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How do we know?

When did we first discover that some standard atomic weights are not constants of nature?

leadIn 1908, the atomic weight of lead was measured and given the value of 207.2. Six years later, the atomic weight of lead from a different source was measured to be 208.4. The inconsistent values were thought to result from the decay of radioactive isotopes of lead. However, in 1936, oxygen’s atomic weight value was found to vary between air and water sources. This time, it was hypothesized that the different atomic weight values were due to variation in the relative abundance of oxygen’s stable isotopes. In 1969, the Commission published uncertainties for all atomic weight values stated:

“The discovery that most chemical elements exist in nature as isotopic mixtures, many of which are known to vary in composition, makes it necessary to modify the historical concept of atomic weights as constants of nature.” 1

Further evidence for natural variation in isotopic abundance grew over the latter half of the 20th century. Mass spectrometry measurements confirmed varying isotopic compositions in other elements. In 2009, the Commission expressed the atomic weight of 10 elements in atomic weight intervals – with lower and upper bounds indicating the lowest and highest atomic weights found in normal materials.

YbChanges are still being made to the accepted standard atomic weights listed on the periodic table, as technological developments improve our ability to take accurate measurements of atomic weights. Using rigorous evaluation criteria, IUPAC reviews published, peer-reviewed atomic weight data to find the best measurement of isotopic abundances. Reports on isotopic data are regularly reassessed by the Commission. For example, in 2015, this process was used to update the atomic weight of Ytterbium from 173.054 to 173.045.

1.IUPAC. Atomic Weights of the Elements 1969. Pure and Applied Chemistry. 1970, 21, 97.

Your turn

Why do atomic weights for some elements vary?

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Some elements have multiple stable or long-lived isotopes. A given sample of a material composed of one element, will contain a particular abundance of its multiple isotopes (and each isotope has a different atomic mass). A sample from a different source may have a different isotope composition (perhaps a relatively lower abundance of some isotopes, and a relatively higher abundance of others). This leads to a different atomic weight of the element.

Why do some elements have varying atomic weights and some do not?

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Variation in atomic weight is attributed to variation in the isotopic composition of the element. Therefore, in order for isotopic composition of an element to vary, there must be at least two or more stable or long-lived isotopes. Some elements have varying atomic weights because they have two or more stable or long-lived isotopes. Some elements only have 1 stable or long-lived isotope.
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