Rhenium

Rhenium is one of the rarest elements in Earth's crust, at about one part per billion. It sits in group 7 with the configuration [Xe]4f14 5d5 6s2, and its seven valence electrons give it a very wide range of oxidation states, from −1 to +7. The most stable is +7, found in the perrhenate ion, ReO₄⁻, and in volatile yellow Re2O7. Strong metallic bonding gives the metal a melting point of 3186 °C, the third highest of any element, and the highest boiling point of all.

Rhenium does not form its own ore deposits. Instead it substitutes for molybdenum in molybdenite, and it is collected from the flue dust when that mineral is roasted, mostly in Chile. Most rhenium goes into nickel-based superalloys for single-crystal jet-engine turbine blades, where a few percent of rhenium slows creep at high temperatures. Platinum-rhenium catalysts are used in catalytic reforming, which converts low-octane naphtha into high-octane petrol.

Discovery

Rhenium was discovered in 1925 by German chemists Walter Noddack, Ida Tacke (later Ida Noddack) and Otto Berg, who detected its X-ray spectral lines in platinum ores and in columbite, and later in gadolinite and molybdenite. They named it after the Rhine (Latin Rhenus). In 1928 they extracted one gram of rhenium from 660 kilograms of molybdenite. It was the last stable element to be discovered.