Uranium

History

Pitchblende, the natural oxide form of uranium, was used as early as 79 AD in the Roman Empire to color ceramic glazes yellow. In 1912, R. T. Gunther of the University of Oxford found yellow glass containing 1% uranium oxide in a Roman villa on Cape Posillipo in the Gulf of Naples, Italy. From the late Middle Ages, pitchblende was extracted from the Habsburg silver mines at Joachimsthal, Bohemia (now Jáchymov in the Czech Republic) and used in local glassmaking.

Until the early 19th century, these mines were the world's only known uranium ore sources. The element was discovered in 1789 by German chemist Martin Heinrich Klaproth in Berlin. He dissolved pitchblende in nitric acid, neutralized it with sodium hydroxide, and precipitated a yellow compound, likely sodium diuranate. Assuming this was an oxide of a new element, he heated it with charcoal to produce a black powder, which he believed was the metal but was actually an oxide.

He named the element 'Uranit' after the planet Uranus, discovered eight years earlier by William Herschel, later changing it to 'Uranium'. In 1841, Eugène-Melchior Péligot, Professor of Analytical Chemistry in Paris, isolated the first sample of uranium metal by heating uranium tetrachloride with potassium. Henri Becquerel discovered radioactivity in 1896 using uranium; in Paris, he left a sample of potassium uranyl sulfate on an unexposed photographic plate in a drawer and found the plate had been fogged by invisible rays from the uranium.

Occurrence

Uranium is present in low levels in all rock, soil, and water, and is the 48th most abundant element in the Earth's crust. Its concentration in the crust is 2 to 4 parts per million, about 40 times that of silver. The crust down to 25 km contains an estimated 10^14 kg of uranium, and the oceans contain about 10^10 kg. Soil concentrations range from 0.7 to 11 parts per million, up to 15 in farmland due to phosphate fertilizers.

Seawater contains 3 parts per billion. Uranium is more abundant than antimony, tin, cadmium, mercury, or silver, and about as abundant as arsenic or molybdenum. It occurs in hundreds of minerals, including uraninite (pitchblende), carnotite, autunite, uranophane, davidite, torbernite, and coffinite. Significant concentrations are found in phosphate rock deposits, lignite, and monazite sands; uranium is recovered commercially from sources with as little as 0.1% uranium.

Compounds

Calcined uranium yellowcake contains a distribution of uranium oxidation species; particles with short residence times in a calciner are less oxidized than those with long retention times. Uranium content is often referenced to U3O8, a standard from the Manhattan Project. The uranium-oxygen system has complex phase relationships, with the most important oxidation states being uranium(IV) and uranium(VI), corresponding to uranium dioxide (UO2) and uranium trioxide (UO3).

Other oxides include uranium monoxide (UO), diuranium pentoxide (U2O5), and uranium peroxide (UO4·2H2O). The most common forms are triuranium octoxide (U3O8) and UO2; both are solids with low water solubility and are stable over a wide range of conditions. U3O8 is the most stable uranium compound and is most common in nature, while UO2 is the typical nuclear reactor fuel. At ambient temperatures, UO3 gradually converts to U3O8.

Salts of many oxidation states are water-soluble; common ionic forms are U3+ (brown-red), U4+ (green), UO2+ (unstable), and UO2^2+ (yellow). Solid compounds like UO and US exist for uranium(II), but no simple ions of that state are known in solution.

U3+ ions liberate hydrogen from water and are highly unstable. The uranyl ion (UO2^2+) forms compounds such as uranyl carbonate, uranyl chloride, and uranyl sulfate, and complexes with organic chelating agents like uranyl acetate. Uranates, containing polyatomic uranium-oxide anions, are generally not water-soluble.

Isotopes

All uranium isotopes are radioactive, as elements with more than 82 protons have no stable isotopes. The two most stable, uranium-238 and uranium-235, are primordial radionuclides with half-lives long enough to have survived since Earth's formation. Natural uranium consists of uranium-238 (99.28% abundance), uranium-235 (0.71%), and uranium-234 (0.0054%), plus trace isotopes uranium-240, uranium-239, uranium-237, uranium-236, and uranium-233. Uranium-238 has a half-life of about 4.5 billion years, decays by alpha emission to thorium-234, and ultimately through 18 members of the uranium series to lead-206.

It is not fissile but is fertile: it can absorb a neutron to become uranium-239, which decays by beta emission to neptunium-239, then to plutonium-239. Plutonium-239 was used as fissile material in the first atomic bomb detonated in the Trinity test on 16 July 1945 in New Mexico. Uranium-235 is fissile and can sustain a nuclear chain reaction with slow neutrons.

Did You Know?

• Uranium metal has a Mohs hardness of 6, sufficient to scratch glass, and a density of 19.1 g/cm³, denser than lead but slightly less than tungsten and gold.
• People working in factories processing phosphate fertilizers, near nuclear weapons test sites, or near coal-fired power plants may have increased exposure to uranium.