Uranium is a metallic chemical element which is classified among the actinides on the periodic table of elements. It is perhaps most famous as the source of fuel for nuclear weapons and power plants, but it has a range of uses as well. Deposits of this element have been found in several regions of the world, and the element is fairly abundant in nature, keeping prices relatively low under normal conditions. Because this element is toxic and radioactive and because it has potentially very dangerous uses, access to the world's supply of uranium is tightly restricted.
When uranium is isolated from the metal ores it occurs in, it is a silvery white, very heavy metal which is extremely reactive and a little bit softer than steel. Because this element is so reactive, it quickly forms a thick gray to black tarnish when exposed to air. There are 14 known isotopes of uranium, and the element also appears in a number of chemical compounds, some of which have industrial uses. You can find this element on the periodic table of elements by looking for the “U” symbol, or atomic number 92.
Humans have actually been aware of this substance for a very long time. As early as the first century CE, uranium oxide was used in glass and pottery colorants. The element itself was isolated in 1789 by Martin Klaproth, who named it for the planet Uranus, which had only recently been discovered. It took another 100 years for people to realize that this element was radioactive, and several more decades elapsed before people recognized radioactivity as a health risk.
This element's reactivity makes it ideally suited to making fuel for nuclear power plants, reactors which power submarines, and of course nuclear weapons. The metal was also used in uranium glass before people realized this usage was dangerous, and it is used in nuclear medicine, research, and in dating of archaeological finds. Several militaries also use a form of uranium in high density penetrating rounds; this usage has been subject to controversy, as such rounds can potentially present a health risk if they are not properly disposed of.
Because uranium is toxic, it needs to be handled very carefully. It can cause severe organ damage, and its dust can irritate mucus membranes such as those found in the lungs. The radioactivity of the metal is also a health risk, as is the explosiveness of the dust. Most people who work around this element are trained to use basic safety precautions to keep their work safe.