Atomic mass unit

The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic masses and molecular masses. It is defined to be 1/12 of the mass of one atom of carbon-12. Accordingly,

Related Topics:
Unit - Mass - Atomic masses - Molecular mass - Atom - Carbon

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:1 u = 1/NA gram = 1/(1000 NA) kg   (where NA is Avogadro's number)

Related Topics:
Gram - Kg - Avogadro's number

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:1 u ≈ 1.66053886 x 10-27 kg

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See 1 E-27 kg for a list of objects which have a mass of about 1 u.

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The symbol amu for atomic mass unit can sometimes still be found, particularly in older works. Atomic masses are often written without any unit and then the atomic mass unit is implied.

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In biochemistry and molecular biology literature (particularly in reference to proteins), the term "dalton" is used, with the symbol "Da". Because proteins are large molecules, they are typically referred to in kilodaltons, or "kDa", with one kilodalton being equal to 1000 daltons.

Related Topics:
Biochemistry - Molecular biology - Protein - Dalton

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The unified atomic mass unit is not an SI unit of mass, although it is (only by that name, and only with the symbol u) accepted for use with SI. See SI website link below.

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The unit is convenient because one hydrogen atom has a mass of approximately 1 u, and more generally an atom or molecule that contains n protons and neutrons will have a mass approximately equal to n u. (The reason is that a carbon-12 atom contains 6 protons, 6 neutrons and 6 electrons, with the protons and neutrons having about the same mass and the electron mass being negligible in comparison.)

Related Topics:
Hydrogen atom - Atom - Molecule - Proton - Neutron

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This is only a rough approximation however, since it does not account for the mass contained in the binding energy of an atom's nucleus; this binding energy mass is not a fixed fraction of an atom's total mass.

Related Topics:
Binding energy - Nucleus

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Another reason the unit is used is that it is experimentally much easier and more precise to compare masses of atoms and molecules (determine relative masses) than to measure their absolute masses. Masses are compared with a mass spectrometer (see below).

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Avogadro's number (NA) and the mole are defined so that one mole of a substance with atomic or molecular mass 1 u will have a mass of precisely 1 gram.

Related Topics:
Mole - Gram

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For example, the molecular mass of water is 18.01508 u, and this means that one mole of water has a mass of 18.01508 grams, or conversely that 1 gram of water contains NA/18.01508 ≈ 3.3428 × 1022 molecules.

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