The atomic mass unit (u) is defined so that a neutral atom of which isotope has a mass of exactly 12 u?

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_