Rare Earths, the group of elements with atomic numbers ranging from 57 through 71. The rare earths are neither earths nor, as a group, particularly rare. They are metals, and are known to occur in considerable quantities in at least 200 minerals. The least abundant member of the group is promethi-um, which occurs only as a fission product in nuclear reactors. These elements were called rare because at the time of their discovery they were believed to be scarce. The name earths was used because the oxides in which these elements are found resemble limestone, magnesia, and similar substances that were once called earths.
Rare earths are more properly called rare-earth elements, or rare-earth metals. They are also referred to as the lanthanide series, after lanthanum, the first member of the group. Individual members of the group are called lanthanides. The lanthanide series occurs between barium and hafnium in the Periodic Table. All members of the series have chemical properties similar to those of lanthanum.
