Atomic mass refers to specific weight of a specific atom or atomic particle. The value for atomic mass also includes sub-atomic particles and their specific masses. As opposed to atomic weight which is also referred to as relative or average atomic mass, the value given for atomic mass is considered specific. Atomic weight involves averaging of the atomic masses of all atoms in a given particle of substance. All these average weights are added up to come up with the atomic weight. Atomic mass meanwhile involves the specific mass of the atom concerned. In general, atomic mass is also defined as 1/12 of the atomic mass of the carbon-12 atom in a state of rest. As a unit, atomic mass may be described in Daltons or unified atomic mass units. In the case of carbon, its atomic mass can be interpreted as 12 Daltons or 12 atomic mass units.
Atoms are basically composed of protons or positively-charged ions, neutrons or neutrally-charged ions, and electrons or negatively-charged ions. All the atomic mass of these components results to the average atomic mass. The actual or specific atomic mass is actually lower than this because of the so-called energy mass loss. This loss of energy involving atomic sub-particles will result to a slightly lower total for the specific atomic mass of a sample atom.
Particles and substances may be composed of atoms and molecules but each of these may also be composed of different isotopes which may lead to different values for specific atomic mass. This is the reason why the term “relative” atomic mass or average atomic mass is commonly used when describing this particular property. The different isotopes provide slight variations in atomic mass and/or atomic weight. With some particles having 2 or more different isotopes, some also measure their atomic mass in using these very details and the result is referred to as relative isotopic mass of the atom.