Historically the term ‘rare earths’ has been applied to the lanthanide group of elements, which range from Lanthanum (atomic number 57) to Lutetium (atomic number 71), plus Scandium (atomic number 21) and Yttrium (atomic number 39), which have similar properties.
The Rare Earth Elements posses varying ionic radii, which produce different properties, therefore, have been broadly classified into two groups: Heavy Rare Earth Elements (HREE) and Light Rare Earth Elements (LREE).
Light REE’s are the ceric sub-group made up of the first six elements of the lanthanide series. They are as follows: Lanthanum (La, atomic number 57), Cerium (Ce, atomic number 58), Praseodymium (Pr, atomic number 59), Neodymium (Nd, atomic number 60), Promethium (Pm, atomic number 61), and Samarium (Sm, atomic number 62). Promethium is not found naturally within the earth’s crust, it only occurs naturally from radioactive decay of Europium-151 or Uranium.
Heavy REE’s, which typically have high value relative to other REE’s, are the following higher atomic numbered elements from the lanthanide series: Europium (EU, atomic number 63), Gadolinium (Gd, atomic number 64), Terbium (Tb, atomic number 65), Dysprosium (Dy, atomic number 66), Holmium(Ho, atomic number 67), Erbium (Er, atomic number 68), Thulium (Tm, atomic number 69), Ytterbium(Yb, atomic number 70) and Lutetium (Lu, atomic number 71). Often included as a Heavy REE due to similar chemical properties are Scandium (Sc, atomic number 21) and Yttrium (Y, atomic number 39).