Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: dn54321 on December 18, 2017, 12:51:29 AM
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I'm not sure if this is in the right section as I've just recently finished highschool, However, I always wondered how scientist were able to measure the atomic number and molecular mass of elements and tabulate it into the periodic table. How were scientist able to measure just how many protons/neutrons were in each element? A great in-depth explanation will help me alot. ^o^
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Initially they didn't measure numbers of protons and neutrons, just compared number of moles with mass. Such measurements are relative, that is, they required to define one of the elements (or better: isotopes) as a reference. At present the reference is C12.
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A key step was Henry Moseley's measurement of characteristic X-ray lines of the elements, and the discovery that the energies varied as the square of "position in the periodic table", thus establishing a physical (as distinct from merely book-keeping) basis for the concept of atomic number:
https://en.wikipedia.org/wiki/Moseley%27s_law
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Compared electrostatic and electromagnetic deflections measure the q/m of ions. Once you know or define q (and have guessed the ionization number), you deduce m.