Protactinium-231

Protactinium has a wealth of halides, with all PaX4 and PaXs known, also Pals. There are also several oxyhalides. 

PaaOs is the usual starting material for the synthesis of the halides  

PaOBr3 has a cross-linked chain structure with 7-coordinate protactinium, whilst yellow-green PaOCl2 has a complicated chain structure involving 7-, 8- and 9- coordination, adopted by several MOX2(M = Th, Pa, U, Np X = Cl, Br, I). 

Protactinium-231, an a-emitter, is the only isotope suitable for macrochemical studies [fj/2 = 32,340 years (36)] and in view of the radiochemical hazards associated with weighable amounts of this isotope, it is necessary to perform all manipulations in glove boxes or, in the case of solution chemistry, in well-ventilated fume hoods. An indication of the toxicity of protactinium-231 is given by the fact that the maximum permissible concentration in air is 10 mg/m whereas that of hydrogen cyanide is 10 mg/m. Details of suitable handling procedures are adequately dealt with in other publications (11, 136). 

Progress in the preparative and structural fields of protactinium chemistry has been rapid during the past 6 years and there is now sufficient information available, particularly in the halide and oxide fields, to permit a more balanced comparison than has previously been possible with the properties of the actinide elements, on the one-hand, and those of niobium and tantalum, on the other. In this connection one must, of course, bear in mind the fact that the ionic radii of protactinium in its various valence states [Pa(V), 0.90 A and Pa(IV), 0.96 A] are appreciably larger than those of niobium or tantalum and this itself will have a considerable influence on the chemical and crystallographic properties of the elements. 

Protactinium exists naturally in the pentavalent state and although it is possible, by employing strongly reducing conditions, to obtain the tetravalent state in solution and in solid compounds this state is, in general, unstable in the presence of oxygen, exceptions being the tetra-fluoride and dioxide, which are stable in the atmosphere. The potential 

The preparation of the metal was first reported by von Grosse (80) who obtained it by bombarding protactinium pentoxide with 35 keV electrons in a high vacuum and by decomposing the pentachloride on a hot wire. No properties were reported for these products and more recently the pure metal has been obtained by reduction of protactinium tetrafluoride with lithium (73) or barium (65,125) vapor at 1300°-1400°C using the double crucible technique and on a larger scale by reduction with barium (106) or 10% magnesium in zinc alloy (107). 

During the past few years numerous new penta- and tetravalent protactinium halides and oxyhalides have been characterized, but of the. possible trivalent compounds only Pals been reported. The presently 

The most stable oxidation states for protactinium are Pa(V) and Pa(IV). The chemical behavior of Pa(V) closely mimics that of Nb(V) and Ta(V), and experimental data are consistent with a 5f(l) rather than a 6d(l) electron configuration for the Pa(IV) species [37]. The electrochemical literature for Pa is mainly focused on the characteristics of the Pa(V)/Pa(IV) couple and the electrodeposition of Pa metal films from aqueous and nonaqueous electrolyte solutions. In aqueous solutions, only Pa(V) and Pa(IV) ions are known to exist, and the standard potential for the Pa(V)/Pa(IV) redox couple is in the range of —0.1 to -0.32 V [38]. 

As mentioned in previous reviews, the po-larography of Pa(V) has been studied in aqueous fluoride, sulfate, citrate, and oxalate media [39]. The results in [NH4]F at pH 7.2 gave the most useful electrochemical data and will be discussed here briefly [40]. At a constant fluoride concentration of 3.84 M, two waves are observed in the polarogram. The first wave is the 

Electrodeposition of Pa metal has been performed from both aqueous and nonaqueous solutions. An isopropanol solution of 10-20 p,gmL Pa from 8M HCl/0.01 M HE/Pa stock was employed for quantitative electrodeposition [41]. The cell consisted of a gold-plated A1 cathode and a Pt wire anode. During deposition the current was maintained at 1 mA, which produced a potential of 400-600 V during the 90-min electrolysis. The progress of the electrolysis was externally monitored by alpha-counting of the electrolysis solution before and during the electrodeposition. Deposition studies of metal from aqueous solutions are more common. Pa was electrodeposited on platinum in 95% yield at tracer concentrations from an electrolyte of [NH4]C1/HC1 [42]. Electrochemical and chemical conditions of the plating process were described for Pu solutions, which served as a model for the other actinide elements studied. Another tracer 

ISOTOPES There are a total of 30 isotopes of protactinium. All are radioactive, and none are stable. Their decay modes are either alpha or beta decay or electron capture. Their half-lives range from 53 nanoseconds to 3.276x10+ ears. 

Energy Levels/Shells/Electrons Orbitals/Electrons 

Protactinium is a relatively heavy, silvery-white metal that, when freshly cut, slowly oxidizes in air. AH the isotopes of protactinium and its compounds are extremely radioactive and poisonous. Proctatinium-231, the isotope with the longest half-life, is one of the scarcest and most expensive elements known. It is found in very small quantities as a decay product of uranium mixed with pitchblende, the ore of uranium. Protactiniums odd atomic number (gjPa) supports the observation that elements having odd atomic numbers are scarcer than those with even atomic numbers. 

Its melting point is just under 1,600°C, its boiling point is about 4,200°C, and its density is 15.37g/cm.  

Because the proportion of protactinium to its ores is of the magnitude of one part in ten milhon, it takes many truckloads of ore to extract a small quantity of the metal. About 30 years ago, approximately 125 grams of protactinium was extracted from over 60 tons of ore 

Marinsky, Jacob A. (1996). The Search for Element 61. In Episodes from the History of the Rare Earth Elements, ed. C. H. Evans. Boston Kluwer Academic Publishers. 

MELTING POINT 1,568°C BOILING POINT Unknown DENSITY 15.37 g/cm  

An isotope of protactinium (having mass number 234 and a half-life of 1.1 minutes) was first identified by Kasimir Fajans and O. Gohring in 1913 as a short-lived member of the naturally occurring decay series and was given the name brevium, meaning brief The existence of protactinium was confirmed in 1918 when another isotope of protactinium (of mass 231 and a half-life of 3.3 X 10 years) was studied independendy by Otto Hahn and 

The element eka-tantalum predicted by Mendeleev is, perhaps, the only one of the radioactive elements that had been discovered earlier than it is generally recognized. We are talking about the element number 91 situated between thorium and uranium. Its long-lived isotope has a considerable half-life (34 300 years) and, therefore, it should be accumulated in the uranium ores moreover, it emits alpha rays. If we look at the accepted date of its discovery (1918) it would be reasonable to ask why it was discovered so late. We shall answer this question later. 

Now let us discuss the family of uranium-238 (see Table 1 and Diagram 1). The notorious element UX discovered by Crookes, which in fact started the hunt for radioelements, is designated as uranium-Xx in Table 1. This name was given to it much later, after the discovery of the radioelement designated as uranium-Xg. 

This was done in mid-March 1913 by K. Fajans and his young assistant 0. Goring who detected a new beta-emitting radioelement with a half-life of 1.17 min and chemical properties similar to those of tantalum. In October of the same year they clearly stated that UXj was a new radioactive element located between thorium and uranium and suggested to name it brevium (from the Greek for short-lived ). 

The symbol UXj took its place in the uranium family but the symbol Bv could hardly be put into box No. 91 of the periodic system though the new element was intensely studied in many laboratories and its discovery was verified by British and German scientists. 

At any rate, the statement that element No. 91 was discovered in 1913 does not seem controversial. But why then does not its history start with this date  

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Before it was known that elements beyond uranium were capable of existence, the heaviest known natural elements, thorium, protactinium and uranium, were placed in a sixth period of the periodic classification, corresponding to the elements hafnium, tantalum and tungsten in the preceding period. It was therefore implied that these elements were the beginning of a new, fourth transition series, with filling of the penultimate n = 6 level (just as the penultimate = 5... 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

However, the quantity of Pa produced in this manner is much less than the amount (more than 100 g) that has been isolated from the natural source. The methods for the recovery of protactinium include coprecipitation, solvent extraction, ion exchange, and volatility procedures. AH of these, however, are rendered difficult by the extreme tendency of protactinium(V) to form polymeric coUoidal particles composed of ionic species. These caimot be removed from aqueous media by solvent extraction losses may occur by adsorption to containers and protactinium may be adsorbed by any precipitate present. 

The actinide elements exhibit uniformity in ionic types. In acidic aqueous solution, there are four types of cations, and these and their colors are hsted in Table 5 (12—14,17). The open spaces indicate that the corresponding oxidation states do not exist in aqueous solution. The wide variety of colors exhibited by actinide ions is characteristic of transition series of elements. In general, protactinium(V) polymerizes and precipitates readily in aqueous solution and it seems unlikely that ionic forms ate present in such solutions. 

The extensive hydrolysis of protactinium in its V oxidation state makes the chemical investigation of protactinium extremely difficult. Ions of protactinium(V) must be held in solution as complexes, eg, with fluoride ion, to prevent hydrolysis. 

Actinide Peroxides. Many peroxo compounds of thorium, protactinium, uranium, neptunium, plutonium, and americium are known (82,89). The crystal stmctures of a number of these have been deterrnined. Perhaps the best known are uranium peroxide dihydrate [1344-60-1/, UO 2H20, and, the uranium peroxide tetrahydrate [15737-4-5] UO 4H2O, which are formed when hydrogen peroxide is added to an acid solution of a uranyl salt. 

Protactinium has a weaker affinity for particles than °Th and is therefore... 

The isolation and identification of 4 radioactive elements in minute amounts took place at the turn of the century, and in each case the insight provided by the periodic classification into the predicted chemical properties of these elements proved invaluable. Marie Curie identified polonium in 1898 and, later in the same year working with Pierre Curie, isolated radium. Actinium followed in 1899 (A. Debierne) and the heaviest noble gas, radon, in 1900 (F. E. Dorn). Details will be found in later chapters which also recount the discoveries made in the present century of protactinium (O. Hahn and Lise Meitner, 1917), hafnium (D. Coster and G. von Hevesey, 1923), rhenium (W. Noddack, Ida Tacke and O. Berg, 1925), technetium (C. Perrier and E. Segre, 1937), francium (Marguerite Percy, 1939) and promethium (J. A. Marinsky, L. E. Glendenin and C. D. Coryell, 1945). 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

Prior to 1940 only the naturally occurring actinides (thorium, protactinium and uranium) were known the remainder have been produced artificially since then. The transactinides are still being synthesized and so far the nine elements with atomic numbers 104-112 have been reliably established. Indeed, the 20 manmade transuranium elements together with technetium and promethium now constitute one-fifth of all the known chemical elements. 

The much rarer element, protactinium, was not found until 1913 when K. Fajans and O. Gohring identified Pa as an unstable member of the decay series ... 

As the parent of actinium in this series it was named protoactinium, shortened in 1949 to protactinium. Because of its low natural abundance its chemistry was obscure until 1960 when A. G. Maddock and co-workers at the UK Atomic Energy Authority worked up about 130g from 60 tons of sludge which had accumulated during the extraction of uranium from UO2 ores. It is from this sample, distributed to numerous laboratories throughout the world, that the bulk of our knowledge of the element s chemistry was gleaned. 

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