The elements from atomic number 89 to atomic number 103 form the actinide series. These elements are all radioactive metals with high electropositivity. They are highly dense, tarnish easily, react with water and dilute acid.
The actinide series consists of actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium and lawrencium. Of these elements, only thorium, uranium and plutonium occur regularly in nature. The other elements are either trace products of uranium decay or wholly synthetic.
Thorium is the most stable element in the actinide series. The half-life of its most stable isotope is 14 billion years, which is longer than the scientifically-accepted age of the universe. Lawrencium is the least stable of the actinides; the half-life of its most stable isotope is 11 hours. All other members of the series fall between these two extremes.
Actinides have limited use in industrial applications, but their primary use is in nuclear reactors. Uranium and thorium are used in nuclear power plants, while plutonium is used in nuclear weapons. Thorium and americium are the only actinides primarily used outside of nuclear reactors and scientific research. Thorium is used for the gas mantle in lanterns; americium is used to manufacture smoke detectors.