Abstract
All elements with atomic number greeter than eighty-three(bismuth) are radioactive. Some of these elements have short half-lives and some have long half-lives. |In radioactive decay, the element which decays is called the parent, and the one which is formed is called the daughter. If the parent has a much longer half-life than its daughter, a special kind of equilibrium called secular equilibrium can be established. Such an equilibrium is formed between the isotopes of lead and bismuth, in which lead-210(Ra.-D) has a half-life of 22.3 years and bismuth-210(Ra-E) has a half-life of 5.0 days.