Reflective gray metal which forms a dark green colored oxide m.p. 931 ºC; 1708 ºF b.p. 3512 ºC; 6354 ºF density (α) 6.773, (β) 6.63 g/cc; (α) 422.825, (β) 413.897 pound/cubic foot	memory peg
	1885 Carl Auer von Welsbach, Austria
	πρασιος + διδυμος (prasios + didymos) = green + twins (Greek), element with green oxide found in Didymium

After he had discovered Lanthanum in 1839, in a sample of ceria, Carl Gustav Mosander (1797-1858) became convinced that there existed a third element in this earth. Its amethyst colour was distinctly different from the white and yellow colours of pure Lanthanum and Cerium salts. Mosander did not wanted to announce his new "discovery" prematurely, but since Theodor Scheerer (1813-1875) in Norway was analyzing gadolinite and could conclude the same thing, Mosander presented his observation at the Scandinavian Meeting of Natural Scientists in Stockholm in July 1842. In fact, Scheerer presented his results at the same meeting but only vaguely suggested that yttria might contain another element.

Because it closely resembled Lanthanum and had been discovered in conjunction with it, Mosander suggested to name the new element Didymium, from the Greek διδυμος (didymos) = twins). Friedrich Wöhler, although he was a good friend of both Berzelius and Mosander, objected the name. In German, Didym sounds rather childish and silly. Also is said that he thought it had been given because Mosander had four children, all born as twins. However, Mosander did not want to change it, since he intentionally looked for a name beginning with D in order to have a symbol unlike those for other metals.
Despite this, the name Didymium was in used for over 40 years.

A number of chemists believed that Didymium was a mixture of elements and in 1879 François Lecoq de Boisbaudran showed that it contained Samarium. In 1885 the Austrian chemist Carl Auer von Welsbach (1858—1929) separated the residual Didymium after removal of Samarium, into two elements (note): Neodymium (new twin) and Praseodymium (green twin), so retaining a part of the original name, with a new suffix. Praseodymium gots its name from πρασιος [prasios] = light green, leek-green, because of the green oxyde. (cf. Neodymium). Lecoq found independently a third element in Didymium: Gadolinium.

John and Gordon Marks suggested in 1994 the name Berzelium (Bz), after Berzelius, according to them the discoverer of this element and the difficult properties (especially the chemistry of separation) of the lanthanides. The Marks brothers found the old names ugly and confusing. They offered alternative names that are equivalent contemporary (at the time and place of discovery) metaphors, both more euphonious and more memorable (note).

See also: Chronological list of discovery of the rare earths, their names in different languages etc. on the Yttrium page

• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 667-699.
• Seltene Erden. Gmelins Handbuch der anorganische Chemie, 8. Aufl.; System-Nummer 39 (1938).
• Lauri Niinistö, Swedish Contributions to the Discovery of Elements: Part 3: The Work of Mosander, Cleve and Nilson. ERES Newsletter, vol. 12, no. 1 (30 June 2001). (on-line).
• Robert Plohn, "Seltene Erden". Zeitschrift 'Die Koralle', Sept. 1929 (on-line).