The concept of relative atomic weight originated from measuring the combining weight of hydrogen with a certain element. In the simplification process H was taken as unity (18th, 19th and 20th century). For example from combining weights of hydrogen and oxygen, one can show that the weight of oxygen is almost 16 times more than H. Chemists did not have any means to measure absolute masses.

Now when Aston invented the mass spectrometer in 1940s, he also measured atomic masses. Aston's book is available here https://archive.org/details/in.ernet.dli.2015.205751/page/n14

1) As per my understanding, a mass spectrometer can measure the absolute mass based on charge-to-mass ratio. Provided we know the charge on the ion which is usually one, one can determine the absolute mass without any regard to chemical principles, purely on the principles of electricity and magnetism. I am wondering how come the current masses measured by chemical means turn out to be almost the same, in terms of numerical values, as measured by an independent instrument such as a mass spectrometer. For example, O-16 isotope is still 16 by MS measurements?

2) Why don't we quote absolute atomic weights given that modern FT-Ion Cyclotron MS can detect extremely delicate mass changes?


  • $\begingroup$ Isn't this because most elements studied in the early 19th century were nearly mono-isotopic? $\endgroup$ – kimchi lover Feb 2 at 17:17
  • $\begingroup$ No, this is not the case, by early 1920s, Theodore Richards (Nobel) has determined atomic weights of most elements. $\endgroup$ – M. Farooq Feb 2 at 17:27
  • $\begingroup$ I'm puzzled by your comment: the 1920s were not in the 19th century. Is your question a historical one at all? $\endgroup$ – kimchi lover Feb 2 at 17:44
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    $\begingroup$ I can't really answer your question, but I'd say it's no coincidence. It's because both are getting at the "underlying truth" as it were. You probably know about it already, but see Prout's hypothesis: "he observed that the atomic weights that had been measured for the elements known at that time appeared to be whole multiples of the atomic weight of hydrogen. He then hypothesized that the hydrogen atom was the only truly fundamental object, which he called protyle..." $\endgroup$ – John Duffield Feb 2 at 18:07
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    $\begingroup$ Since both are evaluating the same physical parameter(s), how could they not be very close? $\endgroup$ – Carl Witthoft Feb 4 at 14:00

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