Discovered | Name | Characteristics | Prepared | Reactions | Uses

Uranium is a member of the actinide series of elements, which stretches from actinium (Ac) to lawrencium (Lr).

The element was discovered in 1789 by Klaproth. (Klaproth also discovered zirconium and verified the discoveries of tellurium and titanium.) Early chemists and mineralogists believed that the mineral pitchblende was a mixture of the ores of iron and zinc. However, in 1789 Klaproth realized that it contained an unknown metal. Indeed, he wrote that "Of late, seventeen metallic substances have been acknowledged as distinct metals, each with a nature peculiar to itself. The [subject] of this essay is to add one to that number. ..." He named the new element uranium in honor of the planet Uranus, which had only recently been discovered.

When Klaproth heated a yellow oxide of uranium with charcoal (a reducing agent), he found a black powder with a metallic luster and assumed it to be metallic uranium. This was accepted for over fifty years by the chemists of the time, but in 1841 Eugene Peligot, a French chemist, showed it to be just an oxide. In an attempt actually to isolate the metal, however, Peligot treated UCl4 with potassium in a platinum crucible. The reaction was so violent that the crucible and its contents glowed white hot. Nonetheless, Peligot finally succeeded in actually isolating uranium metal.

UCl4 + 4 K 4 KCl + U

The element is not all that rare in the earth's crust. In fact, it is slightly more common than tungsten, molybdenum, and beryllium.

Uranium is the last element in the periodic table that is naturally occurring (except for possible traces of neptunium and plutonium). All of the 16 isotopes of uranium are radioactive.

The metal is now most readily produced by reacting uranium(IV) fluoride with magnesium metal.

UF4 + 2 Mg U + 2 MgF2

Although chemists had worked with uranium compounds for almost two centuries, it was not realized that the element is radioactive until Becquerel's work in 1896. We now know that there are 16 radioactive isotopes of uranium with a number of them having long half lives. Isotopes with mass numbers of 234, 235, and 238 exist in nature. The 233 isotope is produced commercially in kilogram quantities from thorium.

232Th + 1n 233Th 233Pa + b

233Pa 233U + b

Natural samples of uranium are nominally 99.2830% by weight 238U, 0.711% 235U, and 0.0054% 233U.

Uranium is a dense, lustrous metal that resembles iron. It is also ductile and malleable. In air it tarnishes quickly, and a freshly exposed surface becomes coated with a layer of dark oxide. When finely divided, the metal burns spontaneously in air (it is pyrophoric).

Acids dissolve the metal, but it does not react with base.

Uranium is toxic not only because of its radioactivity. It is also chemically toxic to about the same degree as arsenic.

The element can have oxidation states of +3 through +6 in its compounds, but +5 is only of minor importance. For example, seven binary fluorides of uranium are known, with UF4 and UF6 of special importance. The hexafluoride is produced by direct fluorination of UF4.

UF4(s) + F2(g) UF6(g)

Uranium(VI) fluoride is produced in very large quantities, since it is a key material in the enrichment of natural uranium in the 235 isotope. The hexafluoride is quite volatile at temperatures not much above room temperature. Therefore, a sample of uranium is converted to UF6, which is then vaporized. The UF6 vapor is pumped through a series of diffusion cells where the UF6 based on the lighter 235 isotope diffuses faster than that based on the heavier 238 isotope. Therefore, a sample of UF6 can be enriched in one or the other isotope.

The chemistry of uranium with oxygen is extremely complex. For example, uranium(IV) oxide, UO2, is a black-brown powder with a very high melting point (2800°C); it occurs in nature as pitchblende. The trioxide, uranium(VI) oxide, exists in six different forms that have different colors. It is obtained quite readily by thermally decomposing various other uranium compounds as, for example, uranyl nitrate [UO2(NO3)2].

Most nuclear reactors built for power generation are based on uranium fuel enriched in 235U. One pound of completely fissioned uranium has the fuel value of over 1500 tons of coal. And finally, most of the internal heat in the earth is thought to be due to the radioactive decay of uranium and thorium.

Depleted uranium - uranium with the amount of the 235U isotope below 0.2% - is used in armor plate, in inertial guidance systems, in gyro compasses, in counterweights in aircraft control surfaces, and for many other purposes.