Iodine- 131 is a radioactive isotope that is used to diagnose and treat some forms of thyroid cancer. Iodine-131 decays to xenon-131. according to the equation below. iodine-131xenon-131+electron The decay is first order with a rate constant of 0.138d1. All radioactive decay is first order. How many days will it take for 37.0% of the iodine-131 in a 0.700M solution of this substance to decay to xenon-131? Select the correct answer below: 3.35 days 7.20 days 0.0457 day 2.68 days