Rhenium (pronounced /'ri?ni?m/) is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust. Rhenium resembles manganese chemically and is obtained as a by-product of molybdenum and copper refinement. Rhenium shows in its compounds a wide variety of oxidation states ranging from -1 to +7. Minor amounts of rhenium are added into tungsten alloys and some rhenium compounds are used as catalysts in the chemical industry. Nickel-based superalloys for the use in jet engines contain up to 6% of rhenium, making it the largest use for rhenium. Because of the low availability and the demand for jet engines, rhenium is among the most expensive metals on Earth, whose price at times exceeds US$12,000 per kilogram. Rhenium, discovered in 1925, was the last naturally occurring stable element to be discovered. Francium was the last identified naturally-occurring element, but it is unstable. Rhenium was named after the river Rhine. 188Rh and 186Rh isotopes are radioactive and are used for treatment of liver cancer. They both have similar penetration depth in tissue (5 mm for 186Rh and 11 mm for 188Rh), but 186Re has advantage of longer lifetime (90 hours vs. 17 hours). Related by periodic trends, rhenium has a similar chemistry with technetium; work done to label rhenium onto target compounds can often be translated to technetium. This is useful for radiopharmacy, where it is difficult to work with technetium - especially the 99m isotope used in medicine - due to its expense and short half-life.