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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

The radioactivity of iodine-131

Problem

Iodine-131 is an unstable radioisotope of iodine that is commonly used for the treatment of hyperthyroidism caused by Grave’s disease. Iodine-131 is readily absorbed by the follicular cells of the thyroid gland via the sodium/iodine symporter. As the atoms of iodine-131 accumulate in the thyroid, they eventually undergo a two-step radioactive decay process that releases high energy electrons and electromagnetic radiation in the form of gamma rays. These highly energetic electrons can penetrate and damage surrounding tissue within 2 mm from the source of emission. Roughly 90 percent of iodine-131 atoms decay into xenon-131 via the decay scheme depicted in Figure 1.
Figure 1. The Decay Scheme of Iodine-131 into Xenon-131
Iodine-131 decays into xenon-131 by a first order process, which means that the rate of decay is directly proportional to the concentration of iodine-131. The half-life, the time required for one half of a given amount of a substance to decay, of a first order process is however independent of the concentration. As the amount of iodine-131 decreases, the amount of xenon-131 increases. A decay curve is used to graphically represent how a certain amount of a substance decreases with respect to time. The decay curve of iodine-131 into xenon-131 is depicted in Figure 2.
Figure 2. The Decay Curve for Iodine-131
Starting from a 200-gram sample of iodine-131, how long will it take for it to decay to 50 grams?
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