Exact versus Relative Atomic Mass ```Name: Jimmy A. Status: educator Grade: other Location: N/A Country: USA Date: May 2007 ``` Question: If actual masses of all the elements are known very accurately, then we should use the actual masses of elements.But we are still using the relative atomic masses of elements.What is the reason that we are still using the relative atomic masses of elements? Replies: Jimmy A. You have raised a much larger issue. The unit of mass -- the kilogram -- is defined in terms of a platinum - iridium cylinders. There are several around the world. It is the last of the fundamental physical constants still based on a physical artifact. So all masses are already relative to begin with. As you point out the actual masses of all, actually just most, of the atomic isotopes are known with more precision (in contrast to "accuracy") than the standard itself. So, although relative, the masses of atomic isotopes are self-consistent -- and for most all applications that is "good enough". There is a lot of research in progress to replace the definition of the "kilogram" to a one based on the other "fundamental constants" because the "artifact" has changed its value with time. There are several candidates under consideration, but there isn't yet a consensus about which method should be used to define the kilogram. There are "pros" and "cons" of the various methods. The details are to long to address here but you can read about the "problem" by searching the term "definition of the kilogram". A couple of sites that I found are: http://www.npl.co.uk/mass/faqs/kilogram.html http://en.wikipedia.org/wiki/Standard_atomic_weight#History Scientists are very careful when "messing with" the definitions of fundamental physical constants. Vince Calder Jimmy, I think we have to be careful with what we mean by "actual mass". One could argue that it is possible to measure the actual mass of a single atom (perhaps by tracking the curve path of a charged ion) - but if we look closely, all these measurements have an embedded number that is still based on the idea of relative atomic masses. Usually, such analyses use the approximation of Avogadro's number (still not known to the units place of significant figures), the estimation of the mass to charge ratio, or a derived molar mass - all of which are based on the agreed upon relative masses. Greg (Roberto Gregorius) Click here to return to the Chemistry Archives

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