rare earth element

Rare earth elements and rare earth metals are a collection of sixteen chemical elements in the periodic table, namely scandium, yttrium, and fourteen of the fifteen lanthanoids (excluding promethium), which naturally occur on the Earth. The former two are included as they tend to occur with the latter in the same ore deposits. Some definitions additionally include the actinoids. The terms "rare earth" and "rare earth metal" are trivial names that fall outside the official IUPAC nomenclature system. Outside of a strict scientific context, however, the terms retain their usability; for instance, the United States Patent and Trademark Office (USPTO) includes the term "rare earth" in the classification of metal alloys and other compounds, as well as distinguishing rare-earth magnets from other types of magnet.
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Noun
1. any element of the lanthanide series (atomic numbers 57 through 71)
(synonym) rare earth, lanthanoid, lanthanide, lanthanon
(hypernym) group, grouping
(hyponym) terbium metal

A metallic element that belongs to Group 3B or to the lanthanide series.

n. ธาตุหายาก (syn. rare-earth metal)