Fluorine-18 (half-life = 110 minutes) is the most commonly used isotope in positron emission tomography (i.e. PET scans). Use the fact that 18F decays by positron emission to answer the following questions.

I. 18F sits
[ Select ]
on
above
below
the belt of stability.
II. When a 18F nucleus decays, the product is
[ Select ]
nitrogen-14
fluorine-17
neon-18
oxygen-18
.

Question

Fluorine-18 (half-life = 110 minutes) is the most commonly used isotope in positron emission tomography (i.e. PET scans). Use the fact that 18F decays by positron emission to answer the following questions.

I. 18F sits 
[ Select ]
on
above
below
 the belt of stability.
II. When a 18F nucleus decays, the product is 
[ Select ]
nitrogen-14
fluorine-17
neon-18
oxygen-18
 .

BlackTom AI · Accepted Answer

To approach this question, we consider the nuclear decay and the position of fluorine-18 on the chart of nuclides.
Option set for I: Where 18F sits relative to the belt of stability includes choices such as above, below, or on the belt. Since 18F is proton-rich compared to the most stable isobars around A=18, it lies on the proton-rich side of the valley of stability, i.e., below the belt for this mass region. Saying it sits above or on the belt would imply it is more neutron-rich or on the stability line, which is not the case here. Therefore, the correct reasoning is that 18F sits below the belt of stability.
Option set for II: When a nucleus decays by positron emission (beta-plus decay), a proton is transformed into a neutron, and the atomic number Z decreases by 1, while the mass number A remains the same. Starting from fluorine-18 (Z = 9, A = 18), the decay product must have Z = 8 and A = 18. An element with Z = 8 and mass number 18 is oxygen-18. Among the listed possibilities (nitrogen-14, fluorine-17, neon-18, oxygen-18), only oxygen-18 matches Z = 8 and A = 18, so that is the correct product. 
Why the other options are incorrect in this context: nitrogen-14 would require a very different combination of Z and A and does not result from a straightforward beta-plus decay of fluorine-18; fluorine-17 would imply a different mass number (A = 17) and is not consistent with a decay that preserves A; neon-18 would imply an increase in Z by +1 relative to fluorine-18, which is the opposite of what occurs in beta-plus decay; and oxygen-18 aligns precisely with the expected Z decrease and same mass number. 
In summary, the two selections reflect (I) the proton-rich location of 18F relative to the stability belt (below), and (II) the decay product as oxygen-18 after positron emission, consistent with beta-plus decay rules.

CHEM 1220 SP2025 (18215) Final Exam- Requires Respondus LockDown Browser

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