atomic mass

atomic mass the mass of a single atom , usually expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus. Each proton or neutron weighs about 1 amu, and thus the atomic mass is always very close to the mass number (total number of protons and neutrons in the nucleus). Atoms of an isotope of an element all have the same atomic mass. Atomic masses are usually determined by mass spectrography (see mass spectrograph ). They have been determined with great relative accuracy, but their absolute value is less certain.

relative atomic mass (r.a.m.) (formerly atomic weight) Mass of an atom of the naturally occurring form of an element divided by ¹/₁₂ of the mass of an atom of carbon-12. The naturally occurring form may consist of two or more isotopes, and the calculation of the r.a.m. must take this into account.

atomic mass number (nucleon number) Number of nucleons (protons and neutrons) in the nucleus of an atom. It is represented by the symbol A. In nuclear notation, such as ⁷₃ Li, the mass number is the upper number and the atomic number (the number of protons) is the lower one. Isotopes of an element have different mass numbers but identical atomic numbers.