‘AMU’ refers to ‘atomic mass unit.’  This particular system of measurement is also referred to as a ‘dalton’ with the symbol ‘Da.’  As a form of measurement, the AMU is used to indicate the so-called mass of an atom or molecule.  When talking about the mass of a particular molecule, for example, it simply refers to the total number of protons and neutrons.  When there are more protons and neurons, the mass and the AMU will also be larger.  As a measurement unit, the AMU is basically 1/12 of the total mass of a carbon-12 atom.  The ’12’ represents the number of protons and neutrons in the carbon molecule. Consequently, 1 atomic mass unit or AMU is also equivalent to one atom of the element hydrogen.

The atomic mass unit is also called a dalton because of the scientist named John Dalton.  Back in 1803, Dalton was the one who established AMU as a unit of measurement. At the time, Dalton used the element hydrogen to establish that it is equivalent to an atomic mass unit.  In simple terms, when a proton is merged with a neutron, the element hydrogen is created, and so it corresponds also to one atomic mass unit.  Over the years, Dalton’s standard was used as a reference to the atomic mass unit measurement of various atoms and molecules.  Later on, another chemist suggested another reference which is oxygen.  For Wilhelm Ostwald, the atomic mass unit or atomic weights of elements can best be measured using oxygen.  By the year 1961, carbon-12 became the new reference for atomic mass units.  By this time the AMU chemical symbol was replaced with ‘U,’ or short for unified atomic mass unit.  It was said that the renaming of AMU to ‘U’ was much needed by the scientific community in order to prevent confusion in terms of documents and literature.