（初めに右欄で英語を１つ選び、続いて選択肢から１つを選んでください。やり直すときは、英語を選び直すことから始めてください。右の［単語帳］を参考にすることもできます。）As defined by IUPAC, rare earth elements or rare earth metals are a collection of seventeen chemical elements in the periodic table, specifically the fifteen lanthanoids plus scandium and yttrium. Scandium and yttrium are considered rare earth elements since they tend to occur in the same ore deposits as the lanthanoids and exhibit similar chemical properties.
Despite their name, rare earth elements (with the exception of the radioactive promethium) are relatively plentiful in the Earth's crust, with cerium being the 25th most abundant element at 68 parts per million (similar to copper). However, because of their geochemical properties rare earth elements are typically dispersed and not often found in concentrated and economically exploitable forms known as rare earth minerals.
It was the very scarcity of these minerals (previously called "earths") that led to the term "rare earth". The first such mineral discovered was gadolinite, a compound of cerium, yttrium, iron, silicon and other elements. This mineral was extracted from a mine in the village of Ytterby in Sweden; many of the rare earth elements bear names derived from this location.