water. ions exhibit a bright blue luminescence. Because it occurs late in the series, the +2 oxidation state can also exist, stabilized by the nearly full 4f electron shell, but occurs only in solids. Thulium's only known oxide is . This oxide is sometimes called "thulia". Reddish-purple thulium(II) compounds can be made by the reduction of thulium(III) compounds. Examples of thulium(II) compounds include the halides (except the fluoride). Some hydrated thulium compounds, such as and are green or greenish-white. Thulium dichloride reacts very vigorously with water. This reaction results in hydrogen gas and exhibiting a fading reddish color. Combination of thulium and chalcogens results in thulium chalcogenides. Thulium reacts with hydrogen chloride to produce hydrogen gas and thulium chloride. With nitric acid it yields thulium nitrate, or . Isotopes The isotopes of thulium range from to . The primary decay mode before the most abundant stable isotope, , is electron capture, and the primary mode after is beta emission. The primary decay products before are element 68 (erbium) isotopes, and the primary products after are element 70 (ytterbium) isotopes. Thulium-169 is thulium's only primordial isotope and is the only isotope of thulium that is thought to be stable; it is predicted to undergo alpha decay to holmium-165 with a very long half-life. The longest-lived radioisotopes are thulium-171, which has a half-life of 1.92 years, and thulium-170, which has a half-life of 128.6 days. Most other isotopes have half-lives of a few minutes or less. Thirty-five isotopes and 26 nuclear isomers of thulium have been detected. Most isotopes of thulium lighter than 169 atomic mass units decay via electron capture or beta-plus decay, although some exhibit significant alpha decay or proton emission. Heavier isotopes undergo beta-minus decay. History Thulium was discovered by Swedish chemist Per Teodor Cleve in 1879 by looking for impurities in the oxides of other rare earth elements (this was the same method Carl Gustaf Mosander earlier used to discover some other rare earth elements). Cleve started by removing all of the known contaminants of erbia (). Upon additional processing, he obtained two new substances; one brown and one green. The brown substance was the oxide of the element holmium and was named holmia by Cleve, and the green substance was the oxide of an unknown element. Cleve named the oxide thulia and its element thulium after Thule, an Ancient Greek place name associated with Scandinavia or Iceland.