Unstable nuclei decay at different rates, with some nuclides decaying faster than others. Half-life and activity are used as quantitative terms for lifetime and rate of decay. The half-life is defined as the time in which half of the original number of nuclei decay. Nuclear decay is a statistical process, and is usually described by an exponential decay law. The decay constant (λ) is a measure of the probability that a given nucleus will decay per unit time, and is related to the half-life by the equation λ = ln(2)/t1/2. By knowing the number of atoms of a radioactive isotope present at a given time, and the decay constant of that isotope, scientists can calculate how many atoms will remain after a certain amount of time has passed.