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A set of base units in the atomic system as in one proposal are the electron rest mass, the magnitude of the electronic charge, the Planck constant, and the permittivity. [6] [9] In the atomic units system, each of these takes the value 1; the corresponding values in the International System of Units [10]: 132 are given in the table.
In 1803 John Dalton proposed to use the (still unknown) atomic mass of the lightest atom, hydrogen, as the natural unit of atomic mass. This was the basis of the atomic weight scale. [13] For technical reasons, in 1898, chemist Wilhelm Ostwald and others proposed to redefine the unit of atomic mass as 1 / 16 the mass of an oxygen atom. [14]
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 (per E = mc 2). Atomic mass is often measured in dalton (Da) or unified atomic mass unit (u). One dalton is equal to 1 ⁄ 12 the mass of a carbon-12 atom in its natural state.
An example of use of a mass number is "carbon-12," which has 12 nucleons (six protons and six neutrons). The actual mass of an atom at rest is often expressed in daltons (Da), also called the unified atomic mass unit (u). 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 ...
Section 3.8 introduces atomic units and gives a table of atomic units of various physical quantities and the conversion factor to the SI units. Section 7.3(v) gives a concise but clear tutorial on practical use of atomic units, in particular how to understand equations "written in atomic units".
However, the names of all SI mass units are based on gram, rather than on kilogram; thus 10 3 kg is a megagram (10 6 g), not a *kilokilogram. The tonne (t) is an SI-compatible unit of mass equal to a megagram (Mg), or 10 3 kg. The unit is in common use for masses above about 10 3 kg and is often used with SI prefixes.
The mass number gives an estimate of the isotopic mass measured in atomic mass units (u). For 12 C, the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of 12 C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number.
Mass; Name of unit Symbol Definition Relation to SI units atomic mass unit, unified: u; AMU Same as dalton (see below) ≈ 1.660 539 068 92 (52) × 10 −27 kg [20] atomic unit of mass, electron rest mass: m e: ≈ 9.109 383 7139 (28) × 10 −31 kg [21] bag : ≡ 60 kg = 60 kg bag (Portland cement) ≡ 94 lb av = 42.637 682 78 kg ...