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What is the difference between atomic theory given by Democritus and John Dalton? Because I have read in many books and figured out that both theories explain the same i.e atom is an indivisible particle and all elements differ in shape and weight of their atoms. So basically what is the difference in both the theories?

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Demokrit was a philosopher, while Dalton was arguing on the basis of (somewhat limited, but still) solid, quantitative evidence. I'm not knocking Demokrit (!), but his was not a scientific statement, let alone a theory. You cannot compare the two, or at least you should not try to compare them from the view of natural science.

Both came to a similar conclusion, but they were reasoning on a completely different basis. Of course, Dalton had the advantage that the idea was already there and known to him.

In Dalton's time, the modern concept of elements was already known and widely accepted. Iron, oxygen, etc., not fire, water and stone. You could say that he knew what he was talking about, regarding atoms. And Dalton and his contemporaries had the numbers they measured.

Demokrit, on the other hand, had no idea about quantitative elemental analysis. Which ultimately was the downfall of his idea: He had no numbers that others could have proved and extended, or disproved. Only when the chemists at Dalton's time were able to make quantitative measurements the atomic theory could become useful. Demokrit had probably theorised that atoms of the same kind had the same weight (we don't know exactly, because we have very little original records from him), but Dalton now had solid evidence, that only made sense if that was indeed the case.

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    $\begingroup$ This may be, but one can still compare the two theories. If you think the question is too non-scientific for Chem.SE, then vote to close with a recommendation it be posted to, e.g., History of Science & Math SE. $\endgroup$
    – hBy2Py
    May 9, 2017 at 17:01
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    $\begingroup$ I don't find this question off-topic, it's just that the answer is that they do not compare very well, if you look from a natural-science point of view. $\endgroup$
    – Karl
    May 9, 2017 at 17:07
  • $\begingroup$ Could they be usefully contrasted? $\endgroup$
    – hBy2Py
    May 9, 2017 at 17:10
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    $\begingroup$ The major differences are simpler than you might think. Democritus hypothesised that everything was made of indivisible particles (essentially the atomic theory of matter) but Dalton knew the number of varieties those particles came in. Neither understood enough about why they differed (that wasn't completely clear until the neutron was discovered in the 1930s.) $\endgroup$
    – matt_black
    Jun 30, 2017 at 0:09
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    $\begingroup$ @anna Can you be a bit more constructive? What is missing? $\endgroup$
    – Karl
    Sep 2, 2018 at 8:26

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