rare-earth metals, in chemistry, group of metals including those of the lanthanide series and actinide series, usually yttrium, sometimes scandium and thorium, and rarely zirconium. Promethium, which is not found in nature, is not usually considered a rare-earth metal. The metals usually occur together in minerals as their oxides (rare earths) and are somewhat difficult to separate because of their chemical similarity. A subgroup of the rare-earth metals, consisting of those with atomic numbers between 57 and 63 and ytterbium, is often called the cerium metals. Misch metal is an alloy of the cerium metals often used in lighter flints, in alloys with other metals (especially magnesium), and to remove residual gases in the manufacture of vacuum tubes. Individual metals may be isolated as their compounds by ion exchange methods, solvent extraction, or fractional crystallization, and chemically or electrolytically reduced to the pure metal. Uses are discussed in articles on individual elements.

rare gas: see inert gas.

rare earths, in chemistry, oxides of the rare-earth metals. They were once thought to be elements themselves. They are widely distributed in the earth's crust and are fairly abundant, although they were once thought to be very scarce. Generally, the name of an earth is formed from the name of its element by replacing -um with -a; e.g., the earth of cerium is ceria. Mixed rare earths are used in glassmaking, ceramic glazes, glass-polishing abrasives, carbon arc-light electrode cores, and catalysts for petroleum refining. Individual purified rare earths have many uses, e.g., in lasers and as color-television picture tube phosphors. Important rare-earth minerals include bastnasite, cerite, euxenite, gadolinite, monazite, and samarskite.

Any of a large class of chemical elements including scandium (atomic number 21), yttrium (39), and the 15 elements from 57 (lanthanum) to 71 (see lanthanides). The rare earths themselves are pure or mixed oxides of these metals, originally thought to be quite scarce; however, cerium, the most plentiful, is three times as abundant as lead in the Earth's crust. The metals never occur free, and the pure oxides never occur in minerals. These metals are similar chemically because their atomic structures are generally similar; all form compounds in which they have valence 3, including stable oxides, carbides, and borides.

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or inert gas

Any of the seven chemical elements that make up the rightmost group of the periodic table as usually arranged: helium, neon, argon, krypton, xenon, radon, and element 118. All are colourless, odourless, and nonflammable and, except for element 118, occur in tiny amounts in the atmosphere (though helium is the most plentiful element in the universe after hydrogen). Their stable electronic configurations, with no unpaired electrons to share, make them extremely unreactive—hence “noble” (i.e., aloof) or inert—though krypton, xenon, and radon, with outer electrons held less firmly, can form compounds (mainly with fluorine). These gases absorb and give off electromagnetic radiation in a much less complex way than other substances, a property exploited in their use in fluorescent lighting devices and discharge lamps. They glow with a characteristic colour when confined in a transparent container at low pressure with an electric current passing through it. Their very low boiling and melting points make them useful as refrigerants for low-temperature research (see cryogenics).

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