The Element Uranium
Atomic Number: 92
Atomic Weight: 238.02891
Melting Point: 1408 K (1135°C or 2075°F)
Boiling Point: 4404 K (4131°C or 7468°F)
Density: 18.95 grams per cubic centimeter
Phase at Room Temperature: Solid
Element Classification: Metal
Period Number: 7
Group Number: none
Group Name: Actinide
Special Notes: Radioactive
What's in a name? Named for the planet Uranus.
Say what? Uranium is pronounced as you-RAY-nee-em.
History and Uses:
Uranium was discovered by Martin Heinrich Klaproth, a German chemist, in the mineral pitchblende (primarily a mix of uranium oxides) in 1789. Although Klaproth, as well as the rest of the scientific community, believed that the substance he extracted from pitchblende was pure uranium, it was actually uranium dioxide (UO2). After noticing that 'pure' uranium reacted oddly with uranium tetrachloride (UCl4), Eugène-Melchoir Péligot, a French chemist isolated pure uranium by heating uranium dioxide with potassium in a platinum crucible. Radioactivity was first discovered in 1896 when Antoine Henri Becquerel, a French physicist, detected it from a sample of uranium. Today, uranium is obtained from uranium ores such as pitchblende, uraninite (UO2), carnotite (K2(UO2)2VO4·1-3H2O) and autunite (Ca(UO2)2(PO4)2·10H2O) as well as from phosphate rock (Ca3(PO4)2), lignite (brown coal) and monazite sand ((Ce, La, Th, Nd, Y)PO4). Since there is little demand for uranium metal, uranium is usually sold in the form of sodium diuranate (Na2U2O7·6H2O), also known as yellow cake, or triuranium octoxide (U3O8).
Since it is naturally radioactive, uranium, usually in the form of uranium dioxide (UO2), is most commonly used in the nuclear power industry to generate electricity. Naturally occurring uranium consists of three isotopes: uranium-234, uranium-235 and uranium-238. Although all three isotopes are radioactive, only uranium-235 is a fissionable material that can be used for nuclear power.
When a fissionable material is struck by a neutron, its nucleus can release energy by splitting into smaller fragments. If some of the fragments are other neutrons, they can strike other atoms and cause them to split as well. A fissionable material, such as uranium-235, is a material capable of producing enough free neutrons to sustain a nuclear chain reaction.
Only 0.7204% of naturally occurring uranium is uranium-235. This is too low a concentration to sustain a nuclear chain reaction without the help of a material known as a moderator. A moderator is a material that can slow down a neutron without absorbing it. Slow neutrons are more likely to react with uranium-235 and reactors using natural uranium can be made using graphite or heavy water as a moderator. Methods also exist for concentrating uranium-235. Once the levels of uranium-235 have been increased to about 3%, normal water can be used as a moderator.
Uranium-238, uranium's most common isotope, can be converted into plutonium-239, a fissionable material that can also be used as a fuel in nuclear reactors. To produce plutonium-239, atoms of uranium-238 are exposed to neutrons. Uranium-239 forms when uranium-238 absorbs a neutron. Uranium-239 has a half-life of about 23 minutes and decays into neptunium-239 through beta decay. Neptunium-239 has a half-life of about 2.4 days and decays into plutonium-239, also through beta decay.
Although it does not occur naturally, uranium-233 is also a fissionable material that can be used as a fuel in nuclear reactors. To produce uranium-233, atoms of thorium-232 are exposed to neutrons. Thorium-233 forms when thorium-232 absorbs a neutron. Thorium-233 has a half-life of about 22 minutes and decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of about 27 days and decays into uranium-233, also through beta decay. If completely fissioned, one pound (0.45 kilograms) of uranium-233 will provide the same amount of energy as burning 1,500 tons (1,350,000 kilograms) of coal.
Uranium is a dense metal that has uses outside of the nuclear power industry. It is used as a target for X-ray production, as ammunition for some types of military weaponry, as a shield against radiation, as a counterweight for aircraft control surfaces and in the gyroscopes of inertial guidance systems.
Uranium compounds have been used for centuries to color glass. A 2,000 year old sample of yellow glass found near Naples, Italy contains uranium oxide. Uranium trioxide (UO3) is an orange powder and has been used in the manufacture of Fiestaware plates. Other uranium compounds have also been used to make vaseline glass and glazes. The uranium within these items is radioactive and should be treated with care.
Estimated Crustal Abundance: 2.7 milligrams per kilogram
Estimated Oceanic Abundance: 3.2×10-3 milligrams per liter
Number of Stable Isotopes: 0 (View all isotope data)
Ionization Energy: 6.194 eV
Oxidation States: +6, +5, +4, +3
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10 5f3
6s2 6p6 6d1