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The atomic mass unit (u) is defined so that a neutral atom of which isotope has a mass of exactly 12 u?
ACarbon-14
BCarbon-12
COxygen-16
DHydrogen-1
Answer & Solution
Correct answer: B. Carbon-12
1. The atomic mass unit needs a fixed reference point to be useful.
2. That reference is a neutral carbon-12 atom, defined to have a mass of exactly 12 u.
3. Carbon-14 has extra neutrons and so does not define the unit itself, and oxygen-16 has a measured mass close to but not exactly 16 u.
4. Hydrogen-1 is not the reference isotope used to fix the scale.
5. So carbon-12 is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.2 Nuclear Forces and Radioactivity_
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