<P> For example, the relative isotopic mass of a carbon - 12 atom is exactly 12 . For comparison, the atomic mass of a carbon - 12 atom is exactly 12 daltons or 12 unified atomic mass units . Alternately, the atomic mass of a carbon - 12 atom may be expressed in any other mass units: for example, the atomic mass of a carbon - 12 atom is about 1.998467052 x 10 kilogram . </P> <P> As in the case of atomic mass, no nuclides other than carbon - 12 have exactly whole - number values of relative isotopic mass . As is the case for the related atomic mass when expressed in unified atomic mass units or daltons, the relative isotopic mass numbers of nuclides other than carbon - 12 are not whole numbers, but are always close to whole numbers . This is discussed more fully below . </P> <P> The atomic mass and relative isotopic mass are sometimes confused, or incorrectly used, as synonyms of standard atomic weight (also known as atomic weight) and the standard atomic weight (a particular variety of atomic weight, in the sense that is a standardized atomic weight). However, as noted in the introduction, atomic weight and standard atomic weight represent terms for (abundance - weighted) averages of atomic masses in elemental samples, not for single nuclides . As such, atomic weight and standard atomic weight often differ numerically from relative isotopic mass and atomic mass, and they can also have different units than atomic mass when this quantity is not expressed in unified atomic mass units (see the linked article for atomic weight). </P> <P> The atomic mass (relative isotopic mass) is defined as the mass of a single atom, which can only be one isotope (nuclide) at a time, and is not an abundance - weighted average, as in the case of relative atomic mass / atomic weight . The atomic mass or relative isotopic mass of each isotope and nuclide of a chemical element is therefore a number that can in principle be measured to a very great precision, since every specimen of such a nuclide is expected to be exactly identical to every other specimen, as all atoms of a given type in the same energy state, and every specimen of a particular nuclide, are expected to be exactly identical in mass to every other specimen of that nuclide . For example, every atom of oxygen - 16 is expected to have exactly the same atomic mass (relative isotopic mass) as every other atom of oxygen - 16 . </P>

What si the difference between mass and weight