The Lanthanides, Rare Earth Metals

Abstract The lanthanides (or lanthanons) are a group of 15 elements of atomic numbers from 57 through 71 in which scandium (atomic number 21) and yttrium (atomic number 39) are sometimes included. The lanthanide series proper is that group of chemical elements that follow lanthanum in its group IIIB column position of the periodic table. Their distinguishing atomic feature is that they fill the 4 f electronic subshell. Actually, only those elements with atomic numbers 58–71 are lanthanides. Most chemists also include lanthanum in the series because, although it does not fill the 4 f subshell, its properties are very much like those of the lanthanides. The elements scandium and yttrium are also known as the “rare earths” because they were originally discovered together with the lanthanides in rare minerals and isolated as oxides, or “earths.” In comparison with many other elements, however, the rare earths are not really rare, except for promethium, which has only radioactive isotopes. Yttrium, lanthanum, cerium, and neodymium are all more abundant than lead in the earth's crust. All except promethium, which probably does not occur in nature, are more abundant than cadmium. The relative abundance and atomic numbers are tabulated and the more common lanthanide compounds are listed. Scandium is a silvery white metallic chemical element, the first member of the first transition‐metal series in the periodic table. The name is derived from Scandinavia, where the element was discovered in the minerals euxenite and gadolinite. In 1876, L. F. Nilson prepared about 2 g of high purity scandium oxide. It was subsequently established that scandium corresponds to the element “ekaboron,” predicted by Mendeleyev on the basis of a gap in the periodic table. Scandium occurs in small quantities in more than 800 minerals and causes the blue color of aquamarine beryl. Yttrium is one of four chemical elements (the others are erbium, terbium, and ytterbium) named after Ytterby, a village in Sweden that is rich in unusual minerals and rare earths. Yttrium is a metal with a silvery luster and properties closely resembling those of rare earth metals. It is the first member of the second series of transition metals. Yttrium is found in several minerals and is produced primarily from the ore material xenotime. Lanthanum is a white, malleable metal; it is the first member of the third series of transition metals, and the first of the rare earths. Lanthanum is found with other lanthanides in the ore minerals monazite, bastnaesite, and xenotime, and in other minerals. It was discovered in 1839 by the Swedish chemist Carl G. Mosander. Scientists have created many radioactive isotopes of lanthanum. The physical and chemical properties of the lanthanides are given. The unique characteristic of the chemistry of the lanthanides is their similarity. The elements occur together in nature in large part due to their chemical similarity. The exception is promethium, which is radioactive and probably occurs naturally only in trace amounts, if at all. The elements are extremely difficult to separate. Modern ion‐exchange and repeated fractional crystallization techniques have been developed that result in the availability of pure (99.99%) materials. The all lanthanides are silvery white, very reactive metals with high melting points. Solubility differences among ionic forms of the lanthanides seem to influence their metabolic fate in biological systems. In general, the toxicity of the lanthanides decreases as the atomic number increases, probably because of the greater solubility and ionic stability of the heavier lanthanide ions. Relatively little is known about the chemistry of scandium, even though it is not particularly rare. Its chemistry resembles that of aluminum in many ways. Yttrium is very similar to scandium. It is also an active metal. The pulmonary toxicity of inhaled lanthanides has been the subject of debate. The relative contributions of radioactive versus stable elements in the development of lanthanide‐associated progressive pulmonary interstitial fibrosis has been questioned. Although contamination of the dust of lanthanides with radioactive materials may accelerate and enhance the pathological response, depending on the form and dose of radioactivity encountered, there is little evidence to suggest that the level of radioactive contamination of occupationally encountered lanthanide dusts is sufficient to be included as a risk factor for pulmonary disease. No standards have been recommended for any of the other lanthanides because either suitable data for setting a standard, such as inhalation studies, or studies on individual lanthanides are lacking. However, because of the accumulating evidence of induction of fibrosis with the lanthanides and their expanding use, the exposure should probably be limited to 1 mg/m 3 .