Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with symbol U and atomic number 92. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly radioactive because all its isotopes are unstable. The most common isotopes of uranium are uranium-238 (which has 146 neutrons) and uranium-235 (which has 143 neutrons). Its density is about 70% higher than that of lead, but not as dense as gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.
Cubes and cuboids of uranium photographed in the 1940s.
Image: U.S. Department of Energy (public domain).
Uranium was named after the planet Uranus, which had been discovered eight years prior to the discovery of uranium. Uranus was named in honour of the Greek god of the sky.
Uranium is more abundant and widespread than most people realise — it occurs in low levels in all rock, soil, and water, and is, for example, more abundant than silver. It is the largest element found on Earth in significant quantities. In the wild, nearly all uranium is the uranium-238 (99.27%) isotope, although there are trace amounts of naturally-occurring uranium-235 and even smaller amounts of uranium-234. Uranium is radioactive and decays by emitting alpha particles (two protons and two neutrons bound together). The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years.
Before it was discovered that uranium is radioactive, it was widely used to colour glass, and pottery and glazes.
The atomic bomb, named “Little Boy”, that was dropped on Hiroshima Japan in 1945, contained 64 kg (140 lb) of highly-enriched U-235. This bomb had an explosive energy of 16 kilotonnes of TNT, it killed an estimated 90,000–166,000 people and destroyed roughly 50,000 buildings.
After enrichment, the barely radioactive U-238 remains behind. Known as depleted uranium, it is used as shielding for radioactive materials, or as “high-density penetrators” — which is military talk for dense pointy projectiles that smash holes through otherwise impenetrable objects.
Uraninite, also known as pitchblende, is the most common ore mined to extract uranium.
A person can be exposed to uranium (or its radioactive daughters such as radon) by inhaling dust in air or by ingesting contaminated water and food. The amount of uranium in air is usually very small; however, people who work in factories that process phosphate fertilizers, live near government facilities that made or tested nuclear weapons, live or work near a modern battlefield where depleted uranium weapons have been used, or live or work near a coal-fired power plant, facilities that mine or process uranium ore, or enrich uranium for reactor fuel, may have increased exposure to uranium.
Houses or structures that are over uranium deposits (either natural or man-made slag deposits) may have an increased incidence of exposure to radon gas.
Most ingested uranium is excreted during digestion. Only 0.5% is absorbed when insoluble forms of uranium, such as its oxide, are ingested, whereas absorption of the more soluble uranyl ion can be up to 5%. However, soluble uranium compounds tend to quickly pass through the body whereas insoluble uranium compounds, especially when inhaled by way of dust into the lungs, pose a more serious exposure hazard. After entering the bloodstream, the absorbed uranium tends to bioaccumulate and stay for many years in bone tissue because of uranium’s affinity for phosphates. Uranium is not absorbed through the skin, and alpha particles released by uranium cannot penetrate the skin.
Incorporated uranium becomes uranyl ions, which accumulate in bone, liver, kidney, and reproductive tissues. Uranium can be decontaminated from steel surfaces and aquifers.
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