Atomic mass unit
The atomic mass unit
, also called the dalton
after the chemist John Dalton
, is a small unit
used to express atomic masses
and molecular masses. It is defined to be 1/12 of the mass of one atom
. 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.
See 1 E-27 kg
for a list of objects which have a mass of about 1 amu.
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.
Avogadro's number NA and the mole are defined so that one mole of a substance with atomic or molecular mass 1 amu will weigh precisely 1 gram. As an equation:
- 1 amu = 1 gram/mole
- 1 gram = NA amu
For example, the molecular mass of water
is 18.01508 amu, and this means that one mole of water weighs 18.01508 grams, or conversely that 1 gram of water contains NA
/18.01508 ≈ 3.3428 × 1022