RE is the chemical periodic table of lanthanides - lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd) , terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and two elements of the lanthanide elements 15 are closely related to - scandium (Sc) and yttrium (Y) a total of 17 kinds of elements, called REE (Rare Earth). Referred to as rare earth (RE or R).
REE originally from Sweden produced relatively rare mineral found in "soil" is the prevailing habits, said water-insoluble substances, so called rare earths.
According to REE atomic electron shell structure and physical and chemical properties, and they can produce different properties in mineral symbiotic situation and characteristics of different ionic radii, seventeen kinds of rare earth elements are usually divided into two groups.
Light rare earth (also known as the cerium group) include: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium.
Heavy rare earth (also known as the yttrium group) include: terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, yttrium.