Rare earth elements are defined as the 15 lanthanoid elements plus scandium and yttrium, and their position in the periodic table is shown in Figure 10.2. However, of these 17 elements, promethium is radioactive and is rarely found in nature and scandium is rarely found together with the rest of the REEs. Rare earth elements possess differing ionic radii, producing varying properties, and have been broadly classified into two groups: heavy rare earth elements (HREEs) and light rare earth elements (LREEs).
Figure 10.2 Rare earth elements in the periodic table
Source: US Geological Survey Fact Sheet 087–02