In support of the International Union of Pure and Applied Chemistry (IUPAC) project “Development of an isotopic periodic table for the educational community”, Commission on Isotopic Abundances and Atomic Weights (CIAAW) presents the IUPAC Periodic Table of the Isotopes, which emphasises the isotopic composition of the elements.
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The IUPAC started the project in 2008 and the table was published in July this year.
IUPAC's committee on chemical education felt that periodic tables which provide mean molecular weights may be misleading to some students. The mass number listed for bromine, for example, is generally 79.9, but of course, there is no such thing as 80Br. Instead bromine is composed of the distinct isotopes 79Br and 81Br which exist in roughly a 1:1 ratio.
Source: Royal Society of Chemistry, Commission on Isotopic Abundances and Atomic Weights
Related Links: Atomic Weights: No Longer Constants of Nature