How is an atomic mass unit (amu) defined?

• A. It is the mass of a single proton.
• B. It is the mass of a hydrogen atom.
• C. It is the average mass of all naturally occurring isotopes of an element.
• 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. Recognize that the amu is not defined as the mass of a single proton or a hydrogen atom, nor as the average mass of isotopes of an element.
3. Recall that the amu is based on the carbon-12 isotope, which is a stable isotope of carbon with 6 protons and 6 neutrons.
4. Know that the atomic mass unit is defined as exactly one twelfth (1/12) of the mass of a single carbon-12 atom.
5. This definition provides a consistent scale for comparing atomic masses across different elements and isotopes.