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THE RARE EARTH AND CHAPTER 24 ACTINOID ELEMENTS

Sc 24.1 The 3 Elements 24.2 The Lanthanoids Y 24.3 The Actinoids 24.4 24.5 Postactinoid Elements La Ce Pr Nd Pm SmEu Gd Tb Dy Ho Er Tm Yb Lu 24.6 Biological Aspects Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Just as the chemistry of the 3d transition metals differs from that of the 4d and 5d transition metals, so the chemistry of the 4f lanthanoids is quite different from that of the 5f actinoids. In addition, the chemistry of the Group 3 elements is so similar to that of the lanthanoids that it is convenient to consider them also in this chapter. Little has been established of the chemistry of the postactinoid elements as the half- lives of all of these elements are so short.

For Chapter 24, see http://www.whfreeman.com/descriptive6e

Context: Enriched and Depleted Uranium Uranium exists in nature as a mixture of three isotopes, each of which has a long half-life. As can be seen from the following table, by

Abundance (%) Half᎑life (years)

U᎑2340.0053 2 ϫ 105 U᎑235 0.71 7 ϫ 108 U᎑238 99.28 4 ϫ 109

691 cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 692692 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

692 CHAPTER 24 / The Rare Earth and Actinoid Elements

far the highest proportion is uranium-238. However, it is the isotopes U-234 and U-235 that are required for use in most nuclear reactors and in nuclear weapons. To separate the isotopes, one route is to pass gaseous uranium(VI) fl uo- ride (see Section 24.4) through a series of membranes. The gas molecules containing the lower-mass uranium isotopes pass through the membrane very slightly faster (this is an application of the law of effusion). By cycling through a large number of membranes, a signifi cant enrichment can be obtained. For most nuclear fi ssion reactors, a concentration of about 0.03 per- cent U-234, 3.5 percent U-235, and 96.5 percent U-238 will suffi ce. This iso- topic mixture is called enriched uranium. Obviously, if U-234 and U-235 are selectively concentrated in the enriched uranium, the remaining portion must be defi cient in these two isotopes. This is known as depleted uranium (DU). In the United States alone, there is estimated to be over 500 000 tonnes of stockpiled DU. It is in munitions that uranium has found a major use. Cost is obviously an advantage, for governments are eager to fi nd a use for all the stockpiled metal. More important, uranium has specifi c technical advantages over other metals. When an artillery shell is fi red at an armored object, such as a tank, the aim is to pierce the armor and penetrate the interior of the vehicle, destroying it and killing the occupants. The penetrability depends in part on the of the metal used for the shell: the greater the density, the greater the kinetic energy of the projectile. has the same density as ura- nium and was formerly used in shells. However, each metal behaves differ- ently when impacted on a surface at fi ve times the speed of sound—the typical impact velocity of a shell. Steel shatters like glass, while tungsten fl ows like a viscous liquid. Uranium, hardened by alloying with , will more easily penetrate a metal object virtually intact. Uranium has a second and equally important military advantage of being pyrophoric. A pyrophoric metal is one whose fi nely divided particles will burn in air (for example, small particles of will burn, as can be seen in “sparklers” and when an iron object is ground or machined). Uranium is highly pyrophoric; thus, the hot uranium particles, some molten, since uranium has a comparatively low of 11308C, will burn intensely in the interior of the vehicle to give a dust of uranium

oxides such as U3O8. DU shells were fi rst used in combat in the 1991 First Gulf War, about 14 000 large-caliber shells being fi red by ground vehicles and about 940 000 rounds of small-caliber shells from aircraft (see following fi gure). The total mass of ura- nium used was about 300 tonnes. DU shells have since been used in all subse- quent major confl icts involving U.S. forces. Both the United States and the United Kingdom deploy DU weapons, and U.S. arms dealers sell DU to 16 coun- tries around the world. As we discuss in Section 24.5, there are considerable concerns about the exposure to people of the uranium oxide dusts remaining following confl icts in which DU has been used. cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 693693 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

24.1 The Group 3 Elements 693

Schematic of a depleted uranium bullet used in the seven-barrel Gatling gun which can fi re 65 rounds This part per second. is fired 302 g of DU

73 mm Propellant

This part is discarded

Flash tube Primer

30 mm

24.1 The Group 3 Elements

Because of similarities with the lanthanoids in chemical behavior, the Group 3 elements (, , and ) are often considered as being part of the same set. To refer collectively to the lanthanoid and Group 3 elements, the term rare earth metals can be used (see Chapter 9, Section 9.8). Both scandium and yttrium are soft, reactive metals that exhibit the 13 . They differ from their transition metal neighbors in that their only oxidation state is a d0 confi guration. Hence, they do not exhibit the range of oxidation states that is characteristic of the transition metals.

Scandium In Chapter 9, Section 9.4, we reviewed the evidence for a link between the chemistry of aluminum and scandium on the basis of the (n) and (n 1 10) rela- tionship. Like the aluminum ion, the scandium(III) ion (such as that in the chloride) hydrolyzes in water to give an acid solution. Scandium is found along with yttrium in the rare mineral thortveitite, con-

sisting of scandium yttrium silicate, (Sc,Y)2Si2O7. However, most scandium is extracted as a by-product from the processing of other metal ores in Ukraine, China, and Russia, with Russia having the largest stockpile. Incorporating a small proportion of scandium into cast aluminum results in an alloy that is stronger and less prone to fatigue. For this reason, the alloy is used in the struc- ture of the Russian MiG-29 fi ghter aircraft. cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 694694 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

694 CHAPTER 24 / The Rare Earth and Actinoid Elements

WORKED EXAMPLE 24.1 In earlier text, it was noted that scandium ion hydrolyzes in a similar manner in water to aluminum ion. Write a balanced equation for the process. Answer The scandium(III) ion will be surrounded in aqueous solution by six water molecules. The overall ion charge can be reduced if a ion is lost to the solvent: 1 2 311 2 1 2 Δ 1 2 1 2 211 2 11 2 ■ [Sc OH2 6] aq 1 H2O l [Sc OH2 5 OH ] aq 1 H3O aq

Yttrium Yttrium is found in the same ores that contain lanthanoids. The fi rst discovery of a rare earth mineral was near the town of Ytterby in , as the names of several of these elements testify: yttrium, , , and . As we described in Chapter 1, in the future, there are likely to be increased

shortages of the rare earths. Bastnäsite, (Ce,La,Y)CO3F, is one of the ores con- taining yttrium. Yttrium is more like a lanthanoid than is scandium in that the yttrium ion is often eight-coordinated, as are many of the lanthanoid ions, whereas the small scandium ion is limited to six-coordination.

24.2 The Lanthanoids

Chemists disagree as to which group of elements actually constitutes the lanthanoids. Some claim to lutetium, while others argue to ytterbium. The problem becomes apparent when we look at the electron confi gurations (Table 24.1). Although most conventional designs of the peri- odic table show lutetium as a lanthanoid, its electron confi guration as an element actually fi ts the pattern for the third transition series: [Xe]6s24f145dn (where n is 1, in this case). However, because all 15 elements from lantha- num to lutetium share common chemical features, it makes more sense to consider them together. The only common ion for each of these elements has the charge 31, and the electron confi gurations for this ion form a simple sequence of 4f orbital fi lling from 0 to 14. Half of the lanthanoids have one or more isolable com- pound in another oxidation state. In Chapter 9, Section 9.8, we noted two lan- thanoids for which the non-31 state is particularly signifi cant: , which The lanthanoids obey the favors the 21 state, and cerium, which favors the 41 state. Oddo-Harkins rule that odd- The ionic radii of the 31 ions decrease smoothly from 117 pm for lantha- numbered– elements num to 100 pm for lutetium. Because the f orbitals do not shield the outer 5s are signifi cantly less abundant and 5p effectively, the increase in nuclear charge causes the ions to than their even-numbered decrease in size. The larger ions have a high coordination number. For exam- 31 neighbors. ple, the hydrated lanthanum ion is a nonahydrate, [La(OH2)9] . Because cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 695695 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

24.2 The Lanthanoids 695

TABLE 24.1 Ground-state electron confi gurations of elements 57 through 71 Element Atom confi guration 31 ion confi guration

Lanthanum [Xe] 6s24f05d1 [Xe] 4f0 Cerium [Xe] 6s24f15d1 [Xe] 4f1 [Xe] 6s24f3 [Xe] 4f2 [Xe] 6s24f4 [Xe] 4f3 [Xe] 6s24f5 [Xe] 4f4 [Xe] 6s24f6 [Xe] 4f5 Europium [Xe] 6s24f7 [Xe] 4f6 [Xe] 6s24f75d1 [Xe] 4f7 Terbium [Xe] 6s24f9 [Xe] 4f8 [Xe] 6s24f10 [Xe] 4f9 [Xe] 6s24f11 [Xe] 4f10 Erbium [Xe] 6s24f12 [Xe] 4f11 [Xe] 6s24f13 [Xe] 4f12 Ytterbium [Xe] 6s24f14 [Xe] 4f13 Lutetium [Xe] 6s24f145d1 [Xe] 4f14

there are differences in the chemistry between the earlier (larger) and later (smaller) lanthanoids, there are separate terms for them: the light rare earth elements, LREE (lanthanum to samarium), and the heavy rare earth ele- ments, HREE (yttrium and europium to lutetium). The LREEs predominate in the Earth’s crust, whereas the HREEs are found more commonly in the Earth’s mantle. The metals themselves are all soft and moderately dense (about 7 g?cm23), and they have melting points near 10008C. Chemically, the metals are about as reactive as the alkaline earths. For example, they all react with water to give the metal hydroxide and hydrogen gas: S 2 M(s) 1 6 H2O(l) 2 M(OH)3(s) 1 3 H2(g) The metal hydroxide dissolves in acid to give the tripositive ion: 1 S 31 M(OH)3(s) 1 3 H (aq) M (aq) 1 3 H2O(l) Tripositive cations of many of the lanthanoids are weakly colored, com- monly green, pink, and yellow. These colors are the result of electron transi- tions within the f orbitals. Unlike the d-d spectra of transition metal ions, the spectra of the lanthanoids do not show major variations for the different ligands. Furthermore, the absorptions are at very precise wavelengths, unlike the broad absorbance bands of the transition metal ions. As mentioned earlier, the common oxidation state of all the lanthanoid

elements is 13; for example, they all form oxides of the type M2O3, where M is the metal ion. The mixed oxides of neodymium and praseodymium absorb cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 696696 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

696 CHAPTER 24 / The Rare Earth and Actinoid Elements

much of the yellow range, and this pinkish tan mixture is sometimes used as a fi lter in sunglasses because the eye is most sensitive to the yellow part of the spectrum.

24.3 The Actinoids

The actinoids are all radioactive. The half-lives of the longest-lived isotopes of these elements are shown in Table 24.2. The values show that, with some irreg- ularities, there is a dramatic reduction in an isotope’s half-life as atomic num- ber increases. Obviously, the long-lived elements (, , uranium, nep- tunium, , and ) have been studied in the most detail. These metals are dense (about 15 to 20 g?cm23) and have high melting points (about 10008C). The actinoids are not as reactive as the lanthanoids; for example, they react with hot, but not cold, water to give the hydroxide and hydrogen gas.

Oxidation States of the Actinoids One might expect the chemistries of the lanthanoid and actinoid elements to be in parallel. But this is not the case. Whereas the 31 oxidation state domi- nates across the lanthanoids, for the actinoids, the situation is much more complex. In fact, for the early actinoids, the chemistries match those of the

TABLE 24.2 Half-lives of the longest-lived isotope of each actinoid element Element isotope Half-life

Actinium-227 22 years Thorium-232 1.4 3 1010 years Protactinium-231 3.3 3 104 years Uranium-238 4.5 3 109 years -237 2.2 3 106 years Plutonium-244 8.2 3 107 years Americium-243 7.4 3 103 years -247 1.6 3 107 years -247 1.4 3 103 years -251 9.0 3 102 years -252 1.3 years -257 100 days -258 51.5 days -259 58 minutes -257 3.6 hours cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 697697 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

24.3 The Actinoids 697

FIGURE 24.1 The periodic H H He table in 1941 showing the Li Be B C NOFNe early actinoids as transition metals. Elements 43, 61, 85, Na Mg Al Si PSCl A and 87 had not then been discovered, and the symbol K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr A was used for and Cb (columbium) for . Rb Sr Y Zr Cb Mo Ru Rh Pd Ag Cd In Sn Sb Te IXe

Cs Ba La Hf Ta WReOs Ir Pt Au Hg TlPb Bi Po Rn

Ra Ac Th Pa UNpPu

CePr Nd SmEu Gd Tb Dy Ho Er Tm Yb Lu

corresponding transition metal groups. This correspondence led the early chemists to place the actinoids as a continuation of 7 (see Figure 24.1). The pattern of the highest common oxidation states of the early actinoids refl ects the loss of all outer electrons, and this pattern parallels that of the tran- sition metals more than that of the lanthanoids. Figure 24.2 shows the pattern in the common oxidation states of the actinoids. For example, uranium has the electron confi guration of [Rn]7s25f36d1. Thus, the formation of the common oxidation state of 16 corresponds to an electron confi guration of [Rn]. Like the lanthanoids, formation of the 31 ion corre- sponds to the loss of the s and d electrons before those of the f orbitals. We mentioned in Chapter 9, Section 9.7, that there are similarities in oxidation states between the early actinoids and the heavier transition metals of the cor- responding groups. The ready loss of the 5f electrons by the early actinoids indicates that these electrons are much closer in energy to the 7s and 6d electrons than the 4f elec- trons are to the 6s and 5d electrons in the lanthanoids. An explanation for this difference can be found in terms of the relativistic effect that we discussed in the context of the so-called inert-pair effect. As a result of the relativistic increase in the mass of the 7s electrons, the 7s orbital undergoes a contraction. Because the electrons in the 5f and 6d orbitals are partially shielded from the nuclear attrac- tion by the 7s electrons, these orbitals expand. As a result, all three orbital sets have very similar energies. In fact, even the middle actinoids very often exhibit the 14 oxidation state, in which a second 5f electron must have been lost.

6 FIGURE 24.2 The most 5 common oxidation numbers 4 of the actinoids. 3 2 1

Oxidation number 0 Ac Th Pa U Np Pu AmCm Bk Cf Es Fm Md No Lr cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 698698 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

698 CHAPTER 24 / The Rare Earth and Actinoid Elements

WORKED EXAMPLE 24.2 Using the following Frost diagram for the early actinoids: (a) Summarize the redox chemistry of americium. (b) Contrast the redox chemistry of americium with that of uranium.

0

Ϫ1 2ϩ AmO2 Ϫ ϩ

2 AmO2 ∫

) 2ϩ

nE Ϫ PuO Ϫ 3 2 e Ϫ Ϫ 4ϩ or 4 Bk ϩ mol 3ϩ PuO F

и U 2 / 2ϩ ∫ Ϫ5 UO (V ϩ 2 G UO

⌬ 2 Ϫ6 U4ϩ 3ϩ Ϫ Cm 7 ϩ Th4 Ϫ8 0 ϩ1 ϩ2 ϩ3 ϩ4 ϩ5 ϩ6 Oxidation state

Answer (a) The thermodynamically most stable oxidation state of americium is 13 (like those of the lanthanoids). The metal itself is strongly reducing. The highest common oxidation state is 16, when americium exists as 21 1 the highly-oxidizing (AmO2) ion. The 15 oxocation, (AmO2) is thermodynamically stable, while the Am41 ion disproportionates. (b) Unlike americium (and the other early actinoids), it is uranium(IV) which is the most thermodynamically stable species. Also, although the formulas of the oxocations are the same, those of uranium are much less oxidizing. ■

Actinium As was true for lanthanum of the lanthanoids, actinium of the actinoids has the noble-gas electron confi guration for its 13 ion. Thus, actinium behaves much more as a continuation of the Group 3 elements, as can be seen from Table 24.3.

TABLE 24.3 A comparison of the pH predominance fi gures for scandium(III), yttrium(III), lanthanum(III), and actinium(III). Very Acidic Acidic Basic Very Basic 31 Scandium Sc (aq) Sc(OH)3(s) 31 Yttrium Y (aq) Y(OH)3(s) 31 Lanthanum La (aq) La(OH)3(s) 31 Actinium Ac (aq) Ac(OH)3(s) cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 699699 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

24.3 The Actinoids 699

Thorium Thorium is found in most rocks and soils, and is about four times as abundant

as . Thorium occurs in several minerals, including thorite, ThSiO4, and the cerium-rich form of , (Ce,La,Pr,Nd,Th,Y)PO4. Thorium’s chemistry is dominated by the 14 oxidation state; this oxidation state possesses the noble- gas core confi guration. As a result, thorium shows a link in behavior to cerium, the corresponding member of the lanthanoids—another example of a periodic pattern (see Chapter 9, Section 9.8). It is important to realize that the half-lives of the natural thorium (and uranium) isotopes are so long that the radiation from these elements and their compounds is quite negligible. Hence, we fi nd these elements in everyday use.

For example, thorium(IV) oxide, ThO2, mixed with 1 percent cerium(IV) oxide, converts heat energy from burning or propane to an intense light. Before the incandescent light bulb, a gauze (gas mantle) of this mixed oxide was placed around a gas fl ame to provide the major source of indoor . Even today, there is a signifi cant demand for these mantles in camping lights. Thorium(IV) oxide ceramic is also used for high-temperature reaction cruci- bles because it will withstand temperatures up to 33008C.

Americium The only actinoid element found in almost every home is americium-241. Because it has such a short half-life, americium-241 does not occur naturally, so it is obtained from nuclear reactor wastes. This isotope is at the heart of all common smoke detectors. It functions by ionizing the air in a sensing chamber, causing an electric current to fl ow. Smoke particles the fl ow of ions, and the drop in current initiates the alarm. Of increasing concern is the disposal of defunct smoke detectors—particularly in areas where incin- eration is used for garbage disposal. It is preferable to contact the manufac- turer to obtain an address to which the old unit can be shipped and the americium-241 recycled. Also, few people realize that smoke detectors have a specifi c lifetime before the radiation level decays below that necessary for the detector to function. This date is usually found on the inside of the unit in tiny print. Typically, the detector life is about 10 years.

Separation of Actinoids To separate radioactive elements, chemists often use the concept of isostruc- tural substitution (see Chapter 5, Section 5.5). For example, uranium ore con- tains very small quantities of . To separate the radium, we fi nd an anion that forms an insoluble compound with the alkaline earth Ra21 ion but that forms soluble compounds with uranium and other actinoids. Sulfate is such an anion (recall that sulfate is highly insoluble). If we tried to precipitate the radium sulfate alone, there would be so little precipitate (even if there was enough to exceed the solubility product), it would be very diffi - cult to collect. By mixing barium ion with the solution before attempting cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 700700 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

700 CHAPTER 24 / The Rare Earth and Actinoid Elements

precipitation, the radium ion is precipitated in some of the barium ion sites of the barium sulfate because the two sulfates are isostructural. This process is known as co-precipitation. With the radium now separated from the other radioactive elements, the barium-radium sulfate can be fi ltered and dried. The radium can then be isolated from the small quantity of barium (com- pared to the original large volume of uranium ore).

24.4 Uranium

Uranium is the one actinoid in large demand, primarily because of its use in nuclear reactors. For this reason, a whole section is devoted to this element. Uranium is found in several ores, including uraninite (commonly called pitch-

blende), mainly UO2 but also containing some U3O8, and oxides of lead, tho- rium, and the rare earth metals, together with decay products of uranium such as radium and . Another source of uranium is carnotite, hydrated potas-

sium uranyl vanadate, K2(UO2)2(VO4)2. The only major uses of uranium in the commercial market are due to its high density, 19.3 g?cm23, which is twice that of lead. For this reason, uranium is used in aircraft as counterweights for the heavy doors. The metal is some- times employed to shield gamma radiation. A very minor use is incorporation in some dental porcelain formulations for false teeth to simulate the fl uores- cence of natural teeth. The chemistry of uranium over much of the aqueous range relates to the 21 21 very stable uranyl ion, [UO2] , which has the linear structure [OPUPO] . 22 Under basic conditions, the diuranate ion, U2O7 , analogous to the dichro- mate ion (see Chapter 9, Section 9.7), is formed. Under less oxidizing condi-

tions, U3O8 predominates, with the crystal of this oxide actually having the 41 61 22 composition [(U )(U )2(O )8]. It is only under the most strongly reducing conditions that UO2 is favored. From the 1880s until the 1920s, household glassware could be purchased containing 2 percent uranium(VI) ion. This glass has a yellow-green opales- cence and is now popular among collectors of glass objects. Sometimes called Vaseline glass, the true uranium-containing glass can be distinguished from imitation by the fact that genuine uranium glass fl uoresces green under ultra- violet light.

Extraction of Uranium The shafts of uranium mines must be ventilated with massive volumes of fresh air to prevent the levels of radon in the mine atmosphere, released by radioac- tive decay of the uranium, from exceeding safe values. One extraction method is to treat the uraninite with an oxidizing agent, such as the iron(III) ion, to

produce uranium(VI) oxide, UO3: S 1 2 UO2(s) 1 H2O(l) UO3(s) 1 2 H (aq) 1 2 e Fe31(aq) 1 e2 S Fe21(aq) cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 701701 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

24.4 Uranium 701

Addition of sulfuric acid produces a solution of uranyl sulfate, which contains 21 the uranyl cation, UO2 : S UO3(s) 1 H2SO4(aq) (UO2)SO4(aq) 1 H2O(l) After removal of impurities, is added to the solution to give a bright

yellow precipitate of ammonium diuranate, (NH4)2U2O7: S 2 (UO2)SO4(aq) 1 6 NH3(aq) 1 3 H2O(l) (NH4)2U2O7(s) 1 2 (NH4)2SO4(aq) This precipitate, often called “yellow cake,” is the common marketable form of uranium.

The Industrial Synthesis of Uranium(VI) Fluoride For use in most types of nuclear reactors and for bomb manufacture, the two common , U-235 and U-238, must be separated. This is usu- ally accomplished by allowing gaseous uranium(VI) fl uoride to diffuse through a membrane; the lower-mass molecules containing U-235 generally pass through more quickly. Again, there are several ways to manufacture this com- pound. One route is to heat the yellow cake to give the mixed oxide uranium(IV)

uranium(VI) oxide, U3O8: Δ 9 (NH4)2U2O7(s) ¡ 6 U3O8(s) 1 14 NH3(g) 1 15 H2O(g) 1 N2(g) The mixed uranium oxide is then reduced with hydrogen to uranium(IV) oxide: S U3O8(s) 1 2 H2(g) 3 UO2(s) 1 2 H2O(g)

A Natural Fission Reactor The values that we use for elements assume The existence of this buried was not that the isotope ratio is constant. But this is not always a sign of visitors from outer space or of a previous civili- true. For example, it was variations in the atomic mass zation. Instead, it was a result of the early uranium com- values for lead—different values for lead from different position on this planet. Uranium-235 has a much shorter sources—that fi rst caused Sir Frederick Soddy, a British half-life than U-238; hence, the proportion of U-235 is chemist, to deduce the existence of isotopes. More steadily decreasing. About 2 billion years ago, when the recently, in a sample of uranium ore, only 0.296 percent Oklo nuclear reaction occurred (an event that lasted of the uranium was found to be uranium-235, much less between 2 3 105 and 1 3 106 years), there was about than the “normal” value of 0.720 percent. 3 percent U-235 in the Oklo rocks. Rainwater is believed This discrepancy might seem to be of little interest, to have leached the uranium salts into pockets, where but in 1972, it brought scientists from around the world the uranium was concentrated enough to initiate the fi s- to the mine site at Oklo in Gabon (in western Africa). sion chain reaction. Equally important, the water acted We know that the U-235 isotope spontaneously fi ssions as a moderator, slowing the emitted so that to give energy and various fi ssion products. When nuclear they could fi ssion a neighboring nucleus and continue chemists and physicists examined the chemical composi- the chain reaction. The discovery of the ancient reaction tion of the ore, they found 15 common fi ssion products. was an interesting event for scientists, even if it did not In other words, at some time in the past, Oklo had been have tabloid newspaper appeal. the site of a nuclear reaction. cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 702702 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

702 CHAPTER 24 / The Rare Earth and Actinoid Elements

The uranium(IV) oxide is treated with hydrogen fl uoride to give green

uranium(IV) fl uoride, UF4: Δ UO2(s) 1 4 HF(g) UF4(s) 1 2 H2O(g) The uranium(IV) fl uoride is then treated with difl uorine gas to oxidize the uranium to the 16 oxidation state: S UF4(s) 1 F2(g) UF6(g) In such a high oxidation state, even this fl uorine compound exhibits covalent properties, such as a sublimation temperature of about 608C. The low of uranium(VI) fl uoride is crucial to the purifi cation of uranium and its isotopic separation. Whereas uranium(IV) fl uoride melts at 9608C, uranium(VI) fl uoride sublimes at 568C. The difference can be interpreted in terms of charge . The uranium(IV) ion has a charge density of 140 C?mm–3, while that of the (theoretical) uranium(VI) ion would be 348 C?mm–3, suffi ciently polar- izing to cause covalent behavior even with fl uoride ion.

24.5 The Postactinoid Elements

The elements beyond the actinoid series are known as the postactinoid ele- ments. Even though the postactinoid elements defi nitively known so far are all transition metals, it is more instructive to consider them in this chapter because they can only be synthesized in nuclear reactions, like most of the actinoid ele- ments. The short half-lives of the postactinoid elements (Table 24.4) have made The short-lived elements with it very diffi cult to study their chemistry. Much of the claimed chemistry in the atomic numbers greater than literature is, in fact, expectations of what the chemistry should be. 100—the later actinoids and Because relativistic effects become more important as the nuclear charge the postactinoids—are becomes larger and the size of the atom increases, chemists are very eager to sometimes called the discover whether the fourth transition metal series has chemical behavior like transfermium elements. the corresponding second and third rows. So far, the parallels seem to hold. For

TABLE 24.4 Half-lives of the longest-lived isotope of some postactinoid transition elements Element isotope Half-life

Rutherfordium-263 10 minutes -262 34 seconds -266 21 seconds -272 10 seconds -269 11 minutes -276 0.72 second -281 1.1 minutes -280 3.6 seconds Copernicum-285 11 minutes cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 703703 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

24.6 Biological Aspects 703

TABLE 24.5 The hybrid Latin-Greek prefi xes for the temporary names of newly discovered elements

0 nil 5 pent 1 un 6 hex 2 bi 7 sept 3 tri 8 oct 4 quad 9 enn

example, forms a chloride, RfCl4, that seems to be similar to the chlorides of the Group 4 elements, and , in their 14 oxida- tion state. Dubnium chemistry, however, shows resemblance to that of both Group 5 transition metals, niobium(V), and the actinoid protactinium(V).

The Postactinoid Main Group Elements At the time of this writing, the names of all but elements 115, 117, and 118 have been settled. For these three elements, while the confl icting claims are being settled, the hybrid Latin-Greek numerical method devised by the International Union of Pure and Applied Chemistry (IUPAC) is used. To name an element, the is broken down into its individual digits, then the numbers are then replaced by hybrid Latin-Greek prefi xes (Table 24.5), and the ending -ium is then added. The symbol is the fi rst letter of each of the parts that con- stitute the whole name.

WORKED EXAMPLE 24.3 Chemists are hoping that the fi rst elements of Period 8 will soon be syn- thesized. Most probably elements 119 or 120, what would be their provi- sional names and symbols? Answer Element 119: the name would be produced by combining the elements un-un-enn-ium; that is, ununennium, symbol Uue. Element 120: the name would be produced by combining the elements un-bi-nil-ium; that is, unbinilium, symbol Ubn. ■

24.6 Biological Aspects

Uranium As many people are aware, there are concerns about the health risks of depleted uranium, particularly for those exposed to the uranium oxide dust produced in a battle environment. These individuals will be both transient combatants and civilians living in the vicinity. For the inhabitants, since the cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 704704 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

704 CHAPTER 24 / The Rare Earth and Actinoid Elements

FIGURE 24.3 Possible bonding model of the 21 O [UO2] ion to DNA. O O N N H U O N O N O H

uranium oxide dust will remain in the environment, contaminating the soil, they will face long-term exposure. There are two potential hazards: radioactive and chemical. Much research is still to be done, but the general consensus is that the radioactive hazard is the lesser one. In terms of gross body exposure, with an average half-life in billions of years, a-emitting uranium represents a negligible health risk. However, the dust will be absorbed on lung surfaces, where there is the potential for it to lodge and cause a long-term irradiation of the lung surface. are chemically toxic, and it is most likely that health risks from exposure to uranium oxide particles result primarily from this mode of action. On ingestion or absorption through the lung surface, uranium is metabolized to 21 the water-soluble uranyl ion, [UO2] (see Section 24.4). This ion can then be absorbed into the bloodstream. Although much of the ion, about 90 percent, is excreted through the urine, the uranyl ion can react with many biological mol- ecules, such as the atoms of a DNA strand (see Figure 24.3), phosphate functions of phospholipids and nucleic acids, and the sulfhydryl (OSH) groups of cysteine in proteins. Illness from uranium intake then seems most likely to originate through disruption of protein function.

KEY IDEAS

■ The Group 3 elements and the lanthanoids share similar ■ As a result of their very short half-lives, very little chemical behavior, such as a 13 oxidation state. chemistry of the postactinoid (transfermium) elements is known. ■ The early actinoids resemble chemically the heavy tran- sition metals of the corresponding group.

EXERCISES

24.1 Write balanced equations for the following chemical 24.3 The europium 21 ion is almost identical in size to reactions: (a) europium with water; (b) uranium(VI) oxide the ion. Which simple europium salts would you with sulfuric acid. expect to be water soluble and which insoluble? 24.2 Although 13 is the common oxidation state of the 24.4 Discuss the reasons for and against including scan- rare earth elements, europium and ytterbium can form an dium and yttrium with the lanthanoids. ion with a 12 charge. Suggest an explanation for this. What 24.5 A solution of cerium(IV) ion is acidic. Write a chem- other oxidation state might terbium adopt? ical equation to account for this. cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 705705 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd

Additional Resources 705

24.6 Calculate the enthalpy change for the reaction 24.7 Suggest a reason why the longest-lived isotopes of S actinium and protactinium have much shorter half-lives UX6(s) UX4(s) 1 X2(g), where X 5 F or Cl, given than those of thorium and uranium. ¢ ™ 21 fH (UF6(s)) 522197 kJ?mol 24.8 Suggest a reason why nobelium is the only actinoid ¢ ™ 21 fH (UF4(s)) 521509 kJ?mol for which the 12 oxidation number is most common. ¢ ™ 21 fH (UCl6(s)) 521092 kJ?mol 24.9 Convincing chemical reasons suggested that the ¢ ™ 21 early actinoids fi t with the transition metals. Give one of the fH (UCl4(s)) 521019 kJ?mol most important reasons and, in particular, mention the diu- Explain the difference in your two calculated values. ranate ion in your discussion.

BEYOND THE BASICS 24.10 Lanthanum forms only a trifl uoride, whereas cerium 24.13 Write the probable electron confi guration of forms both a trifl uoride and a tetrafl uoride. Identify the rea- meitnerium. son for the difference on the basis of Born-Haber cycle calcu- 24.14 Suggest the probable formula of the oxide of sea- lations for each of the four possibilities (LaF3, LaF4, CeF3, and borgium if it were synthesized. Explain your reasoning. 21 CeF4). Assume the MX3 lattice energy to be 25000 kJ?mol 21 24.15 In weapon projectiles, uranium is alloyed with tita- and the MX4 lattice energy to be 28400 kJ?mol . Obtain the other necessary values from the data tables in the appropriate nium. Explain why this combination of metals will make a appendices. satisfactory alloy. 24.11 Using the Frost diagram in Worked Example 24.2, 24.16 An isotope of element 112, ununbium, is formed by comment on the redox chemistry of berkelium. the nuclear reaction of lead-208 with -70. If the other product is a , what isotope of Uub was formed? 21 1 24.12 The standard potential for the PuO2 (aq)/PuO2 (aq) Why was zinc-70 chosen as a “projectile” over the more 1 1 half-reaction is 1.02 V, while that for the PuO2 (aq)/ common isotopes of zinc? Pu41(aq) half-reaction is 11.04 V. Calculate the equilibrium 1 24.17 An isotope of element 111, roentgenium, is formed constant for the disproportionation of the PuO2 (aq) ion. Under what pH conditions could the disproportionation be by the nuclear reaction of -209 with -64. If the minimized? other product is a neutron, what isotope of Rg was formed?

ADDITIONAL RESOURCES For answers to odd-number questions: www.whfreeman.com/descriptive6e cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 706706 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd