The rare earths


The 17 rare earth elements, or lanthanides, consist of elements 21 (scandium), 39 (yttrium), and 57 (lanthanum) to 71 (lutetium). The rare earths were so named because of their low concentration in minerals which were scarce. However, some of the elements are not as rare as once thought. Today, the rare earths are used in color television picture tubes, glass polishes, ceramic glazes, protective goggles, lasers, and superconductors. Most recently, some have been used in diagnostic imaging in the field of nuclear medicine.

The story of their discovery is probably the most confusing and complex of any of the elements. The search for and identification of the rare earth elements constituted an integral part of the development of science and technology in the late 19th and early 20th centuries. Several factors made their identification difficult: the chemical and physical properties of the different elements are quite similar; the elements were isolated as "earths" or oxides of the elements; and the methods of separation and purification depended on laborious fractional precipitation and crystallization techniques. Compounding the difficulties of separation were a lack of good methods for identification and assessment of purity and a lack of knowledge of the number of rare earth elements that existed.

A University of New Hampshire (UNH) professor, Charles James (1880-1928), all but forgotten today, was well known and highly respected by his contemporaries for his contribution to rare earth chemistry. The very first paper published by James in the Journal of the American Chemical Society, "A New Method for the Separation of the Yttrium Earths," indicates the direction of his research. Keenly motivated to improve the methods of separation and endowed with imagination and determination, James experimented with the preparation of many different salts of the rare earths derived from simple inorganic to complex organic acids. Letters in the UNH archives show that he supplied numerous workers worldwide with samples of the compounds that he had prepared, including the sample of terbium that Henry Moseley, an English scientist, used to determine its atomic number. James's use of bromates and double magnesium nitrates for fractional crystallization became known as the "James Method." Developed in the early 1900s, his method was used widely by others and remained the best for separating rare earth elements until the advent of ion exchange in the 1940s.

James's work with the rare earths is part of a long history that illuminates more areas of chemical progress than does any other group of elements. It stretches from the discovery of the first rare earth mineral in 1787 by Carl Axel Arrhenius to the unequivocal identification of the last in 1947 by Jacob Marinsky, Lawrence Glendenin, and Charles Coryell. The story involves the discovery of emission spectroscopy by Gustav Robert Kirchhoff and Robert Wilhelm Bunsen, the development of the periodic table by Dmitri Ivanovich Mendeleev and Lothar Meyer, and the discovery of X-rays by Wilhelm Conrad Roentgen. It includes the formulation of atomic numbers by Moseley, atomic structure by Neils Bohr and Ernest Rutherford, and work on the atomic bomb during World War II. Over the course of a century, two minerals -- ytterbite and cerite -- were separated into eight and seven stable elements, respectively, as shown in the diagram below.


Separation of the Lanthanides


 

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The rare earths | Discovery | Chemistry at the University of New Hampshire: 1866-1928
Conant Hall | Charles James | Landmark designation and acknowledgments

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