History of the Lanthanides

Lanthanides have historically been called rare earths because it was originally believed that these elements were sporadically distributed in nature. Actually, they are relatively abundant in the Earth s crust, even 

The optically active electrons involve the 4f shell, buried deep within the Xe-core of electrons. Because of the shielding effect of the outer closed shells (5s25p6), the chemistry of each lanthanide is quite similar to the others. Thus, the separation of each rare earth, free from the others, has proven to be very difficult. The most stable valence state is Ln3+, where three (3) electrons (4fi and Gs ) have been removed. Most separations were accomplished in the 1800 s by repeated fractional crystallization. Sometimes, literally thousands of recrystallization steps were required before a reasonably pure product could be Isolated and demonstrated to be pure. A number of false starts have been recorded where the investigator thought he had a pure product, only to be shown that he did not. The first rare earth to be documented, gadoliniiun - Gd, was actually named around 1805. But, the product was later shown to be a mixture of rare earths. Gd was finally isolated in a pure form some 75 years later. It was named after the Swedish investigator, J. GadoUn (1794) who characterized the first rare earth ore. 

The following table, presented on the next page, lists the lanthanides and the dates of their first isolation and characterization. Promethium is radioactive, with a very short half-life, and does not occur in Nature at all. It was first S3mthesized in an atomic reactor and is now used in human circulatory anal3 s via CAT scans when it is injected into the body. 

Y Sc are not lanthanides, but their chemical properties are so nearly identical that they occur in the same ore bodies and have the same separation difficulties as those of the lanthanides. Hence, they are included in the rare earth category. 

---