Rare earth metals, also known as rare earth elements, are the general name of 17 elements of scandium, yttrium and lanthanide in Group IIB of the periodic table of elements, which are commonly expressed by R or re. From the discovery of the first rare earth element yttrium in 1794 to the discovery of the rare earth element promethium in the nature in 1972, it took 178 years for people to find all 17 rare earth elements in the nature. The luster of rare earth metals is between silver and iron. Rare earth metals have strong chemical activity.