<P> The well - known standard atomic weight is an application of this relative atomic mass values from different samples . It is expected range of the relative atomic mass values, with the various sources being terrestrial (taken from Earth). These standard atomic weights are what chemists loosely and so incorrectly call "atomic weights" (incorrectly, because they are not from a single sample). They are the most published form of the relative atomic mass, because they are not sample - specific, but cover a broad range of expected Earth samples . </P> <P> The continued use of the term "atomic weight" (of any element), as opposed to "relative atomic mass" has attracted considerable controversy, since at least the 1960s, mainly due to the technical difference between weight and mass in physics . Both terms are officially sanctioned by IUPAC . The term "relative atomic mass" now seems to be gaining as the preferred term over "atomic weight", although in the case of "standard atomic weight", this shorter term (as opposed to the more correct "standard relative atomic mass") continues to be used . </P> <P> Relative atomic mass is determined by the average atomic mass, or the weighted mean of the atomic masses of all the atoms of a particular chemical element found in a particular sample, which is then compared to the atomic mass of carbon - 12 . This comparison is the quotient of the two weights, which makes the value dimensionless (no unit appended). This quotient also explains the word relative: the sample mass value is made relative to carbon - 12 . </P> <P> It is a synonym for atomic weight (and is not to be confused with relative isotopic mass). Relative atomic mass is frequently used as a synonym for the standard atomic weight and these will have overlapping values if the relative atomic mass used is that for an element from Earth under defined conditions . However, relative atomic mass (atomic weight) covers more than standard atomic weights, and is more general term that may more broadly refer to non-terrestrial environments and highly specific terrestrial environments that deviate from Earth - average or have different certainties (number of significant figures) than do the standard atomic weights . </P>

Which isotope is used as relative scale to find atomic mass