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×23rd Jun 2019 @ 3 min read

The atomic mass constant is used to define the atomic mass unit (or the unified atomic mass). It equals one atomic mass unit. The atomic mass unit is the standard unit used to quantify mass on the atomic scale. Also, the atomic mass unit, the unified mass unit, and the dalton are all synonymous units and are interchangeably used.

The International Union of Pure and Applied Chemistry (IUPAC) defines the atomic mass constant as “the one-twelfth of the mass of a carbon-12 atom in its nuclear and electronic ground state.”

The atomic mass constant is equal to one atomic mass unit (or one unified mass unit).

The symbol for the atomic mass constant is *m*_{u}.

Note: *m* should be in lowercase because *m* in uppercase is the symbol for the molar mass constant (*M*_{u}).

It has the unit of mass, kg.

The above definition can be mathematically expressed as:

Also, the atomic mass constant equals one unified mass unit (1 u). So, we can modify the above equation as

The approximate value of *m*_{u} is 1.661 10^{−27} kg. The more precise values are mentioned below.

The atomic mass constant (*m*_{u}) and the molar mass constant (*M*_{u}) are related by the Avogadro’s constant (*N*_{A}).

According to the Committee on Data for Science and Technology (CODATA), the recommended value of *M*_{u} is 0.999 999 999 65(30) g mol^{−1}. This number can be approximated to 1 g mol^{−1} for practical purposes. Thus, substituting *M*_{u} ≈ 1 in the above equation, we have

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