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Worth the Weight: New Table Aims to Clarify Variable Atomic Weight Values

The new table includes both standard and conventional atomic weights values to clarify that many atomic weights have natural variation and to provide single values for chemical education use

Those left confused by recent updates to the table of standard atomic weights, whose values appear on the periodic table of elements, have reason to celebrate.

Three scientists from the U.S. Geological Survey, the International Union of Pure and Applied Chemistry (IUPAC), and the Brookhaven National Laboratory have prepared a new table meant to clarify atomic weights.

For the first time, a single table containing both four-digit standard atomic weight values and conventional atomic weight values, for those elements with standard atomic weights given as intervals, is available, making it easier for teachers to demonstrate that the atomic weights of many elements have natural variation and are not constants of nature. Additionally, students and others can select a single value for molecular calculations.

Pie Diagram for Sulfur Showing Standard and Conventional Atomic Weight
For sulfur, the four-digit conventional atomic weight is 32.06 and the standard atomic weight is the interval [32.06, 32.08]. (Public domain.)

The atomic weights of more than half of the elements have some variability. To indicate this, in 2009 and 2011 the Commission on Isotopic Abundances and Atomic Weights, of the IUPAC, replaced single-value standard atomic weight values with atomic weight intervals for 12 elements, whose variations are well known: hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, bromine, and thallium. For example, the four-digit standard atomic weight of sulfur became the interval [32.06, 32.08].

This change, while representative of the true atomic weights of elements, presented its own problem: teachers and students did not know what value to use in classroom problems, like molecular calculations. With the new table, teachers can easily demonstrate to students that several chemical elements have variable atomic weight values depending upon their source, and when a single value is needed, such as for molecular calculations, the same row in the table indicates that the single-value conventional atomic weight can be used.

“As a young student of chemistry, I was taught that the atomic weights of the elements on the periodic table were constants of nature,” said Tyler Coplen, Director of the Reston Stable Isotope Laboratory. “It took me decades to discover that standard atomic weights of a dozen elements are variable and should be displayed as intervals to highlight these natural variations.”

“This new table presents the best of both worlds—single values for use in calculations and the more complex intervals for stable isotope applications,” said Coplen. For example, interval values can be used to highlight the variability of atomic weights arising from variations in abundances of stable isotopes in different sources of elements, such as variations in atomic weights of hydrogen and oxygen in precipitation, rivers, and groundwater.

Map showing the atomic wight of hydrogen in rivers of the United States
The atomic weight of hydrogen of river water is variable across the United States and depends on the source of the water. (Public domain.)

Word and Excel versions of this Table of Standard and Conventional Atomic Weights 2016 Abridged to Four Significant Digits are provided in the USGS data release:

Coplen, T. B., Meyers, F., Holden, N. E., 2016. Standard and conventional atomic weights 2016 abridged to four significant digits. U.S. Geological Survey Data Release,


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