erbium (ûrˈbēəm) [key] [from Ytterby, a town in Sweden], metallic chemical element; symbol Er; at. no. 68; at. wt. 167.259; m.p. 1,529°C; b.p. 2,863°C; sp. gr. 9.05 at 25°C; valence +3. Erbium is a soft, malleable, lustrous, silvery metal. It is a member of the lanthanide series in Group 3 of the periodic table. With other rare earths its oxide occurs in the mineral gadolinite, found in Sweden. Natural erbium is a mixture of 6 stable isotopes; in addition, 10 radioactive isotopes are known. Erbium does not oxidize in air as rapidly as some of the other rare-earth metals. Erbia is a rose-colored oxide of erbium; it has been used to a very limited extent in glazes and glass as a coloring agent. The discovery of erbium is generally credited to Carl G. Mosander, although he did not succeed in isolating the element. In 1843 he separated from gadolinite three oxide fractions that he called yttria, erbia, and terbia. Later, what he had called terbia became known as erbia and was shown to contain five distinct rare earths, now called erbia, scandia, holmia, thulia, and ytterbia. Fairly pure erbium oxide was first isolated in 1905; fairly pure erbium was isolated in 1934.