Which of the following best defines the atomic mass unit (amu)?

• A. It is equal to the mass of a single proton.
• B. It is the average mass of all naturally occurring isotopes of an element.
• C. It is the mass of a hydrogen atom.
• D. It is defined as one-twelfth the mass of a carbon-12 atom.

Answer: (D)

Verified step by step guidance

1. Understand that the atomic mass unit (amu) is a standard unit of mass used to express atomic and molecular weights.
2. Recall that the amu is not defined by the mass of a proton, hydrogen atom, or an average of isotopes, but rather by a specific reference standard.
3. Recognize that the reference standard for the amu is based on the carbon-12 isotope, which is a stable isotope of carbon.
4. Know that the atomic mass unit is defined as exactly one-twelfth (1/12) the mass of a single carbon-12 atom.
5. This definition allows for a consistent and precise scale to compare the masses of different atoms and molecules.