![]() ![]() This is frequently used as a synonym for the standard atomic weight and it is not incorrect to do so since the standard atomic weights are relative atomic masses, although it is less specific to do so. The relative atomic mass (A r) (also known as atomic weight and average atomic mass) is the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance. The atomic mass of an uncommon isotope can differ from the relative atomic mass or standard atomic weight by several mass units. For elements with more than one common isotope the difference even to the most common atomic mass can be half a mass unit or more (e.g. In the case of many elements that have one dominant isotope the actual numerical difference between the atomic mass of the most common isotope and the relative atomic mass or standard atomic weights can be very small such that it does not affect most bulk calculations but such an error can be critical when considering individual atoms. The atomic mass is defined as the mass of an atom, which can only be one isotope at a time and is not an abundance-weighted average. The atomic mass is sometimes incorrectly used as a synonym of relative atomic mass, average atomic mass and atomic weight however, these differ subtly from the atomic mass. The atomic mass may be considered to be the total mass of protons, neutrons and electrons in a single atom (when the atom is motionless). The atomic mass (m a) is the mass of an atom at rest, most often expressed in unified atomic mass units.
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