Lanthanides (Encyclopedia of Science)
The lanthanides are the chemical elements found in Row 6 of the periodic table between Groups 3 and 4. They follow lanthanum (La), element #57, which accounts for their family name. The lanthanides include the metals 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), and lutetium (Lu).
Lanthanides as rare earth elements
At one time, the lanthanides were called the rare earth elements. The name suggests that chemists once thought that the elements were present in Earth's crust in only very small amounts. As it turns out, with one exception, that assumption was not correct. (That exception is promethium, which was first discovered in the products of a nuclear fission reaction in 1945. Very small amounts of promethium have also been found in naturally occurring ores of uranium.)
The other lanthanides are relatively abundant in Earth's crust. Cerium, for example, is the twenty-sixth most abundant element. Even thulium, the second rarest lanthanide after promethium, is more abundant than iodine.
The point of interest about the lanthanides, then, is not that they are so rare, but that they are so much alike. Most of the lanthanides occur together in nature, and they are very...
(The entire section is 975 words.)
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