I've forgotten most of my chemistry, but if i'm not wrong, most elements, especially radioactive ones, decompose overtime, but they don't decompose totally. Say if element x has a half-life of 50years. In 50years time, perhaps 100kg of element x, becomes 50kg of element x. in 100 years, it becomes 25kg, then 12.5kg, and so fore. Please correct me if i'm wrong.