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

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

Answer: (B)

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 definition of 1 amu is based on the carbon-12 isotope, which is a stable isotope of carbon with 6 protons and 6 neutrons.
3. Recognize that 1 amu is defined as exactly one-twelfth the mass of a single carbon-12 atom, meaning: 1 amu = (mass of carbon-12 atom) / 12.
4. Note that this definition provides a consistent and precise scale for comparing atomic masses, rather than using the mass of hydrogen, a proton, or an average of isotopes.
5. Therefore, the best definition of 1 amu is that it is one-twelfth the mass of a carbon-12 atom.