Dmitri Mendeleev arranged his first periodic table according to the number of protons and valency electrons. How did he, and others of his age, know of these subatomic particles and electron shell capacities? Nowadays, we have technology such as electron microscopes that can help us see (referring to another Physics Stack Exchange post regarding whether we can actually see atoms) these subatomic particles, but how did chemists and scientists in general know?

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    $\begingroup$ He used atomic weight, and not number of protons ... See Mendeleev. $\endgroup$ Jul 28, 2015 at 15:44
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    $\begingroup$ Previously from Physics. $\endgroup$
    – HDE 226868
    Jul 28, 2015 at 16:52
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    $\begingroup$ Even now we can't use electron microscopes to see or count protons. We know how many protons by charge number. $\endgroup$ Jun 9, 2020 at 18:46

1 Answer 1


Of course Mendeleev had no idea about protons or electrons. It is he who discovered the integer which we call atomic number, and which was later found to be the number of protons.

The story of this discovery is approximately this. He was preparing to his chemistry course where he was supposed to give a survey of elements. He had to choose some logical order. For this he made little paper cards on which he wrote the element names and their main properties (like chemical valency). Then he tried to arrange the cards in some logical order.

Finally he found that if the cards are ordered according to the atomic weight, then their chemical properties change roughly periodically. This was called the periodic law. It suggested that the elements are arranged in a table...

There were several exceptions to ordering according to the weight. With the corresponding changes the table looked so nice, that Mendeleev was convinced that he discovered some law. The table also had some gaps. Mendeleev had courage to predict that these gaps correspond to the elements which are not discovered yet. He PREDICTED the existence of new elements and predicted some of their properties.

When these new elements were discovered, this essentially proved that the law was correct.

At the time of discovery this was a purely empirical law. Explanation of this law had to wait for the discovery of quantum mechanics and spin and Pauli Principle.

EDIT. These sources make it more precise: he did not do this in one evening while preparing to a lecture, but worked on a textbook (as a part of his duties as a professor of chemistry). The sources give his personal recollections, unfortunately all only in Russian:



  • $\begingroup$ Can you give a reference that says this work arose from his preparation to teach a course? For comparison, I've seen over the years several sources discuss Dedekind's formulation of a definition of real number as an effect of his desire to give a clearer foundation for analysis in his teaching, but I hadn't read that Mendeleev was led to the periodic table while preparing to teach a class. The page molomo.ru/myth/mendeleev.html (see 5th "myth") reports Mendeleev saying that he was led to the periodic table after thinking about it over a 20-year period. $\endgroup$
    – KCd
    Aug 3, 2015 at 19:26
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    $\begingroup$ This was well-known when I studied chemistry at school, but this was long ago and far away. I will search for a reference. Of course, this story may be a simplification. He may have started thinking on this while preparing the course, and finishing 20 years later. But I will try to find a reference. $\endgroup$ Aug 3, 2015 at 21:21
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    $\begingroup$ @KCd: Ok, here is a source: hemi.nsu.ru/ucheb145.htm $\endgroup$ Aug 3, 2015 at 21:31
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    $\begingroup$ Thanks for the links in your answer. I agree that the second one makes it clear in his own words (or at least his son's paraphrasing?) that the discovery was made in its precise form while writing a textbook, not for preparing any particular lectures. $\endgroup$
    – KCd
    Aug 4, 2015 at 7:28
  • $\begingroup$ As for references in English you may consider the book by Gordin amazon.com/Well-ordered-Thing-Dmitrii-Mendeleev-Periodic/dp/… . He also wrote a joint paper on this A Well-Ordered Thing: Dmitrii Mendeleev and the Shadow of the Periodic Table, Michael D. Gordin and Ursula Klein, Physics Today, 06/2005; 58(6):60-62. DOI: 10.1063/1.1996479 $\endgroup$ Aug 18, 2015 at 8:18

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