ytterbium (ĭtûrˈbēəm) [key] [for Ytterby, a town in Sweden], metallic chemical element; symbol Yb; at. no. 70; at. wt. 173.054; m.p. 819°C; b.p. about 1,194°C; sp. gr. about 7.0; valence +2 or +3. Ytterbium is a soft, malleable, ductile, lustrous silver-white metal. Although it is one of the rare-earth metals of the lanthanide series in Group 3 of the periodic table, in some of its chemical and physical properties it more closely resembles calcium, strontium, and barium. It exhibits allotropy; at room temperature a face-centered cubic crystalline form is stable. The metal tarnishes slowly in air and reacts slowly with water but rapidly dissolves in mineral acids. It forms numerous compounds, some of which are yellow or green. The oxide (ytterbia, Yb2O3) is colorless. It is widely distributed in a number of minerals, e.g., gadolinite, and is recovered from monazite but has no commercial uses. Its discovery is credited to J. C. G. de Marignac, who in 1878 separated a substance he called ytterbia. In 1907, Georges Urbain showed that this substance contained lutetium in addition to ytterbium. At about this same time C. A. von Welsbach independently discovered ytterbium and called it aldebaranium.