Uranium

With the exception of tiny amounts of neptunium, uranium is the heaviest element found on Earth—that is, the element with the highest atomic number and atomic weight. It has held that distinction ever since it was first recognized as an element by the German chemist Martin H. Klaproth in 1789, who named it uranium in honor of the new planet that had recently been discovered: Uranus. Until 1896, when Henri Becquerel discovered radioactivity, uranium remained a dull, uninteresting metal that found occasional use in making yellow glass. But then it acquired the distinction of being one of only two known elements that possessed the mysterious property of being radioactive. (The other element was thorium.) When nuclear fission was discovered in 1938, uranium suddenly became the most fateful element in the periodic table. Because of its ability to undergo nuclear fission with the release of huge amounts of energy, it became a brand-new source of power, which people would use for both peaceful and destructive purposes.

Aside from its nuclear properties of radioactivity and fission, uranium is literally dull; freshly cut uranium metal is silvery white, but it soon develops a dull gray color in air because of a thin coating of black uranium oxide.

In spite of its radioactivity, uranium has a few useful applications because it is so heavy. Having a density of 19.0 grams per cubic centimeter, it is almost as dense as gold (19.3) and platinum (21.5). But it is much cheaper for two reasons: it is much more plentiful on Earth (40 or 50 times as abundant as silver) and it is a byproduct of the nuclear power industry after the very valuable uranium-235 isotope has been removed. It therefore finds some military uses in which a lot of weight is needed in a small space, such as for counterweights in aircraft control systems, ballast for missile reentry vehicles, and shielding against radiation.

Chemically, uranium is a member of the actinide series of elements, which runs from atomic number 89 (actinium) to atomic number 103 (lawrencium). Those with atomic numbers higher than uranium's 92 are the transuranium elements. Uranium's most important features lie not in its chemistry, but in its radioactivity and its ability to undergo nuclear fission.