of 23.72 h. All of the remaining radioactive isotopes have half-lives of less than 3 hours, and most of these have half-lives below 8 minutes. Tungsten also has 11 meta states, with the most stable being 179mW (t1/2 6.4 minutes). Chemical properties Tungsten is a mostly non-reactive element: it does not react with water, is immune to attack by most acids and bases, and does not react with oxygen or air at room temperature. At elevated temperatures (i.e., when red-hot) it reacts with oxygen to form the trioxide compound tungsten(VI), WO3. It will, however, react directly with fluorine (F2) at room temperature to form tungsten(VI) fluoride (WF6), a colorless gas. At around 250 °C it will react with chlorine or bromine, and under certain hot conditions will react with iodine. Finely divided tungsten is pyrophoric. The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 to +6. Tungsten typically combines with oxygen to form the yellow tungstic oxide, WO3, which dissolves in aqueous alkaline solutions to form tungstate ions, . Tungsten carbides (W2C and WC) are produced by heating powdered tungsten with carbon. W2C is resistant to chemical attack, although it reacts strongly with chlorine to form tungsten hexachloride (WCl6). In aqueous solution, tungstate gives the heteropoly acids and polyoxometalate anions under neutral and acidic conditions. As tungstate is progressively treated with acid, it first yields the soluble, metastable "paratungstate A" anion, , which over time converts to the less soluble "paratungstate B" anion, . Further acidification produces the very soluble metatungstate anion, , after which equilibrium is reached. The metatungstate ion exists as a symmetric cluster of twelve tungsten-oxygen octahedra known as the Keggin anion. Many other polyoxometalate anions exist as metastable species. The inclusion of a different atom such as phosphorus in place of the two central hydrogens in metatungstate produces a wide variety of heteropoly acids, such as phosphotungstic acid H3PW12O40. Tungsten trioxide can form intercalation compounds with alkali metals. These are known as bronzes; an example is sodium tungsten bronze. In gaseous form, tungsten forms the diatomic species W2. These molecules feature a sextuple bond between tungsten atoms — the highest known bond order among stable atoms. History In 1781, Carl Wilhelm Scheele discovered that a new acid, tungstic acid, could be made from scheelite (at the time called tungsten). Scheele and Torbern Bergman suggested that it might