Rare earth element

Rare earth elements ("REEs") are a set of seventeen chemical elements. They include the fifteen lanthanides plus scandium and yttrium.^[2] Scandium and yttrium are rare earth elements since they are often found in the same ore deposits as the lanthanides and have similar chemical properties.

Although they are called rare, rare earth elements are not extremely rare on Earth. Cerium, one of the lanthanides, is the 25th most common element in the Earth's crust. They are called "rare" because they are spread very evenly over the Earth, so it is hard to find much in one place. Most rare earth elements are not found concentrated or in pure forms. Promethium is rare, because it is radioactive, and decays.

1. ↑ "News and events". US Department of Agriculture. Archived from the original on 2012-04-22. Retrieved 2012-03-13.
2. ↑ Connelly N.G., ed. (2005). Nomenclature of inorganic chemistry: IUPAC recommendations 2005 (PDF). Cambridge: RSC Publ. ISBN 0-85404-438-8. Archived from the original (PDF) on 2008-05-27. Retrieved 2012-03-13.