Erbium, UR bee uhm (chemical symbol, Er), is one of the rare earth metals. Its atomic number (number of protons in its nucleus) is 68. It has a relative atomic mass of 167.259. An element's relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of carbon 12, the most abundant form of carbon. Erbium had six known isotopes, forms with the same number of protons but different numbers of neutrons. The Swedish scientist Carl Mosander discovered it in 1843. Erbium is found associated with the heavier rare earths, such as europium and gadolinium, chiefly in the yttrium minerals. Erbium of high purity is available commercially. The metal has a grayish-silver color. It melts at 1529 °C and boils at 2868 °C. It has a density of 9.045 grams per cubic centimeter at 25 °C. Erbium forms a rose-pink oxide used to make pink glass. See also Rare earth; Yttrium.