Atomic mass, also known as atomic weight, is the weighted average mass of an atom of an element, measured in atomic mass units (amu). It reflects the mass of an atom relative to the mass of carbon-12, which is assigned a value of exactly 12 amu. Atomic mass takes into account the relative abundances of the different isotopes of an element in nature and their respective masses.
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The rest mass of a chemical atom in its ground state, a physical quantity expressed in SI mass units or the unified atomic mass unit. Here means the atomic mass of a neutral chemical atom of isotope , not the mass of its bare atomic nucleus and not a population average.[ref]
Atomic mass includes electrons and depends on nuclear binding energy; it is not just mass number times a universal nucleon mass. The atomic mass of neutral carbon-12 in its ground state is exactly by definition of the unified atomic mass unit. That exactness establishes the reference scale, rather than making the atomic masses of all isotopes exact integers in the same unit.