Final answer:
An atomic mass unit (amu) is defined as one-twelfth of the mass of a carbon-12 atom, used as a standard for measuring atomic masses.
Step-by-step explanation:
The atomic mass unit (amu) or unified atomic mass unit (u) is closely related to the mass of a carbon-12 atom. It is defined as one-twelfth of the mass of a carbon-12 atom, which has 6 protons and 6 neutrons in its nucleus. Since carbon-12 is used as the standard for measuring atomic masses, 1 amu is equivalent to 1.660539 × 10-27 kilograms, and the mass of a carbon-12 atom is exactly 12 amu. This provides a convenient scale for comparing the masses of different atoms, so the mass of other atoms can be expressed as a multiple of the mass of carbon-12. For instance, the mass of an oxygen-16 atom is approximately 16 amu.