1. The problem statement, all variables and given/known data A sample of iodine contains 1 atom of the radioactive isotope iodine 131 (131I) for every 5 * 107 atoms of the stable isotope iodine-127. Iodine has a proton number of 52 and the radioactive isotope decays into xenon 131 (131Xe) with the emission of a single negatively charged particle. (a) What particle is emitted when iodine 131 decays? Write the nuclear equation which represents the decay. (b) The diagram shows how the activity of a freshly prepared sample of the iodine varies with time. Use the graph to determine the decay constant of iodine 131. Give your answer in s-1. (c) Determine the number of iodine atoms in the original sample. Answers: (b) 1.0 * 10-6 s-1, (c) 1.0 * 1018 2. The attempt at a solution (a) Is it something like: 13152I → 12752I → 131?Xe? (b) N = N0 e-λ t, where the decay consant is λ, N = remaining number of atoms, N0 = original number of atoms, t = time. We have N0 = 5 * 107 atoms, how do we find N?