The atomic mass unit, also called the dalton after the chemist John Dalton, 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. The abbreviations "u", "amu" and "Da" are used for this unit; often, atomic masses are written without any unit and then the amu is implied.
The value is
- 1 amu ≈ 1.6605387 × 10-27 kilograms.
The unit is convenient because one hydrogen atom weighs approximately 1 amu, and more generally an atom or molecule that contains n protons and neutrons will have a mass approximately equal to n amu. This is only a rough approximation however, since it doesn't account for the mass contained in the binding energy of the nucleus.
Another reason the unit is used is that it is much easier to compare masses of atoms and molecules (determine relative masses) than to measure their absolute masses. Finding the mass of a given molecule in amus is thus easier than to express 1 amu in terms of kilograms.
- 1 amu = 1 gram/mole
- 1 gram = NA amu