Lanthanides

Although once called the rare earths, most lanthanides are not particularly rare in the earth's crust. Today, with the exception of promethium, the lanthanides are known to have abundances comparable to many other elements. The 15 elements, together with their chemical symbols, are 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), and lutetium (Lu). Thulium, one of the scarcest lanthanides, has an abundance in the earth's crust of 0.2 parts per million (ppm), and is more abundant than arsenic or mercury. The most abundant is cerium (46 ppm), which is more abundant than tin. Promethium, which is radioactive, is found only in trace amounts in uranium ores. Small amounts have been isolated from the spent fuel of nuclear reactors. The lanthanide elements, cerium through lutetium, have corresponding atomic numbers of 58 through 71.

The discovery of the lanthanides spanned more than a century of work, beginning in the late 1700s. In 1794, the Finnish chemist Johan Gadolin (1760-1852) studied ytterbia, which he believed was a new element. More than a decade later, the English chemist Sir Humphry Davy (1778-1829) showed that ytterbia was a compound, composed of oxygen and a metal, rather than an element. Because many of the lanthanides occur together in the same minerals, and due to their similar properties, separation of the lanthanides proved a challenge to nineteenth century chemists. This often led to confusion, since it was difficult to distinguish one element from another or from its mineral precursor. The mid-nineteenth century invention of the spectroscope, an instrument that measures light emission and absorption from heated substances, assisted with unravelling lanthanide identification. With this instrument it is possible to analyze light from the Sun and the stars, and we now know that lanthanides are present in other parts of our solar system and even beyond it.