A 70 kg human body typically contains 140 g of potassium. Potassium has a chemical atomic mass of 39. 1 u and has three naturally occurring isotopes. One of those isotopes, 40k, is radioactive with a half-life of 1. 3 billion years and a natural abundance of 0. 12%. Each 40k decay deposits, on average, 1. 0 mev of energy into the body.