Problem 97 - Chemistry: An Atoms-Focused Approach

Question

Predict the most likely mode of decay for the following isotopes used as imaging agents in nuclear medicine: (a) \(^{197} \mathrm{Hg}\) (kidney); (b) \(^{75} \mathrm{Se}\) (parathyroid gland); (c) \(^{18} \mathrm{F}\) (bone).

Step-by-Step Solution

Answer

Based on the analysis of the radioactive isotopes used in nuclear medicine, predict the most likely mode of decay for each isotope: (a) \(^{197} \mathrm{Hg}\): (b) \(^{75} \mathrm{Se}\): (c) \(^{18} \mathrm{F}\):

Step 1: Analyze \(^{197}\mathrm{Hg}\)

To predict the most likely decay mode for \(^{197}\mathrm{Hg}\), we need to look at its atomic number (number of protons) and its mass number (protons plus neutrons). For mercury (Hg), the atomic number is 80 and the mass number is 197. Therefore, we have 80 protons and 117 neutrons. Since the number of neutrons is significantly greater than the number of protons, this isotope is likely to undergo beta decay (particularly beta-minus decay) to balance the ratio of protons to neutrons.

Step 2: Analyze \(^{75}\mathrm{Se}\)

Next, we analyze the isotope \(^{75}\mathrm{Se}\). Selenium (Se) has an atomic number of 34. With a mass number of 75, this means there are 41 neutrons within the nucleus. In this case, the number of neutrons is slightly higher than the number of protons. Therefore, this isotope is also likely to undergo beta-minus decay in order to balance the proton-to-neutron ratio.

Step 3: Analyze \(^{18}\mathrm{F}\)

Finally, we examine the isotope \(^{18}\mathrm{F}\). Fluorine has an atomic number of 9, meaning there are 9 protons. With a mass number of 18, this gives us a total of 9 neutrons in the nucleus. Since the proton-to-neutron ratio is 1:1, this isotope is unable to undergo alpha or beta-minus decay. Instead, it will undergo beta-plus decay or electron capture to reduce the number of protons and increase the number of neutrons, achieving a stable proton-to-neutron ratio.

Step 4: Summary

In conclusion, the most likely modes of decay for each isotope in nuclear medicine are as follows: (a) \(^{197} \mathrm{Hg}\): Beta-minus decay (b) \(^{75} \mathrm{Se}\): Beta-minus decay (c) \(^{18} \mathrm{F}\): Beta-plus decay or electron capture

Key Concepts

Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons in their nuclei. This difference in neutrons gives isotopes varying mass numbers. For example, the isotopes referenced in nuclear medicine imaging are specific forms of elements like mercury, selenium, and fluorine.
- Each isotope is identified by its element symbol (e.g., Hg, Se, F) and its unique mass number, such as ^{197} - The stability of an isotope depends on its proton-to-neutron ratio. Isotopes with imbalanced ratios might undergo nuclear decay to reach a more stable form.
This instability is exactly why isotopes are useful in medical imaging. As they decay, they emit radiation that can be captured for imaging purposes.

Beta Decay

Beta decay is a type of radioactive decay where an unstable atomic nucleus transforms by emitting a beta particle. Beta particles are either electrons (beta-minus decay) or positrons (beta-plus decay). This decay process helps isotopes achieve a more balanced proton-to-neutron ratio.
- **Beta-minus decay** occurs when a neutron in the nucleus converts into a proton, emitting an electron and an antineutrino. This increases the atomic number by 1. For example, the isotopes ^{197}Hg and ^{75}Se are likely to undergo beta-minus decay as they have more neutrons than protons.
- **Beta-plus decay** happens when a proton converts into a neutron, releasing a positron and a neutrino. This decreases the atomic number by 1. Isotopes like ^{18}F, with a 1:1 proton-to-neutron ratio, may undergo beta-plus decay to achieve stability, though electron capture could also occur in this scenario.
Beta decay is essential in nuclear medicine, allowing scientists and doctors to trace and visualize the decay process to understand and diagnose different conditions.

Nuclear Medicine Imaging Agents

Nuclear medicine imaging agents are special radioactive isotopes purposefully used to diagnose or treat medical conditions. Due to their radioactive properties, they undergo decay, emitting radiation that can be detected and imaged. This allows medical professionals to examine bodily organs' structure and function.
- **Imaging Agents**: These isotopes are selected based on their decay properties. For instance, ^{197}Hg for kidney imaging, ^{75}Se for parathyroid gland diagnostics, and ^{18}F for bone imaging. Each is chosen for its specific ties to different parts of the body and its decay characteristics.
- **Detecting Radiation**: As the isotopes decay during the medical imaging process, the radiation emitted is detected by special cameras (like gamma cameras) to create images visible for diagnosis.
Nuclear medicine imaging agents are pivotal in modern diagnostics, offering powerful insights into various diseases and conditions through the innovative application of radioactive isotopes.