Atomic mass ($m_{\mathrm{a}}$)

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 $m_i$ means the atomic mass of a neutral chemical atom of isotope $i$, 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 $12\mathrm{u}$ 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.

The unified atomic mass unit and dalton are names for the same mass unit:where the carbon-12 chemical atom is free, at rest, and in its nuclear and electronic ground state. Its conversion to SI mass units is determined experimentally. See The International System of Units (SI), ninth edition, SI Brochure (Table 8, footnote (f), printed p. 146: dalton and unified atomic mass unit).

The mass deficit of a bound atomic nucleus relative to its separated protons and neutrons. If their rest masses are $m_p$ and $m_n$, the atomic nucleus has rest mass $M_{\mathrm{nuc}}$, and the positive nuclear binding energy is $B$, thenwith $c$ the speed of light. Releasing energy when the nucleons bind reduces the rest mass of the bound system without changing the exact count $A=Z+N$. A neutral atomic mass also includes $Z$ electron rest masses, reduced by the energy required to separate those bound electrons. Consequently $A-m_{\mathrm{a}}/\mathrm{u}$ is not the definition of the nuclear mass defect: the reference unit and constituent rest masses must also be accounted for.

For a chemical element in a specified population, its relative atomic mass or atomic weight is the dimensionless ratio $A_{\mathrm{r}}=\bar{m}/\mathrm{u}$, with $\bar{m}$ the mean neutral atomic mass weighted by isotopic abundance. For the illustrative neon population, $A_{\mathrm{r}}\approx 20.1800$, whereas $\bar{m}\approx 20.1800\mathrm{u}$ has mass units.^[ref]

A recommended atomic weight applicable to normal materials, rather than the result for every possible enriched sample. The tabulated neon value $20.1797(6)$ and the arithmetic result above need not agree in every last displayed digit. Some chemical elements have intervals of standard atomic weight because their isotopic abundances vary across normal materials.^[ref]