Relative atomic mass (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., and in the table below.
The term "atomic weight" (of the element) is also used to describe this physical quantity, and is synonymous with it. However, its continued use has attracted considerable controversy since at least the 1960s (see below).
Read more about Relative Atomic Mass: Definition, Naming Controversy, Determination of Atomic Weight, Periodic Table With Atomic Weights
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