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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)
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Update: June 2012
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