The atomic mass of an element is the mass of the number of particles in its nucleus given in atomic mass units. Although it is sometimes referred to as atomic weight, this is inaccurate, since weight is a measurement that includes the effect of gravity. One atomic mass unit (amu) is 1.66 x 10^{- 24} grams. This is equal to one-twelfth the mass of an atom of carbon- 12. This standard was adopted internationally in 1961, replacing an arbitrarily assigned value of 16.000 amu for the atomic mass of an atom of oxygen.

The instrument most often used to measure atomic mass is the mass spectrometer (spectrograph). In this instrument, atoms are vaporized and then changed to positively charged particles by knocking off electrons. These charged particles are passed through a magnetic field that causes them to be deflected different amounts, depending on the size of the charge and...