Atomic weight (symbol: Ar) is a dimensionless physical quantity, the
ratio of the average mass of atoms of an element (from a given source) to 1/12
of the mass of an atom of carbon-12 (known as the unified atomic mass unit).
The IUPAC definition
of atomic weight is:
An atomic weight (relative atomic mass) of an
element from a specified source is the ratio of the average mass per atom of
the element to 1/12 of the mass of an atom of12C.
Atomic weight (symbol:
Ar) is a dimensionless physical quantity, the ratio of the average mass of atoms
of an element (from a given source) to 1/12 of the mass of an atom of carbon-12
(known as the unified atomic mass unit). The term is usually used, without
further qualification, to refer to the standard atomic weights published at
regular intervals by the International Union of Pure and Applied Chemistry (IUPAC)
and which are intended to be applicable to normal laboratory materials. These
standard atomic weights are reprinted in a wide variety of textbooks, commercial
catalogues, wall charts etc, and in the table below. The fact "relative atomic
mass of the element" may also be used to describe this physical quantity, and
indeed the continued use of the term "atomic weight" has attracted considerable
controversy since at least the 1960s
Atomic weights, unlike atomic masses (the masses of individual atoms), are not
physical constants and vary from sample to sample. Nevertheless, they are
sufficiently constant in "normal" samples to be of fundamental importance in
chemistry.
The definition deliberately specifies "An atomic
weight…", as an element will have different atomic weights depending on the
source. For example, boron from Turkey has a lower atomic weight than boron
from California, because of its different isotopic composition. Nevertheless,
given the cost and difficulty of isotope analysis, it is usual to use the
tabulated values of standard atomic weights which are ubiquitous in chemical
laboratories.
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