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I was reading about the history of the periodic table and the description of an element’s atomic number as its proton number. I couldn’t find the source again but it was stated that they figured out each element’s proton number because it’s equal to that element’s nucleus’ electric charge. So, how do you measure an element’s nucleus’ electric charge?

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    $\begingroup$ They didn't have the means for that back then - it would involve fully ionising it. That atomic number is its proton number was found thanks to Moseley's law $\endgroup$
    – Mithoron
    Commented Apr 15 at 13:47
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    $\begingroup$ Atoms are neutral. If one knows the number of electrons, one knows the number of protons. See how Mosely relate3d spectral lines to number of electrons, for example. $\endgroup$ Commented Apr 15 at 13:51
  • $\begingroup$ How does one fully ionize an atom and was that technology invented later? @mithoron $\endgroup$ Commented Apr 15 at 17:14
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    $\begingroup$ You just pump it enough energy, with laser, maser, or whatever. Then you could tell it's charge from behaviour in magnetic field. Accelerators use fully stripped nuclei all the time. $\endgroup$
    – Mithoron
    Commented Apr 15 at 17:39
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    $\begingroup$ Might be worth rephrasing to "how did early chemists work out the number of protons in the nucleus". $\endgroup$
    – matt_black
    Commented Apr 15 at 23:20

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The allusion to Moseley's contributions in the comments is only partially correct in a historical sense. It was Antonius van den Broek (a name rarely mentioned in general chemistry) who proposed that the atomic number corresponds to the charge of the atomic nucleus.

See his single paragraph influential letter: Van den Broek, Antonius. "The Number of Possible Elements and Mendeléff's “Cubic” Periodic System." Nature 87.2177 (1911): 78-78. Link

The Number of Possible Elements and Mendeléeff's "Cubic" Periodic System. According to Rutherford's theory of "single scattering" ("On the Scattering of $\alpha$ and $\beta$ Particles by Matter and the Structure of the Atom," Phil. Mag., May, I9II), and to Barkla's "Note on the Energy of Scattered X-Radiation" (ibid.), the numbers of electrons per atom is half the atomic weight; thus, for $U$, about 120 . Now, a reconstruction of Mendeléeff's " cubic" periodic sy:stem, as suggested in his famous paper "Die Beziehungen zwischen den Eigenschaften der Elemente und ihren Atomgewichten " (Ostw. Klass., No. 68, pp. 32, 36, 37, and 74), gives a constant mean difference between consecutive atomic weights $=2$, and thus, from $H$ to $U, > 120$ as the number of possible elements (van den Broek, "Das Mendelejeff'sche 'Kubische' Periodische System der Elemente und die Einordnung der Radioelemente in dieses System," Physik. Zeitschr. I2, p. 490). Hence, if this cubic periodic system should prove to be correct, then the number of possible elements is equal to the number of possible permanent charges of each sign per atom, or to each possible permanent charge (of both signs) per atom belongs a possible element.

Noordwijk-Zee, June 23. A. VAN DEN BROEK.

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    $\begingroup$ The key point of Moseley's work is that he showed that something observable (the frequency of X-ray emissions from nuclei) told us something about the composition of the nucleus. This made sense if atomic number (previously based on ordering elements by mass with some adjustments for chemical similarity) was based on some observable nuclear feature that matched the atomic number. Other known observations combined suggested this feature was the nuclear charge. Actually creating fully ionised nuclei wasn't necessary. $\endgroup$
    – matt_black
    Commented Apr 16 at 9:02
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    $\begingroup$ @AChem May you add the name of journal of the publication, or/and a doi? $\endgroup$
    – Buttonwood
    Commented Apr 19 at 16:28

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