Lutetium, lutecium, cassiopeium—different names and spellings for the same chemical element

Lutetium (pronounced ‘loo-tee-shi-uhm’) is a metallic chemical element—the last member of the lanthanide series. The name derives from Lutetia, the ancient name for Paris [1], where it was discovered (in not so ancient times) in 1907 by G. Urbain. Independently, it was discovered by C. James at the University of New Hampshire, USA [2] and by Carl Auer von Welsbach in Austria [3]. Lutecium is an older spelling, which was changes to lutetium in 1949.

The atomic number of lutetium is 71 and the atomic symbol is Lu. However, one may also find the symbol Cp in the (older) literature and table works. Cp derives from cassiopeium, the name given by von Welsbach in reference to the northern-sky star constellation Cassiopeia. The element was called cassiopeium by German-speaking chemists during the first half of the twentieth century and frequently occurs in its traditional German spelling: Kassiopeium.

The key isotopes of lutetium are ¹⁷⁵Lu and ¹⁷⁶Lu with natural abundance values of 97.4% and 2.59% [2], respectively. Compare these values with the relative abundance given in a German-language publication [4]: ¹⁷⁵Cp:¹⁷⁶Cp=100:2,58±0.07 (notice the use of symbol Cp and of a comma as decimal point).

Keywords: chemistry, history, nomenclature, rare-earth elements (German: Seltene Erden)

References and museum tour
[1] Handbook of Chemistry and Physics, 88th Edition, 2007-2008; Section 4: the elements.
[2] John Emsley: The Elements. Third Edition. Oxford University Press, Oxford, UK, 1998.
[3] Dr. Carl Freiherr Auer von Welsbach: www.althofen.at/AvW_Museum/Seiten_e/rundgang_e.html.
[4] J. Mattauch and H. Lichtblau: Ein bemerkenswertes Isotop des Cassiopeiums. Z. Phys. A 1938, 111 (7-8), pp. 514-521. DOI: 10.1007/BF01329513.