cc24TheRareEarthandActinoidElements.indd24TheRareEarthandActinoidElements.indd PagePage 691691 26/09/1326/09/13 5:505:50 PMPM f-500f-500 //207/WHF00221/work/indd207/WHF00221/work/indd THE RARE EARTH AND CHAPTER 24 ACTINOID ELEMENTS Sc 24.1 The Group 3 Elements 24.2 The Lanthanoids Y 24.3 The Actinoids 24.4 Uranium 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 isotopes 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 isotope (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 density of the metal used for the shell: the greater the density, the greater the kinetic energy of the projectile. Tungsten 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 titanium, 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 iron 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 melting point 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 (scandium, yttrium, and lutetium) 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 oxidation state. They differ from their transition metal neighbors in that their only oxidation state is a d0 electron 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 hydrogen 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 Sweden, as the names of several of these elements testify: yttrium, terbium, erbium, and ytterbium. 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 cerium to lutetium, while others argue lanthanum 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: europium, 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–proton elements num to 100 pm for lutetium.