The actinides

2 The actinides. The actinides metals are electropositive and very reactive they are pyrophoric in finely divided form. They tarnish rapidly in air forming an oxide protective coating in the case of Th, but more slowly for the other actinides. The metals react with most non-metals. With steam or boiling water, oxide is formed on the surface of the metal and H2 evolves in this way hydrides are produced that react rapidly with water and facilitate further attack on the metals. The oxidation states observed in the chemistry of lanthanides and actinides are shown in Fig. 5.9. Notice the predominant oxidation state III for the lanthanides 

Just as the lanthanides form a series of closely related elements following La in which the characteristic ions M have from 1 to 14 4f electrons, so the actinide series might be expected to include the 14 elements following the prototype Ac (which, like La, is a true member of Group III), with from 1 to 14 electrons entering the 5f in preference to the 6d shell. In fact there are probably no 5f electrons in Th and the number in Pa is uncertain, and these elements are much more characteristically members of Groups IV and V respectively than are the corresponding lanthanides Ce and Pr. Thus the chemistry of Th is essentially that of Th(iv), whereas there is an extensive chemistry of Ce(iii) but only two solid binary compounds of Ce(iv), namely, the oxide and fluoride. In contrast to Pa, the most stable oxidation state of which is v, there are no compounds of Pr(v). 

The radii of actinide ions and decrease with increasing atomic number in much the same way as do those of the lanthanides, the radius of being about 0-10 A greater than that of M (e.g. L03 A, 0-93 A). The 

All the actinides are believed to have the electron configuration (7s)2 and a variable occupation of the 5f and 6d shells (Table 98). 

These ionic radii, derived from X-ray diffraction data, should be compared with those of the lanthanides (p. 421). The size of an ion depends largely upon the quantum number of the outermost electrons and the effective nuclear charge (p. 89). In the 3+ ions of these elements the outermost electrons are in a completed 6p shell the effective nuclear charge rises with atomic number because the screening effect of extra electrons in the 5f level fails to compensate entirely for the increased nuclear charge. The existence of a contraction, similar to the lanthanide contraction, affords further support for the idea that the 5f level is being filled in passing onwards from actinum. The contraction is more rapid in the actinides. 

The principal source of thorium is monazite (p. 425), a phosphate of cerium and lanthanum with up to 15% of thoria. It is dissolved in concentrated sulphuric acid and the thorium phosphate precipitated with magnesium oxide. The washed phosphate heated with sodium carbonate gives crude thoria, ThOg, which is converted to the soluble oxalate and separated from the insoluble oxalates of cerium and lanthanum. After ignition to oxide the nitrate is made, purified by recrystallisation, and again calcined to thoria. 

An alternative separation of thoria, based on the low solubility of lanthanides in a mixture of phosplioric and sulphuric acids, is to treat monazite at 225° with concentrated sulphuric acid and to leave most of the lanthanide sulphates behind by extracting the semi-solid mass with water. Tlie crude thorium sulphate is crystallised by concentrating the liquor, washed with cold concentrated sulphuric acid to remove phosphoric acid and redissolved in 25% sulphuric acid. The solution of the sulphate is boiled with ammonium carbonate to precipitate basic thorium carbonate. This is washed with a very little dilute nitric acid and calcined to thoria. 

The metal may be purified (especially from ThOg) by the thermal dissociation of Thl4 on a hot filament (1100-1200°), the liberated iodine being recirculated to react with more crude metal powder (Van Arkel and De Boer). 

A study has been made of butanol and a-methylbenzylamine adducts of the chiral shift reagents derived from Eu, Gd, Tb, Er, Ho, or Tm and 3-t-butylhydroxymethylene-(+)-camphorate. In the absence of added substrates, the shift reagents were dimeric in CCl.  

A large contact term contribution to the induced shifts was observed in the n.m.r. spectra of pyridine and y-picoline in the presence of [Eu(dpm)3], [Pr(dpm)3], and [Eu(fod)3]. Both Fermi contact and pseudo-contact shifts have been induced in pyridine, 3,5-lutidine, and isoquinoline by [Eu(dpm)3]. An evaluation of the effect of [M(dpm)3] (M = Eu, Er, Ho, Dy, Nd, Tb, Pr, and Yb) on the n.m.r. spectrum of isoquinoline established that [Eu-(dpm)3] and its Nd analogue had the greatest influence, the Dy, Ho, and Tb complexes were intermediate, and the Er and Yb complexes had least effect. 

The equilibrium between two chair forms of (7) was determined using [Eu(fod)3] this caused a large variation of 7(HP) attributed to an increase in the concentration of the axial phosphoryl conformer. 

This section deals with the structure and chemistry of actinide compounds and complexes, excluding derivatives of uranyl and related systems. 

Structural Studies.—A review of the loose connection between the electronic behaviour and structure, and chemistry, of the transuranic elements has appeared. In Am2(S04)3,2H20, each Am atom is eight-co-ordinate,  

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Evidence other than that of ion-exchange favours the view of the new elements as an inner transition series. The magnetic properties of their ions are very similar to those of the lanthanides whatever range of oxidation states the actinides display, they always have -1-3 as one of them. Moreover, in the lanthanides, the element gado-... 

In the actinides, the element curium, Cm, is probably the one which has its inner sub-shell half-filled and in the great majority of its compounds curium is tripositive, whereas the preceding elements up to americium, exhibit many oxidation states, for example -1-2, -1-3. -1-4, -1-5 and + 6, and berkelium, after curium, exhibits states of -1- 3 and -E 4. Here then is another resemblance of the two series. 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Element 103, lawrencium, completes the actinides. Following this series, the transition elements should continue with the filling of the 6d orbitals. There is evidence for an element 104 (eka-hafnium) it is believed to form a chloride MCl4, similar to that of hafnium. Less positive evidence exists for elements 105 and 106 attempts (so far unsuccessful) have been made to synthesise element 114 (eka-lead). because on theoretical grounds the nucleus of this elemeni may be stable to decay by spontaneous fusion (as indeed is lead). Super-... 

Thor, Scandinavian god of war) Discovered by Berzelius in 1828. Much of the internal heat the earth produces has been attributed to thorium and uranium. Because of its atomic weight, valence, etc., it is now considered to be the second member of the actinide series of elements. 

Planet pluto) Plutonium was the second transuranium element of the actinide series to be discovered. The isotope 238pu was produced in 1940 by Seaborg, McMillan, Kennedy, and Wahl by deuteron bombardment of uranium in the 60-inch cyclotron at Berkeley, California. Plutonium also exists in trace quantities in naturally occurring uranium ores. It is formed in much the same manner as neptunium, by irradiation of natural uranium with the neutrons which are present. 

Albert Einstein) Einsteinium, the seventh transuranic element of the actinide series to be discovered, was identified by Ghiorso and co-workers at Berkeley in December 1952 in debris from the first large thermonuclear explosion, which took place in the Pacific in November, 1952. The 20-day 253Es isotope was produced. 

Dmitri Mendeleev) Mendelevium, the ninth transuranium element of the actinide series discovered, was first identified by Ghiorso, Harvey, Choppin, Thompson, and Seaborg in early in 1955 during the bombardment of the isotope 253Es with helium ions in the Berkeley 60-inch cyclotron. The isotope produced was 256Md, which has a half-life of 76 min. This first identification was notable in that 256Md was synthesized on a one-atom-at-a-time basis. 

Experiments seem to show that the element possesses a moderately stable dipositive (11) oxidation state in addition to the tripositive (111) oxidation state, which is characteristic of the actinide elements. 

Lawrencium behaves differently from dipositive nobelium and more like the tripositive elements earlier in the actinide series. 

Filling up the 4/ orbital is a feature of the lanthanides. The 4/ and 5d orbitals are of similar energy so that occasionally, as in La, Ce and Gd, one electron goes into 5d rather than 4f. Similarly, in the actinides, Ac to No, the 5/ subshell is filled in competition with 6d. 

The actinide elements are a group of chemically similar elements with atomic numbers 89 through 103 and their names, symbols, atomic numbers, and discoverers are given in Table 1 (1-3) (see Thorium and thorium compounds Uranium and uranium compounds Plutonium and plutonium compounds Nuclear reactors and Radioisotopes). 

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. 

Thorium, uranium, and plutonium are well known for their role as the basic fuels (or sources of fuel) for the release of nuclear energy (5). The importance of the remainder of the actinide group Hes at present, for the most part, in the realm of pure research, but a number of practical appHcations are also known (6). The actinides present a storage-life problem in nuclear waste disposal and consideration is being given to separation methods for their recovery prior to disposal (see Waste treati nt, hazardous waste Nuclear reactors, waste managet nt). 

Isotopes sufficiently long-Hved for work in weighable amounts are obtainable, at least in principle, for all of the actinide elements through fermium (100) these isotopes with their half-Hves are Hsted in Table 2 (4). Not all of these are available as individual isotopes. It appears that it will always be necessary to study the elements above fermium by means of the tracer technique (except for some very special experiments) because only isotopes with short half-Hves are known. 

Special techniques for experimentation with the actinide elements other than Th and U have been devised because of the potential health ha2ard to the experimenter and the small amounts available (15). In addition, iavestigations are frequently carried out with the substance present ia very low coaceatratioa as a radioactive tracer. Such procedures coatiaue to be used to some exteat with the heaviest actinide elements, where only a few score atoms may be available they were used ia the earHest work for all the transuranium elements. Tracer studies offer a method for obtaining knowledge of oxidation states, formation of complex ions, and the solubiHty of various compounds. These techniques are not appHcable to crystallography, metallurgy, and spectroscopic studies. 

The close chemical lesemblance among many of the actinide elements permits their chemistry to be described for the most part in a correlative way... 

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 reduction potentials for the actinide elements ate shown in Figure 5 (12—14,17,20). These ate formal potentials, defined as the measured potentials corrected to unit concentration of the substances entering into the reactions they ate based on the hydrogen-ion-hydrogen couple taken as zero volts no corrections ate made for activity coefficients. The measured potentials were estabhshed by cell, equihbrium, and heat of reaction determinations. The potentials for acid solution were generally measured in 1 Af perchloric acid and for alkaline solution in 1 Af sodium hydroxide. Estimated values ate given in parentheses. 

Fig. 5a. Standard (or formal) reduction potentials of actinium and the actinide ions in acidic (pH 0) and basic (pH 14) aqueous solutions (values are in volts...

Solid Compounds. The tripositive actinide ions resemble tripositive lanthanide ions in their precipitation reactions (13,14,17,20,22). Tetrapositive actinide ions are similar in this respect to Ce . Thus the duorides and oxalates are insoluble in acid solution, and the nitrates, sulfates, perchlorates, and sulfides are all soluble. The tetrapositive actinide ions form insoluble iodates and various substituted arsenates even in rather strongly acid solution. The MO2 actinide ions can be precipitated as the potassium salt from strong carbonate solutions. In solutions containing a high concentration of sodium and acetate ions, the actinide ions form the insoluble crystalline salt NaM02(02CCH2)3. The hydroxides of all four ionic types are insoluble ... 

Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 8 (13,17,18,22). The binary compounds with carbon, boron, nitrogen, siUcon, and sulfur are not included these are of interest, however, because of their stabiUty at high temperatures. A large number of ternary compounds, including numerous oxyhaUdes, and more compHcated compounds have been synthesized and characterized. These include many intermediate (nonstoichiometric) oxides, and besides the nitrates, sulfates, peroxides, and carbonates, compounds such as phosphates, arsenates, cyanides, cyanates, thiocyanates, selenocyanates, sulfites, selenates, selenites, teUurates, tellurites, selenides, and teUurides. 

In general, the absorption bands of the actinide ions are some ten times more intense than those of the lanthanide ions. Fluorescence, for example, is observed in the trichlorides of uranium, neptunium, americium, and curium, diluted with lanthanum chloride (15). 

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