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

• A. It is the mass of an electron and equals approximately 9.11 × 10^{-28} grams.
• B. It is the average mass of all naturally occurring isotopes of an element.
• C. It is defined as one twelfth the mass of a carbon-12 atom and equals approximately 1.66 × 10^{-24} grams.
• D. It is the mass of a single proton and equals exactly 1 gram.

Answer: (C)

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 defined relative to the carbon-12 isotope, which serves as the reference standard.
3. Know that one atomic mass unit is defined as exactly one twelfth of the mass of a carbon-12 atom.
4. Recognize that this definition gives the amu a value of approximately \$1.66 \times 10^{-24}$ grams.
5. Compare this with other options: the mass of an electron is much smaller (~\$9.11 \times 10^{-28}$ g), the average atomic mass refers to weighted isotopic averages, and the mass of a proton is close to 1 amu but not exactly 1 gram.