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atomic weight

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, wallcharts etc. 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. Wikipedia

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 of <sup>12</sup>C.

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. Wikipedia

"(Atomic weight) Is the rate of vibration." (John Dalton)

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Page last modified on Wednesday 04 of August, 2010 01:18:26 MDT