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 0.012%. each 40k decay deposits, on average, 1.0 mev of energy into the body.