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The atomic mass (ma 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]
Atomic units. The atomic units are a system of natural units of measurement that is especially convenient for calculations in atomic physics and related scientific fields, such as computational chemistry and atomic spectroscopy. They were originally suggested and named by the physicist Douglas Hartree. [1]
The mass of an atom or other particle can be compared more precisely and more conveniently to that of another atom, and thus scientists developed the dalton (also known as the unified atomic mass unit). By definition, 1 Da (one dalton) is exactly one-twelfth of the mass of a carbon-12 atom, and thus, a carbon-12 atom has a mass of exactly 12 Da.
This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately 1.66 × 10 −27 kg. [61] Hydrogen-1 (the lightest isotope of hydrogen which is also the nuclide with the lowest mass) has an atomic weight of 1.007825 Da. [ 62 ] The value of this number is called the atomic mass .
The dalton or unified atomic mass unit (symbols: Da or u) is a unit of mass defined as 1 12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. [1][2] It is a non-SI unit accepted for use with SI. The atomic mass constant, denoted mu, is defined identically, giving mu = 1 12 ...
Mass number. A = (Relative) atomic mass = Mass number = Sum of protons and neutrons. N = Number of neutrons. Z = Atomic number = Number of protons = Number of electrons. A = Z + N {\displaystyle A=Z+N\,\!} Mass in nuclei. M'nuc = Mass of nucleus, bound nucleons. MΣ = Sum of masses for isolated nucleons.
In particle physics and physical cosmology, Planck units are a system of units of measurement defined exclusively in terms of four universal physical constants: c, G, ħ, and kB (described further below). Expressing one of these physical constants in terms of Planck units yields a numerical value of 1. They are a system of natural units ...
Other units of mass are also in use. Historical units include the stone, the pound, the carat, and the grain. For subatomic particles, physicists use the mass equivalent to the energy represented by an electronvolt (eV). At the atomic level, chemists use the mass of one-twelfth of a carbon-12 atom (the dalton).