At the end of the 18th century, people called insoluble solid oxides "clay". Rare earths are a group of metals that are difficult to dissolve in water. They were called "rare earths" because they were rare and unusual when they were first discovered. These metal oxides are called rare earth elements. Rare earth elements include all 15 elements of the lanthanide series in the periodic table, as well as scandium and yttrium from the third subgroup.
It took more than 150 years from 1794, when Finnish scientist Gadolin separated the first rare earth element, yttrium, to 1947, when Americans Marinskii and others synthesized the last rare earth element, promethium, in the laboratory, before people finally identified all 17 rare earth elements. It is now known that rare earth elements include all 15 elements of the lanthanide series: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), as well as scandium (Sc) and yttrium (Y) from the third subgroup. Because scandium and yttrium often coexist with lanthanide elements in mineral deposits and have similar chemical properties, they are also considered rare earth elements. Their common feature is that electrons first fill the outermost orbit, and then fill the 4f and 5d orbits in turn.
Rare earth elements are typical metals, with chemical reactivity second only to alkali metals and alkaline earth metals, but more reactive than other metals. Rare earth elements are soft, have a silvery-white luster and excellent electrical conductivity. Their active chemical properties enable them to form compounds with oxygen, sulfur, and halogens. They are not easy to preserve in humid air and are soluble in dilute acids.
Due to their active chemical properties, the discovery, preparation, and separation of rare earth elements are not easy. For example, the last discovered rare earth element, promethium, was carefully separated and purified by ion exchange from the products of uranium fission experiments by three scientists, Marinskii and others. At that time, people thought that this element did not exist in nature at all and was a truly "rare earth" element. It was not until 1965 that a phosphate factory in Finland discovered promethium in nature during the processing of phosphorite. Of course, its content is indeed very rare, only reaching trace levels. The content levels are ranked from high to low as large, small, trace, and trace.