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 SI symbol for atomic mass unit is "u". The symbol "amu" can sometimes 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".

The value and SI symbol are defined at the SI website as:

1 u ≈ 1.6605402 x 10^-27 kg

See 1 E-27 kg for a list of objects which have a mass of about 1 u.

The unit is convenient because one hydrogen atom weighs 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. 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 u in terms of kilograms.

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 weigh precisely 1 gram. As an equation:

1 u = 1 gram/mole

or equivalently

1 gram = N_A u

For example, the molecular mass of water is 18.01508 u, 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.

Measuring Relative Atomic Masses

The relative atomic mass is measured with a mass spectrometer. After placing a sample of the element to be measured in the mass spectrometer it is bombarded with electrons which turns the atoms into positive ions. An electric field is then used to accelerate these positive ions, afterwhich the ions are deflected using a magnetic field. As a result the various isotopes are separated out due to the ions of lighter isotopes being deflected more than those heavier. This produces a mass spectrum.

This spectrum provides two things:

1. Relative isotopic masses in the sample
2. Abundances of the isotopes

Using Mess Spectrum Data to Calculate Relative Atomic Mass

A simple calculation may be used to calculate the relative atomic mass of the sample. This is demonstrated in the following example.

Therefore, the relative atomic mass of the Carbon sample is:

(70/100 x 11) + (30/100 x 13)

7.7 + 3.9

= 11.6 [this is not the true atomic mass of carbon, it is merely illustrative]