Atomic mass unit is defined as?

• A. One amu is defined as 1/12 the mass of a carbon-12 atom.
• B. One amu equals 1 gram.
• C. One amu equals mass of a proton.
• D. One amu equals mass of a neutron.

Answer: (A)

Mass on the atomic scale uses a defined reference so that atomic and molecular masses can be compared precisely. This reference is a carbon-12 atom, and the unit is set so that its mass is exactly 12 amu. Consequently, one amu is 1/12 of the mass of carbon-12. This provides a stable, universal scale for all atoms and molecules. Protons and neutrons are each close to 1 amu, but not exactly; the amu is defined independently of their exact masses, which is why the carbon-12 standard is used. In kilograms, one atomic mass unit is about 1.66054 x 10^-27 kg, and while everyday mass uses grams, a gram corresponds to roughly 6.022 x 10^23 amu. So the precise definition is 1 amu equals 1/12 the mass of a carbon-12 atom.