Chemistry

An Overview about the Chemical Element Uranium



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Uranium

Symbol: U

Atomic Number: 92

Atomic Mass: 238.0289 amu (atomic mass units)

Melting Point: 1132.0 C (1405.15 K, 2069.6 F)

Boiling Point: 3818.0 C (4091.15 K, 6904.4 F)

Number of Protons: 92

Number of Electrons: 92

Number of Neutrons: 146

Classification: Rare Earth

Crystal Structure: Orthorhombic

Density @ 293 K: 18.95 grams per cubic centimeter

Color: silver

The rare earth element uranium was discovered by the German chemist Martin Heinrich Klaproth. He believed the substance he extracted from the mineral ore pitchblende was pure uranium but in fact it was uranium dioxide. In 1841 Eugene-Melchoir Peligot, a French chemist, prepared pure uranium by heating a sample of uranium dioxide in a platinum crucible with potassium. Uranium is named after the planet Uranus which in turn was named after the Greek god Ouranos who was both the son and husband of Gaia.

All uranium's isotopes are radioactive. It was in a sample of uranium that the French physicist Antoine Henri Becquerel first discovered radioactivity in 1896. Natural uranium consists of three isotopes uranium-238 (99.2742%, half-life 4.468 billion years), uranium-235 (0.7204%, half-life 703.88 million years) and uranium-234 (0.0054%, half-life 24.55 thousand years). Numerous other isotopes of uranium have been produced with mass numbers ranging from 218 to 242.

Uranium shows three temperature dependant crystal modifications alpha up to 688 C (941 K, 1234.4 F), beta up to 776 C (1049 K, 1428.8 F) and gamma at higher temperatures. Pure uranium metal oxidizes in air. It will be dissolved by acids but not alkalis. Finely divided uranium can be attacked by cold water. Uranium metal is malleable, ductile and slightly softer than steel. It is also slightly paramagnetic.

Uranium is a rare earth element of the actinide or actinoid series. It is the highest naturally occurring element. All elements with atomic numbers greater than 92 are man-made.

Uranium oxide has been used to give a green or yellow coloration to glass. Such glass also shows fluorescence. This type of glass is known as "Vaseline glass". The Romans made such glass and examples of this type were found in the ruins of Pompeii. Vaseline glass is radioactive and should be handled with caution.

Uranium-238 levels are used by geologists to date the age of igneous rocks such as granite.

It is in the field of nuclear energy that most uranium is used.

Uranium-235 is the only naturally occurring fissionable isotope. In nuclear fission if a fissionable isotope is hit by a neutron it will spilt into smaller atomic fragments. Some of these fragments will be neutrons that can split other atoms this is a nuclear chain reaction.

Naturally occurring uranium does not contain enough uranium-235 to maintain a chain reaction. If natural uranium is used to power a nuclear reactor then a moderator must also be used. Heavy water or graphite are used as moderators in nuclear reactors powered by natural uranium,

There are methods such as gaseous diffusion that can concentrate uranium-235. Once a concentration of uranium-235 greater than 2% is achieved it can maintain a chain reaction using ordinary water as a moderator.

Uranium-238 can be transformed by a series of steps into fissionable plutonium-239. These steps start with the addition of a neutron to form uranium-239 then the addition of a beta particle forms neptunium-239 a further beta addition yields plutonium-239. These steps take place in breeder reactors.

Uranium-233 is a man made fissionable isotope produced from thorium-232 by a similar radiation process as that that produces plutonium. Thorium-232 -> thorium-233 -> protactinium-233 -> uranium-233. Uranium-233 can then be used to fuel nuclear reactors.

A pound of totally fissionable uranium in a nuclear reactor can yield the same amount of power as 1500 tons of coal in a fossil fuelled power station. In 1990 there were 429 nuclear power stations in operation worldwide. These power stations produced as estimated 311000 megawatts of power.

The production of enriched uranium for the nuclear industry also produces depleted uranium. This is not as radioactive as natural uranium. Its high density of 19.1 grams per cubic centimeter has given it a number of uses. Depleted uranium is used by the military to make armor plating and armor-piercing bullets. Away from military applications depleted uranium is used to form counter weights for aircraft as well as shielding for radiation therapy and radiography equipment.

The military scientists also use uranium for the production of nuclear explosive devices.

Uranium has a few other uses such as the production of high energy X-rays. Uranium acetate is used by analytical chemists and uranium nitrate has been used as a toner in photography.

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