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The '''unified atomic mass unit''' ('''u'''), or '''dalton''' ('''Da'''), is a small unit of mass used to express atomic and molecular masses. It is defined to be one twelfth of the mass of an unbound atom of the carbon-12 nuclide, at rest and in its ground state.

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.
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.   

The unified atomic mass unit, or dalton, is not an SI unit of mass, although it is accepted for use with SI under either name.

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.The mass of the electron is approximately 1/1836 of the mass of the proton) This is an approximation, 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. The differences which result from nuclear binding are generally less than 0.01 u, however. Chemical element masses, as expressed in u, would therefore all be close to whole number values (within 2% and usually within 1%) were it not for the fact that atomic weights of chemical elements are averaged values of the various stable isotope masses in the abundances which they naturally occur. [http://www.sisweb.com/referenc/source/exactmaa.htm] For example, chlorine has an atomic weight of 35.45 u because it is composed of 76% ³⁵Cl (34.96 u) and 24% ³⁷Cl (36.97 u).

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.

Avogadro's number (N_A) 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.
For example, the molecular mass of a water molecule containing one ⁶O isotope and two ¹H isotopes is 18.0106 u, and this means that one mole of this monoisotopic water has a mass of 18.0106 grams. Water and most molecules consist of a mixture of molecular masses due to naturally occurring isotopes. For this reason these sort of comparisons are more meaningful and practical using molar masses which are generally expressed in g/mol, not u. In other words the one-to-one relationship between daltons and g/mol is true but in order to be used accurately for any practical purpose any calculations must be with isotopically pure substances or involve much more complicated statistical averaging of multiple isotopic compositions.