Uranium

Although uranium is indeed radioactive—the discovery of radioactivity occurred during a study of uranium's properties—it has a very long half-life, which means that it emits its radiations at a rather leisurely pace. Also, it emits mostly alpha particles, which do not travel very far through the air and will not even penetrate the skin. Its radiations are therefore not very harmful, and uranium and its compounds can be handled with a reasonable amount of care, like any other highly poisonous chemicals.

The half-life of the most abundant uranium isotope, uranium-238, is 4.47 × 10⁹ years, or about 4.5 billion years, which happens to be equal to the age of Earth as a planet. This fact allows scientists to use the disintegration of uranium as a sort of clock to determine the ages of rocks and other geological features of Earth.

Uranium-238 is the "parent" atom of a series of radioactive isotopes that we find associated with it in uranium ores. Through radioactive disintegrations, the uranium has been producing these "daughter" isotopes ever since the ore was laid down where we find it today. Uranium-235, which has a half-life of 7.04 × 10⁸ (700 million) years, is the parent of another radioactive series. Among the daughters in these two series are various radioactive isotopes of radium, radon, and other elements. Both series of disintegrations proceed by producing consecutive radioactive isotopes until they wind up as stable isotopes of lead. Thus, the uranium isotopes are slowly turning into lead at a steady rate that is well-known from their half-lives. By measuring the relative amounts of uranium and lead isotopes in a uranium-containing rock, scientists can calculate how old it is.

One of the disintegration products of uranium is radium, the element of atomic number 88. Radium was discovered by Marie Curie (1867-1934), who isolated it from a uranium ore. Another important disintegration product of uranium-238 is radon-222, which has a half-life of 3.8 days. Radon is a gas (a rare gas) that can diffuse out of uranium in the ground and seep into people's houses. Radon can cause lung cancer because when inhaled, it can emit alpha particles directly in the lung, where they can do the most damage. Testing houses for radon gas has become an important precaution ever since this hazard was uncovered only 10 or 15 years ago.