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    Frequent Questions

    1. Why do we need so many digits in atomic weights?
    2. Are the atomic weights constants of nature?
    3. Why do some elements have no standard atomic weight?
    4. Atomic weight or atomic mass?
    5. Why are there no units for atomic weights?
    6. What was the biggest atomic weight revision?

    1. Why do we need so many digits in atomic weights?

    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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    2. Are the atomic weights constants of nature?

    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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    3. Why do some elements have no standard atomic weight?

    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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    4. Atomic weight or atomic mass?

    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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    5. Why are there no units for atomic weights?

    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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    6. What was the biggest atomic weight revision?

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

    Edna Krabappel: Who can tell me the atomic weight of bolognium?

    Ghostbusters II (Columbia Pictures, 1989)

    Dr. Raymond Stantz: Think there's a connection between this Vigo character and the...slime?
    Dr. Egon Spengler: Is the atomic weight of cobalt 58.9?