Atomic mass

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Atomic mass
Stylized lithium-7 atom: 3 protons, 4 neutrons, and 3 electrons (total electrons are ~1⁄4300th of the mass of the nucleus). It has a mass of 7.016 Da. Rare lithium-6 (mass of 6.015 Da) has only 3 neutrons, reducing the atomic weight (average) of lithium to 6.941.
Common symbols	ma, m
SI unit	kilogram (kg)
Other units	dalton (Da)
Intensive?	yes
Behaviour under coord transformation	scalar

The atomic mass (m_a or m) is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1 Da is defined as 1⁄12 of the mass of a free carbon-12 atom at rest in its ground state.^[1] The protons and neutrons of the nucleus account for nearly all of the total mass of atoms, with the electrons and nuclear binding energy making minor contributions.^[2] Thus, the numeric value of the atomic mass when expressed in daltons has nearly the same value as the mass number. Conversion between mass in kilograms and mass in daltons can be done using the atomic mass constant ${\displaystyle m_{\rm {u}}={{m({\rm {^{12}C}})} \over {12}}=1\ {\rm {Da}}}$.

The formula used for conversion is:^[3][4]

${\displaystyle 1\ {\rm {Da}}=m_{\rm {u}}={M_{\rm {u}} \over {N_{\rm {A}}}}={M(^{12}\mathrm {C} ) \over {12\ N_{\rm {A}}}}=1.660\ 539\ 066\ 60(50)\times 10^{-27}\ \mathrm {kg} ,}$

where ${\displaystyle M_{\rm {u}}}$ is the molar mass constant, ${\displaystyle N_{\rm {A}}}$ is the Avogadro constant,^[5] and ${\displaystyle M(^{12}\mathrm {C} )}$ is the experimentally determined molar mass of carbon-12.^[6]

The relative isotopic mass (see section below) can be obtained by dividing the atomic mass m_a of an isotope by the atomic mass constant m_u yielding a dimensionless value. Thus, the atomic mass of a carbon-12 atom is 12 Da by definition, but the relative isotopic mass of a carbon-12 atom is simply 12. The sum of relative isotopic masses of all atoms in a molecule is the relative molecular mass.

The atomic mass of an isotope and the relative isotopic mass refers to a certain specific isotope of an element. Because substances are usually not isotopically pure, it is convenient to use the elemental atomic mass which is the average (mean) atomic mass of an element, weighted by the abundance of the isotopes. The dimensionless (standard) atomic weight is the weighted mean relative isotopic mass of a (typical naturally occurring) mixture of isotopes.

The atomic mass of atoms, ions, or atomic nuclei is slightly less than the sum of the masses of their constituent protons, neutrons, and electrons, due to binding energy mass loss (per E = mc²).