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Ch.21 - Nuclear Chemistry
All textbooksBrown 14th EditionCh.21 - Nuclear ChemistryProblem 55
Chapter 21, Problem 55

Iodine-131 is a convenient radioisotope to monitor thyroid activity in humans. It is a beta emitter with a half-life of 8.02 days. The thyroid is the only gland in the body that uses iodine. A person undergoing a test of thyroid activity drinks a solution of NaI, in which only a small fraction of the iodide is radioactive. (a) Why is NaI a good choice for the source of iodine? (b) If a Geiger counter is placed near the person’s thyroid (which is near the neck) right after the sodium iodide solution is taken, what will the data look like as a function of time?

Verified step by step guidance
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Step 1: Understand why NaI is a good choice for the source of iodine. Sodium iodide (NaI) is a good choice because it is soluble in water, allowing it to be easily ingested and absorbed by the body. The iodide ion (I-) is readily taken up by the thyroid gland, which is the only gland in the body that uses iodine, making it an effective way to deliver iodine-131 to the thyroid for monitoring purposes.
Step 2: Consider the properties of iodine-131. Iodine-131 is a beta emitter with a half-life of 8.02 days. This means that every 8.02 days, half of the iodine-131 will decay, emitting beta particles in the process. This property is crucial for understanding how the radioactivity will change over time.
Step 3: Predict the initial data from the Geiger counter. Right after the sodium iodide solution is taken, the Geiger counter will detect a high level of radioactivity near the thyroid due to the presence of iodine-131. This is because the iodine-131 is concentrated in the thyroid gland, and the beta particles emitted by the decay of iodine-131 are detected by the Geiger counter.
Step 4: Describe how the data will change over time. As time progresses, the radioactivity detected by the Geiger counter will decrease. This is due to the radioactive decay of iodine-131, which follows an exponential decay pattern. The rate of decrease is determined by the half-life of iodine-131, which is 8.02 days.
Step 5: Explain the expected pattern of the data. The data from the Geiger counter will show an exponential decay curve. Initially, the count rate will be high, but it will decrease by half every 8.02 days. This pattern will continue until the radioactivity is no longer detectable or is at a background level.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Radioisotopes and Their Properties

Radioisotopes, like iodine-131, are unstable isotopes that emit radiation as they decay to a more stable form. Iodine-131 specifically emits beta particles and has a half-life of 8.02 days, making it suitable for medical applications such as monitoring thyroid activity. Its radioactive nature allows for the detection of its presence in the body using instruments like Geiger counters.
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Physical Properties

Sodium Iodide (NaI) as a Source of Iodine

Sodium iodide (NaI) is an effective source of iodine because it is highly soluble in water, allowing for easy absorption in the body. When ingested, NaI dissociates into sodium and iodide ions, with the iodide being readily taken up by the thyroid gland. This property is crucial for ensuring that the radioactive iodine-131 can be effectively utilized for monitoring thyroid function.
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Geiger Counter and Radiation Detection

A Geiger counter is an instrument used to detect and measure ionizing radiation, including beta particles emitted by radioactive substances. When placed near the thyroid after ingestion of NaI containing iodine-131, the Geiger counter will register radiation levels that initially spike as the radioactive iodine accumulates in the thyroid. Over time, as the iodine-131 decays, the detected radiation will decrease, reflecting its half-life and the diminishing presence of the isotope.
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Related Practice
Textbook Question

Based on the following atomic mass values: 1H, 1.00782 amu; 2H, 2.01410 amu; 3H, 3.01605 amu; 3He, 3.01603 amu; 4He, 4.00260 amu—and the mass of the neutron given in the text, calculate the energy released per mole in each of the following nuclear reactions, all of which are possibilities for a controlled fusion process:

(a) 21H + 31H → 42He + 10n

(b) 21H + 21H → 32He + 10n

(c) 21H + 32He → 42He + 11H

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Textbook Question

Iodine-131 is a convenient radioisotope to monitor thyroid activity in humans. It is a beta emitter with a half-life of 8.02 days. The thyroid is the only gland in the body that uses iodine. A person undergoing a test of thyroid activity drinks a solution of NaI, in which only a small fraction of the iodide is radioactive. (c) A normal thyroid will take up about 12% of the ingested iodide in a few hours. How long will it take for the radioactive iodide taken up and held by the thyroid to decay to 0.01% of the original amount?

Textbook Question

Why is it important that radioisotopes used as diagnostic tools in nuclear medicine produce gamma radiation when they decay? Why are alpha emitters not used as diagnostic tools?