Atomic mass unit

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 ≈ 1.6605402 × 10^-27 kilograms.

The unit is convenient because 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.

Avogadro's number N_A 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 = N_A 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 N_A/18.01508 ≈ 3.3428 × 10²² molecules.