In a radioactive sample, the number of parent atoms that have not decayed is proportional to the rate at which the parent atoms are decaying. Those familiar with calculus may recognize this situation as one governed by an equation using the transcendental number e. The constant e has a value of approximately 2.718. In general, the equation describing radioactive decay is
where n is the number of atoms remaining, is the number of atoms originally in the sample, and l is a decay constant equal to