They are often found in minerals with thorium, and less commonly uranium. All isotopes of promethium are radioactive, and it does not occur naturally in the earth's crust, except for a trace amount generated by spontaneous fission of uranium-238. The water-soluble compounds are mildly to moderately toxic, but the insoluble ones are not. These elements and their compounds have no biological function other than in several specialized enzymes, such as in lanthanide-dependent methanol dehydrogenases in bacteria. They react with steam to form oxides and ignite spontaneously at a temperature of 400 ☌ (752 ☏). These metals tarnish slowly in air at room temperature and react slowly with cold water to form hydroxides, liberating hydrogen. The term 'rare-earth' is a misnomer because they are not actually scarce, although historically it took a long time to isolate these elements. Scandium and yttrium are considered rare-earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties, but have different electronic and magnetic properties.
Compounds containing rare earths have diverse applications in electrical and electronic components, lasers, glass, magnetic materials, and industrial processes. The rare-earth elements ( REE), also called the rare-earth metals or rare earths or, in context, rare-earth oxides, and sometimes the lanthanides (although yttrium and scandium, which do not belong to this series, are usually included as rare earths), are a set of 17 nearly indistinguishable lustrous silvery-white soft heavy metals.
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