Decay of iodine-131
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Decay of iodine-131
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[post_date] => 2025-01-09 21:42:13
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
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
[post_title] => Decay of iodine-131
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[question] => 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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[description] => Reason for the Correct Answer:
Determine the half-life of iodine-131 by analyzing the graph provided in the passage.
The half-life of iodine-131 is 8 days. This is because it takes 8 days for a given amount of iodine to reduce to one half its original value.
Starting from a 200-gram sample of iodine-131, it takes two half-life to decay to 50 grams. 200–>100–>50
Two half-life of iodine-131 is equal to 16 days. 2 x 8 days
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[answers] => Array
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[each_answer] => A. 16 days
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[each_answer] => B. 8 days
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[each_answer] => C. 24 days
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[3] => Array
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[each_answer] => D. 32 days
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Which type of radioactive decay is responsible for generating the highly energetic electron that is produced when an atom of iodine-131 decays into an atom of xenon-131?
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[answer] => 2
[description] => Reason for the Correct Answer:
The total mass of the isotope does not affect the wavelength of emission, only the
When iodine-131 decays into xenon-131, a neutron is transformed into a proton.
As a result of this transformation, an electron is emitted from the nucleus.
This type of radioactive decay is defined as beta decay.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Positron Emission
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[each_answer] => B. Beta Decay
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[each_answer] => C. Alpha decay
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[each_answer] => D. Electron capture
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => There are three radioisotopes of iodine that play an important role in nuclear medicine. These isotopes include I-123, I-125, and I-131. These radioisotopes differ from one another by the number of which type of subatomic particle?
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
The isotope listed in the question stem differ in their mass number, the number of protons and neutrons.
An element is defined by their number of protons. All iodine isotopes will have 53 protons in their nucleus.
Therefore, iodine isotopes differ in their number of neutrons.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Electrons
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[each_answer] => B. Electrons
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[each_answer] => C. Neutrons
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[each_answer] => D. Protons
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[quiz_unique_key] => 2377279144
[question] => Approximately how much energy is expended when an atom of iodine-131 decays into a stable xenon-131 atom?
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[answer] => 3
[description] => Reason for the Correct Answer:
Analyze the decay scheme provided in figure 1.
Iodine-131 decays into xenon-131 via a two-step process. Each step is associated with an amount of energy, which is provided in figure 1.
The sum of these two energies is equal to the amount of energy expended when a single atom of iodine-131 decays into xenon-131.
606 keV + 364 keV = 970 keV
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[each_answer] => A. 364 keV
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[each_answer] => B. 606 keV
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[each_answer] => C. 970 keV
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[each_answer] => D. 242 keV
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[question] => Which of the following is a true statement regarding the atomic mass of Xenon-131?
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[answer] => 4
[description] => Reason for the Correct Answer:
A proton weighs slightly less than a neutron.
Although xenon-131 and iodine-131 have the same mass number, xenon-131 weighs less than iodine-131 because it has an extra proton.
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[each_answer] => A. Xenon-131 weighs more than iodine-131 because it has an extra proton.
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[each_answer] => B. Xenon-131 weighs more than iodine-131 because it has one fewer proton.
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[each_answer] => C. Xenon-131 weighs less than iodine-131 because it has one fewer proton.
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[each_answer] => D. Xenon-131 weighs less than iodine-131 because it has an extra proton.
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