Atomic mass is the mass of a single atom or an individual isotope. The atomic weight is the average mass of all naturally occurring isotopes of an element.
The atomic mass is expressed in units of atomic mass unit, or amu, and consists primarily of the protons and neutrons in the atom's nucleus, making it closely match the mass number of an atom. When an element has two or more isotopes, the atomic weight is expressed as an interval with an upper and lower number. Both atomic masses and atomic weights are determined by mass spectrometry, a technique where electrically charged particles are separated by their masses.