Values from CRC Handbook of Chemistry and Physics, 100th Edition (Internet Version 2019), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.Standard atomic weights are from Wieser, M. E., et al., Pure Appl. Chem. 85, 1047, 2013 plus changes made through 2018 by the International Union of Pure and Applied Chemistry (IUPAC) Commission on Isotopic Abundances and Atomic Weights. Each atomic weight had previously been given as a single value with an uncertainty that took into account both the measurement uncertainty and the variation in isotopic abundance in samples of the element from different terrestrial sources. For a variety of reasons, this fails to give complete information on the natural variability in isotopic abundance of several elements. Therefore, beginning in 2009 the IUPAC recommendations expressed the atomic weights of several elements as intervals rather than single numbers plus uncertainties. The symbol for these intervals is [a, b], where a is the lower bound of values found in normal terrestrial materials and b the upper bound. For the other elements in the table, a single recommended atomic weight value is given; the number in parentheses following the value gives the uncertainty in the last digit.
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