# Frequent Questions

Accurate values of atomic weights have an important place in science.
The 1914 Nobel Prize for Chemistry was awarded to Theodore W. Richards for "his work in accurate determination of atomic weights of many elements", and this recognized the need for accurate atomic weights of elements.

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Yes, but only for those elements whose standard atomic weight is determined by only one isotope (there are nineteen such elements).
For all elements with more than one isotope, their atomic weight depends on the relative amounts of the isotopes, which can vary significantly in nature.

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Many elements have no stable isotopes and do not have a characteristic terrestrial isotopic composition. For these elements, there is no standard atomic weight.

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The name *atomic mass* refers to a mass of a *single* atom.

In contrast, the name *atomic weight* refers to the weighted average mass of *all* atoms of an element, considering the amount of each isotope.
Furthermore, *atomic weight* is defined as the ratio of the average masses of all isotopes and the atomic mass unit (1/12 of carbon-12 atom mass)

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Because the *atomic weight* is defined as the ratio of two mass-related quantities – it is the ratio of the average atomic mass of an element over 1/12 of the mass of the carbon-12 atom.
As long as both of these masses are measured in the same scale, the resulting ratio does not depend on the unit of mass employed.

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Starting from 1951, CIAAW has recommended atomic weights with the associated uncertainty. Since then, only once has the recommended atomic-weight value been placed outside the uncertainty bounds of the prior recommendation.
This happened in 2007, when the standard atomic weight of zinc was changed to 65.38 ± 0.02 from 65.409 ± 0.004.

Historically, the largest change in the atomic weight value by CIAAW occurred in 1909, when the atomic weight of the newly discovered xenon was changed to 130.7 from 128.0.

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The current values of standard atomic weights and isotopic compositions of the elements were adopted by the CIAAW in 2021.

The latest *Standard atomic weights* Report was published in *Pure Appl. Chem.* **94** aop (2022).

*Isotopic compositions of the elements* were last published in *Pure Appl. Chem.* **88** 293-306 (2016).

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No. Dalton (Da) is the unit of atomic mass whereas g/mol is the SI unit for molar mass.
In the past, definitions of dalton and mole were both linked to carbon-12 which is why molar masses of elementary entities (in g/mol) were numerically
identical to their atomic masses (in Da).

Since 2019, however, the definition of the mole has changed and is no longer tied to carbon-12. Therefore, the molar mass of unbound carbon-12 atoms is no
longer 12 g/mol exactly. Rather, it has acquired a small uncertainty - less than one part in 10^{9} - which is of no consequence to chemists.

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*The Simpsons*, Lisa Gets an "A" (FOX, 1998)

Ghostbusters II (Columbia Pictures, 1989)