|
|
| General |
| Name, symbol,
number |
uranium, U, 92 |
| Chemical series |
actinides |
| Group, period,
block |
n/a, 7,
f |
| Appearance |
silvery gray metallic;
corrodes to a spalling
black oxide coat in air
 |
| Standard atomic weight |
238.02891(3) g·mol−1 |
| Electron configuration |
[Rn] 5f3 6d1 7s2 |
| Electrons per shell |
2, 8, 18, 32, 21, 9, 2 |
| Physical properties |
| Phase |
solid |
| Density (near r.t.) |
19.1 g·cm−3 |
| Liquid density at m.p. |
17.3 g·cm−3 |
| Melting point |
1405.3 K
(1132.2 °C, 2070 °F) |
| Boiling point |
4404 K
(4131 °C, 7468 °F) |
| Heat of fusion |
9.14 kJ·mol−1 |
| Heat of vaporization |
417.1 kJ·mol−1 |
| Heat capacity |
(25 °C) 27.665 J·mol−1·K−1 |
Vapor pressure
| P/Pa |
1 |
10 |
100 |
1 k |
10 k |
100 k |
| at T/K |
2325 |
2564 |
2859 |
3234 |
3727 |
4402 |
|
| Atomic properties |
| Crystal structure |
orthorhombic |
| Oxidation states |
3+,4+,5+,6+[57]
(weakly basic oxide) |
| Electronegativity |
1.38 (Pauling scale) |
| Ionization energies |
1st: 597.6 kJ/mol |
| 2nd: 1420 kJ/mol |
| Atomic radius |
175 pm |
| Van der Waals radius |
186 pm |
| Miscellaneous |
| Magnetic ordering |
paramagnetic |
| Electrical resistivity |
(0 °C) 0.280 µΩ·m |
| Thermal conductivity |
(300 K) 27.5 W·m−1·K−1 |
| Thermal expansion |
(25 °C) 13.9 µm·m−1·K−1 |
| Speed of sound (thin rod) |
(20 °C) 3155 m/s |
| Young's modulus |
208 GPa |
| Shear modulus |
111 GPa |
| Bulk modulus |
100 GPa |
| Poisson ratio |
0.23 |
| CAS registry number |
7440-61-1 |
| Selected isotopes |
|
|
| References |
|
Uranium (IPA: /jʊˈreɪniəm/)is a
white/black metallic chemical element in the
actinide series of the periodic table that has the
symbol U and atomic number 92. It has 92
protons and electrons, 6 of them valence electrons. It can have between 141 and 146 neutrons, with 143 and 146 in its most common isotopes. Uranium has the highest atomic weight of the
naturally occurring elements (see plutonium). Uranium is approximately 70% more
dense than lead and is weakly radioactive. It occurs naturally in low concentrations (a few parts per million) in soil, rock and water, and is commercially extracted from uranium-bearing
minerals such as uraninite (see uranium mining).
In nature, uranium atoms exist as uranium-238 (99.275%), uranium-235 (0.711%), and a very small amount of uranium-234 (0.0058%).
Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of
uranium-235 is 704 million years,[1] making them useful in dating the age of the Earth (see
uranium-thorium dating, uranium-lead
dating and uranium-uranium dating). Along with thorium and plutonium, uranium is one of the three fissile elements, meaning it can easily break apart to become lighter elements. While uranium-238 has a
small probability to fission spontaneously or when bombarded with fast neutrons, the much higher probability of uranium-235 and
to a lesser degree uranium-233 to fission when bombarded with slow neutrons generates the
heat in nuclear reactors used as a source of power, and provides the fissile
material for nuclear weapons. Both uses rely on the ability of uranium to produce a
sustained nuclear chain reaction. Depleted
uranium (uranium-238) is used in kinetic energy penetrators and
armor plating.[2]
Uranium is used as a colorant in uranium glass, producing orange-red to lemon yellow
hues. It was also used for tinting and shading in early photography. The 1789
discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, who
named the new element after the planet Uranus. Eugène-Melchior Péligot was the first person to isolate the metal, and its radioactive
properties were uncovered in 1896 by Antoine Becquerel. Research by Enrico Fermi and others starting in 1934 led to its use as a fuel in the nuclear power industry and in
Little Boy, the first
nuclear weapon used in war. An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of nuclear weapons that used enriched uranium and uranium-derived plutonium. The security of those weapons and their fissile
material following the breakup of the Soviet Union in 1991 along
with the legacy of nuclear testing and nuclear accidents is a concern for public health and safety.
Characteristics
An induced nuclear fission event involving uranium-235
When refined, uranium is a silvery white, weakly radioactive metal, which is slightly softer than steel,[3] strongly electropositive and a poor
electrical conductor.[4] It is malleable, ductile, and slightly paramagnetic.[3] Uranium metal has very high density, being
approximately 70% more dense than lead, but slightly less dense than gold.
Uranium metal reacts with nearly all nonmetallic elements and their compounds, with
reactivity increasing with temperature.[5] Hydrochloric and nitric acids dissolve uranium, but nonoxidizing acids attack the element very slowly.[4] When finely divided, it can react with
cold water; in air, uranium metal becomes coated with a dark layer of uranium oxide.[3] Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry.
Uranium was the first element that was found to be fissile. Upon bombardment with
slow neutrons, its uranium-235 isotope becomes a very short-lived uranium-236 isotope, which immediately divides into two smaller
nuclei, releasing nuclear binding energy and more
neutrons. If these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain
reaction occurs and, if there is nothing to absorb some neutrons and slow the reaction, the reaction is explosive. As
little as 15 lb (7 kg) of uranium-235 can be used to make an atomic bomb.[6] The first atomic bomb worked by this principle (nuclear fission).
Applications
Military
The major application of uranium in the military sector is in high-density penetrators. This ammunition consists of
depleted uranium (DU) alloyed with 1–2% other elements. At high impact speed, the
density, hardness, and flammability of the projectile enable destruction of heavily armored targets. Tank armor and the removable
armor on combat vehicles are also hardened with depleted uranium (DU) plates. The use of DU became a contentious
political-environmental issue after the use of DU munitions by the US, UK and other countries during wars in the Persian Gulf and
the Balkans raised questions of uranium compounds left in the soil (see Gulf War
Syndrome).[6]
Depleted uranium is also used as a shielding material in some containers used to store and transport radioactive
materials.[4] Other uses of DU
include counterweights for aircraft control surfaces, as ballast for missile re-entry
vehicles and as a shielding material.[3] Due
to its high density, this material is found in inertial guidance devices and
in gyroscopic compasses.[3] DU is preferred over similarly dense metals due to its ability to be easily
machined and cast as well as its relatively low cost.[7] Counter to popular belief, the main risk of exposure to DU is chemical poisoning by
uranium oxide rather than radioactivity (uranium being only a weak alpha emitter).
During the later stages of World War II, the entire Cold
War, and to a much lesser extent afterwards, uranium was used as the fissile explosive material to produce
nuclear weapons. Two major types of fission bombs were built: a relatively simple device
that uses uranium-235 and a more complicated mechanism that uses uranium-238-derived plutonium-239. Later, a much more complicated and
far more powerful fusion bomb that uses a plutonium-based device in a uranium casing to cause a mixture of tritium and deuterium to undergo nuclear
fusion was built.[8]
Civilian
The most visible civilian use of uranium is as the thermal power source used in
nuclear power
plants.
The main use of uranium in the civilian sector is to fuel commercial nuclear power
plants; by the time it is completely fissioned, one kilogram of uranium can theoretically produce about
20 trillion joules of energy
(20×1012 joules); as much electricity as 1500 tonnes of coal.[2] Generally this is in the form of enriched
uranium, which has been processed to have higher-than-natural levels of uranium-235 and can be used for a variety of
purposes relating to nuclear fission.
Commercial nuclear power plants use fuel that is typically enriched to around 3%
uranium-235,[2] though
some reactor designs (such as the CANDU reactors) can use unenriched uranium fuel. Fuel
used for United States Navy reactors is typically highly enriched in uranium-235 (the
exact values are classified). In a breeder
reactor, uranium-238 can also be converted into plutonium through the following
reaction:[3] 238U(n, gamma) →
239U -(beta) → 239Np -(beta) → 239Pu.
Prior to the discovery of radiation, uranium was primarily used in small amounts for yellow
glass and pottery dyes (such as uranium glass and in Fiestaware). Uranium was also used in photographic chemicals
(esp. uranium nitrate as a toner),[3] in lamp filaments, to improve the appearance of dentures, and in the leather and wood industries for stains and dyes. Uranium salts are mordants of silk or wool. Uranyl acetate and uranyl formate are used as stains in transmission electron microscopy, to increase the contrast of biological specimens in
ultrathin sections and in negative staining of viruses, isolated cell organelles and macromolecules.
The discovery of the radioactivity of uranium ushered in additional scientific and practical uses of the element. The long
half-life of the isotope uranium-238 (4.51×109 years) makes it well-suited for use
in estimating the age of the earliest igneous rocks and for other types of radiometric dating (including uranium-thorium dating
and uranium-lead dating). Uranium metal is used for X-ray targets in the making of high-energy X-rays.[3]
History
Pre-discovery use
The use of uranium in its natural oxide form dates back to at least the year 79, when it was
used to add a yellow color to ceramic glazes.[3] Yellow glass with 1% uranium oxide was found in a Roman
villa on Cape Posillipo in the Bay of Naples,
Italy by R. T. Gunther of the University of Oxford
in 1912.[9] Starting in
the late Middle Ages, pitchblende was extracted from the
Habsburg silver mines in Joachimsthal,
Bohemia (now Jáchymov in the Czech Republic) and was
used as a coloring agent in the local glassmaking industry.[10] In the early 19th century, the world's only known source
of uranium ores were these old mines.
Discovery
The discovery of the element is credited to the German chemist
Martin Heinrich Klaproth. While he was working in his experimental laboratory
in Berlin in 1789, Klaproth was able to precipitate a yellow compound (likely sodium diuranate) by dissolving pitchblende in nitric acid and
neutralizing the solution with sodium hydroxide.[10] Klaproth mistakenly assumed the yellow substance was the
oxide of a yet-undiscovered element and heated it with charcoal to obtain a black powder, which
he thought was the newly discovered metal itself (in fact, that powder was an oxide of uranium).[10][11] He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier by William
Herschel.[12]
In 1841, Eugène-Melchior Péligot, who was Professor of Analytical Chemistry
at the Conservatoire National des Arts et Métiers (Central
School of Arts and Manufactures) in Paris, isolated the first sample of uranium
metal by heating uranium tetrachloride with potassium.[13][10] Uranium was not seen as
being particularly dangerous during much of the 19th century, leading to the development of various uses for the element. One
such use for the oxide was the aforementioned but no longer secret coloring of pottery and glass.
Antoine Becquerel discovered radioactivity
by using uranium in 1896.[5]
Becquerel made the discovery in Paris by leaving a sample of uranium on top of an unexposed photographic plate in a drawer and noting that the plate had become 'fogged'.[14] He determined that a form of
invisible light or rays emitted by uranium had exposed the plate.
Fission research
A team led by Enrico Fermi in 1934 observed that bombarding uranium with
neutrons produces the emission of beta rays
(electrons or positrons; see beta particle).[15] The fission products were at first mistaken for new elements of atomic numbers 93 and 94,
which the Dean of the Faculty of Rome, Orso Mario Corbino, christened ausonium and
hesperium, respectively.[16][17][18][19] The
experiments leading to the discovery of uranium's ability to fission (break apart) into
lighter elements and release binding energy were conducted by Otto Hahn and Fritz Strassmann[15] in Hahn's laboratory in Berlin. Lise
Meitner and her nephew, physicist Otto Robert Frisch, published the physical
explanation in February 1939 and named the process 'nuclear fission'.[20] Soon after, Fermi hypothesized that the fission of uranium might release
enough neutrons to sustain a fission reaction. Confirmation of this hypothesis came in 1939, and later work found that on average
about 2 1/2 neutrons are released by each fission of the rare uranium isotope uranium-235.[15]
Further work found that the far more common uranium-238 isotope can be transmuted into plutonium, which, like uranium-235, is also
fissionable by thermal neutrons.
On 2 December 1942, another team led by Enrico Fermi was
able to initiate the first artificial nuclear chain reaction. Working in a lab
below the stands of Stagg Field at the University of
Chicago, the team created the conditions needed for such a reaction by piling together 400 tons (360 tonnes) of
graphite, 58 tons (53 tonnes) of uranium
oxide, and six tons (five and a half tonnes) of uranium metal.[15] Later researchers found that such a chain reaction could either be controlled to produce
usable energy or could be allowed to go out of control to produce an explosion more violent than anything possible using
chemical explosives.
Bombs and reactors
Two major types of atomic bomb were developed in the Manhattan Project during
World War II: a plutonium-based device (see
Trinity test and 'Fat Man') whose plutonium was derived
from uranium-238, and a uranium-based device (nicknamed 'Little Boy') whose fissile material
was highly enriched uranium. The uranium-based Little Boy device became the first
nuclear weapon used in war when it was detonated over the Japanese city of Hiroshima on 6 August 1945.
Exploding with a yield equivalent to 12,500 tonnes of TNT, the blast and thermal
wave of the bomb destroyed nearly 50,000 buildings and killed approximately 75,000 people (see Atomic bombings of Hiroshima and Nagasaki).[14]
Four light bulbs lit with electricity generated from the first artificial electricity-producing
nuclear reactor,
EBR-I (1951)
Experimental Breeder Reactor I at the Idaho National Laboratory(INL) near Arco, Idaho became
the first functioning artificial nuclear reactor on 20 December 1951. Initially, four 150-watt light bulbs were lit by the reactor, but improvements eventually enabled it to power
the whole facility (later, the whole town of Arco became the first in the world to have all its electricity come from nuclear power).[21] The world's first commercial scale nuclear power station, Calder
Hall in England, began generation on 17 October 1956.[22] Another early power reactor
was the Shippingport Reactor in Pennsylvania, which began electricity production in 1957. Nuclear power was used for the first time for
propulsion by a submarine, the USS
Nautilus, in 1954.[15]
Fifteen ancient and no longer active natural nuclear fission reactors
were found in three separate ore deposits at the Oklo mine in Gabon,
West Africa in 1972. Discovered by French physicist Francis
Perrin, they are collectively known as the Oklo Fossil Reactors.
The ore they exist in is 1.7 billion years old; at that time, uranium-235 constituted about three percent of the total uranium on
Earth.[23] This is high enough to permit a
sustained nuclear fission chain reaction to occur, providing other conditions are right. The ability of the surrounding sediment
to contain the nuclear waste products in less than ideal conditions has been cited by
the U.S. federal government as evidence of their claim that the Yucca Mountain facility
could safely be a repository of waste for the nuclear power industry.[23]
Cold War legacy and waste
U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2006
During the Cold War</
