As we will see in Section 9.4, a radioactive substance decays exponentially: The mass at time is m (t) = m (0) ekr, where m (0) is the initial mass and is a negative constant. The mean life of an atom in the substance is for the radioactive carbon isotope, 14C, used in radiocarbon dating, the value of is –0.000121. Find the mean life of a 14C atom.