t r a c e r s between a t h i o n y l t r i fluoracetonate-benzene phase and the aqueous phase. The r e s u l t s f o r Fm may be expressed as f o l l o w s : 3+

Fm

3+

Fm

3+

+ H 0 2

^± Fm0H

+ 2H Cit 3

34

Fm " + 2HoCit

^ ^

2+

+

+ H ; 2

+

F m ( H C i t ) ~ + 5H ; 2

+

FmCit2"" + 6H ;

log

K = -3.80

+0.2

log

3 = 11-17

log

$2

2

= 1

2

-

4

0

Compared to the other a c t i n i d e ions i n v e s t i g a t e d , Fm formed stronger complexes with c i t r a t e and hydrox^l ions because of i t s smaller i o n i c r a d i u s . The smaller radius i s a d i r e c t consequence of the increased nuclear charge and p a r t i a l s h i e l d i n g of the outermost 6_p e l e c t r o n s by the inner f_ e l e c t r o n s . The r e d u c t i o n of F m to F m was f i r s t reported i n 1972 by N. B. Mikheev and coworkers (14). The reduction was accomplished with Mg metal i n the presence of Sm which was coreduced i n an aqueous-ethanol s o l u t i o n . I d e n t i f i c a t i o n of the d i v a l e n t s t a t e of Fm was e s t a b l i s h e d by determining the extent of i t s c o c r y s t a l l i z a t i o n with SmCl2 and t h i s was compared to the amount of t r a c e r Sr also c a r r i e d with SmC^- A milder reductant, Eu , f a i l e d to reduce Fm , which placed the standard reduction p o t e n t i a l of Fm between E u and S m or -0.43 t o -1.55 V r e l a t i v e to the standard Pt,^!!!*" e l e c t r o d e . Later work (15) by these s c i e n t i s t s 3+

2+

3+

3+

3+

2 +

2+

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

244

LANTHANIDE

A N D ACTINIDE CHEMISTRY

A N D SPECTROSCOPY

narrowed the range to betweem -0.64 and -1.15 V and most r e c e n t l y , they were able to estimate the p o t e n t i a l was the same as the Yb -> Y b couple w i t h i n 0.02 V, o r -1.15 V (16). The reduct i o n o f Fm to a d i v a l e n t i o n with SmCl2 bas a l s o been observed r e c e n t l y by Hulet e t a l . (17). In f u r t h e r work r e l a t e d to the d i v a l e n t s t a t e , the e l e c t r o d e p o t e n t i a l f o r the r e d u c t i o n of F m to Fm° has been measured by Samhoun and David (18). Over a period of years, they developed and r e f i n e d a r a d i o p o l a r o g r a p h i c technique f o r determining h a l f wave p o t e n t i a l s a t a dropping-Hg cathode. In a d d i t i o n to Fm, they have measured e i t h e r the I I I -> 0 o r I I -> 0 p o t e n t i a l f o r a l l transplutonium a c t i n i d e s except No and L r (18,24). The p o l a r o graph f o r Fm i s shown i n F i g u r e 1. The e l e c t r o c h e m i c a l r e a c t i o n taking place a t a r e v e r s i b l e e l e c t r o d e can be deduced from the slope of the polarographic wave. S p e c i f i c a l l y , the number o f e l e c t r o n s exchanged at the e l e c t r o d e , based on the Nernst equat i o n , i s obtained from t h i s slope. From t h e i r a n a l y s i s of the polarograms, there were three e l e c t r o n s i n v o l v e d i n the e l e c t r o chemical r e d u c t i o n of the t r i v a l e n t ions of the elements Am through Es and only two e l e c t r o n s f o r the r e d u c t i o n of Fm. This implies that F m was f i r s t reduced to F m before being f u r t h e r reduced to the metal. The I I I -> I I r e d u c t i o n step i s not d e t e c t ed by t h i s r a d i o p o l a r o g r a p h i c technique because both the I I I and II ions are i n the s o l u t i o n phase; whereas, the measured parameter i s the d i s t r i b u t i o n of the t r a c e r between the aqueous and Hg phase. The half-wave p o t e n t i a l s measured by t h i s method i n c l u d e the amalgamation p o t e n t i a l of the metal-mercury r e a c t i o n . The potent i a l f o r the o v e r a l l process f o r Fm, i . e . 3+

2 +

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2+

3+

2+

Fm

2+

+ 2e~

=

Fm(Hg),

was found to be -1.474 V with reference to the standard hydrogen electrode. The amalgamation p o t e n t i a l was estimated to be 0.90 V by using the metal r a d i i as a c o r r e l a t i n g parameter and i n t e r p o l a t i n g w i t h i n a s e r i e s o f d i v a l e n t elements with known amalgamation p o t e n t i a l s (19). This c o r r e l a t i o n i s shown i n Figure 2. The standard e l e c t r o d e p o t e n t i a l i s then given as -2.37 V f o r the Fm + 2e~ = Fm° r e a c t i o n . The a u t h o r s estimated 5 mV accuracy for the measured half-wave p o t e n t i a l seems reasonable, but there i s a much l a r g e r u n c e r t a i n t y i n the estimated amalgamation potential. Because the amalgamation p o t e n t i a l represents a l a r g e c o r r e c t i o n i n o b t a i n i n g the standard p o t e n t i a l , c a u t i o n should be e x e r c i s e d i n combining t h i s standard p o t e n t i a l with other data to c a l c u l a t e a d d i t i o n a l thermodynamic p r o p e r t i e s . 2+

1

Mendelevium 2

The i s o t o p e ^ M d ± n e a r l y always employed f o r chemical s t u d i e s of t h i s element. Besides having a convenient h a l f l i f e s

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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HULET

Chemistry

-1.70

of the Heaviest

-1.75

Actinides

-1.80

245

-1.85

Potential relative to see (V) Figure 1. Distribution of fermium as a function of applied voltage between mercury in a dropping mercury cathode and 0.1M tetramethyl ammonium perchlorate at pH = 2.4. The slope of the logarithmically transformed line indicates the number of electrons exchanged in the electrolysis reaction (24).

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

246

L A N T H A N I D E A N D ACTINIDE

Amalgamation potential A A = ( E y ) - E ° (0 - II) 2

CHEMISTRY

A N D SPECTROSCOPY

2

2

1.4

n

1

r

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Ral 1.2

Fm-Es

1.0

Sm Cf

• Ca

0.8

> I 0.6

0,4 -

>»Be

0.2

0.0. 1.0

J 1.2

1.4

1.6

1.8

2.0

i

L 2.2

2.4

Atomic radii — A

Figure 2. Amalgamation potentials, A*, derived from experimental data are plotted as a function of the atomic (metallic) radii. The amalgamation potential for fermium is obtained by using an estimated radius (19).

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

12.

HULET

Chemistry

of the

Heaviest

247

Actinides

of 77 min, t h i s n u c l i d e can be made with m i l l i b a r n c r o s s s e c t i o n s by a number of n u c l e a r r e a c t i o n s between l i g h t and heavy ions with a c t i n i d e t a r g e t n u c l e i . We have found that the bombardment of f r a c t i o n s of a microgram of 254jr ^ t h intense a l p h a - p a r t i c l e beams w i l l produce ~1()6 atoms of 256^d ^ two hours of i r r a d i a t i o n time. The 2 5 6 ^ ^ t e a s i l y detected through spon­ taneous f i s s i o n a r i s i n g from the ingrowth of i t s e l e c t r o n - c a p t u r e daughter 256jr ^ d i f f i c u l t y with using s p o n t a n e o u s - f i s s i o n counting to determine the Md content of samples i s that the growth and decay of f i s s i o n r a d i o a c t i v i t y i n each sample must be followed with time i n order to r e s o l v e the amounts of Md and Fm i n i t i a l l y present. However, a l p h a - p a r t i c l e s of a d i s t i n c t i v e energy coming from a 10% alpha-decay branch can a l s o be used to identify 256 i n a mixture of a c t i n i d e t r a c e r s . Mendelevium metal was found to be more v o l a t i l e than other a c t i n i d e metals as described i n the s e c t i o n on fermium (_7). There are no experimental v e r i f i c a t i o n s of the e l e c t r o n i c s t r u c ­ ture of Md, but t h i s has been c a l c u l a t e d by s e v e r a l methods to be 5f_ 7£ i n which the ground s t a t e l e v e l i s 7/? (5)· The s e p a r a t i o n of Md from the other a c t i n i d e s can be accom­ p l i s h e d e i t h e r by r e d u c t i o n of Md3+ to the d i v a l e n t s t a t e (20) or by chromatographic separations with Md remaining i n the t r i positive state. H i s t o r i c a l l y , Md^ has been separated i n columns of cation-exchange r e s i n by e l u t i o n with α-hydroxyisobutyric acid s o l u t i o n s ( 9 ) . This method i s s t i l l widely used even though e x t r a c t i o n chromatography r e q u i r e s l e s s e f f o r t and a t t e n t i o n to technique. Horwitz and coworkers (10) developed a h i g h l y - e f f i c i ­ ent and r a p i d s e p a r a t i o n of Md by employing HNO^ e l u t i o n s of columns of s i l i c a powder saturated with an organic e x t r a c t a n t , bis(2-ethylhexyl)phosphoric acid. The s e p a r a t i o n of Md from Es and Fm could be completed i n under 20 minutes and had the advant­ age of p r o v i d i n g f i n a l s o l u t i o n s of Md f r e e of complexing agents that might be an i n t e r f e r e n c e i n subsequent experiments. When the d i v a l e n t s t a t e of Md was f i r s t d i s c o v e r e d , e x t r a c ­ t i o n chromatography was used to prove that the behavior of Md2 was d i s s i m i l a r to that of E s ^ and Fm^ (20). The e x t r a c t a n t , b i s ( 2 - e t h y l h e x y l ) p h o s p h o r i c a c i d (HDEHP), has a much lower a f f i n ­ i t y f o r d i v a l e n t ions than i t does f o r the t r i - and t e t r a v a l e n t ones. Thus, the e x t r a c t i o n of Md2 i s much poorer than the e x t r a c t i o n of the neighboring t r i p o s i t i v e a c t i n i d e s as i n d i c a t e d by the r e s u l t s shown i n Table 2. This became the b a s i s f o r a s e p a r a t i o n method i n which t r a c e r Md i n 0.1 M HC1 i s reduced by f r e s h Jones' Reductor i n the upper h a l f of an e x t r a c t i o n column c o n t a i n i n g HDEHP absorbed on a fluorocarbon powder i n the lower half. Mendelevium, i n the d i p o s i t i v e s t a t e , i s r a p i d l y e l u t e d with 0.1 M HC1 whereas the other a c t i n i d e s are r e t a i n e d by the extractant. The s e p a r a t i o n i s q u i c k l y performed, but the Md con­ t a i n s small amounts of Z n 2 from the Jones' Reductor and a l s o E u 2 , which was added p r i o r to the e l u t i o n to prevent r e o x i d a t i o n of Md2 by the e x t r a c t a n t . s w

n

s

m

o

o

n

e

t

o

s

Downloaded by CORNELL UNIV on September 8, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0131.ch012

m#

M d

F

+

+

+

+

+

+

+

+

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

Downloaded by CORNELL UNIV on September 8, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0131.ch012

248

LANTHANIDE

A N D ACTINIDE

CHEMISTRY

A N D SPECTROSCOPY

Table 2. Comparison of the extraction behavior of tracer einsteinium, fermium, and mendelevium after treatment with various reducing agents. The column-elution method of extraction chromatography was used with the extractant HDEHP adsorbed on a column bed of afluoroplasticpowder (20)

CONDITIONS FOR REDUCTION

STANDARD POTENTIAL OF REDUCING AGENT (volts)

Zn(Hg) A M A L G A M , 80° -20 min, 0.1 M HCI; Zn(Hg) AMALGAM IN UPPER HALF OF EXTRACTION COLUMN

% NON-EXTRACTED BY HDEHP COLUMN Md

Es-Frr

+0.763

77