Robert Klug, age 13, of Peoria, Illinois, for his question:
What is meant by rare earth elements?
The Periodic Table of Elements lists the rare earths in a family of their own. In atomic structure they share family features, they look alike and they act alike. In fact, these 15 elements are so much alike that it is not easy to tell them apart and for a long time, separating them was extremely difficult.
We think of earth materials as resembling the crumbly soil. But in their pure form, the rare earth materials are silvery metals and not earthy at all. Neither are they rare. The world supply of the rarest rare earth almost equals that of iodine. Several are more plentiful than platinum and one is more plentiful than tin. The atomic family of the 15 rare earths seem to have been misnamed. However, in the last century when the family was charted, the name seemed suitable. Nature never keeps them in their silvery metallic form, but always combined with other elements in the form of rather soft, rocky ores. Some of these oxide ores do resemble crumbly earths and at first they were mistaken for the pure elements.
Modern chemistry is, of course, a precise science and a brief review of its past is necessary to explain how these fairly common metallic elements came to be classified as the family of rare earths. Naturally, you are more interested in their precise properties as known to modern science. The atoms of these elements are rather large and heavy. Their atomic numbers run from 57 to 71, their atomic weights range from 138.91 to 174.97.
The chemical properties of an atom depend largely on its shells of orbiting elec¬trons, especially its outer shell. The 15 rare earths all have five complete shells and a sixth incomplete shell of two electrons. All are softish silvery metals with a talent for adding toughness when alloyed with other metals. In nature they combine with oxygen and all 15 occur in the same mineral ores. Separating them was a costly process and the rare earths were simply extracted together. This mixture of soft, metallic elements turned out to be an exceedingly tough alloy. Since the 1950's, we have used a process called ion exchange to separate the close knit family. Different elements now can be used in precise quantities to toughen aluminum and magnesium alloys, to purify and improve steel and either to clarify or add color to glass.
The strange names of the rare earth elements were coined from a strange assort¬ment of origins. Holmium, erbium and europium are place names with Latin accents. So are terbium, thulium, ytterbium and lutetium. Neodymium, samarium and gadolinium borrow their names from other minerals. Lanthanum means "hidden" and dysprosium means "hard to get." Praseodymium means "green." Cerium is named for an asteroid and promethium for a Greek hero.
Though ion exchange processes now make it practical to exploit the talents of the separate rare earths, we still use them together in the old mixture. This alloy is called "misch metal," or "mixed metal," and we use about 600 tons of it every year. Some is added to iron to make hard flints for cigarette lighters. A mixture of rare earth metals also is combined with carbon in the durable parts required in those powerful carbon arc lights they use in lighting movie sets.