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Dalton showed in 1803 that matter was made of atoms rather than continuously distributed. But phosphorous, hydrogen, and other elements were identified before this discovery. How could they have known that these were elements before it was shown that matter was made of atoms?

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    $\begingroup$ For the ancient chemist, an element was a substance that had never been decomposed into smaller fragments. So if nobody had been able to decompose a substance by electricity, by heat, or any other process, this substance was considered as an element. Phosphorous and Hydrogen had never been decomposed in Dalton times. So they were put in this category. $\endgroup$
    – Maurice
    Jan 19 at 20:58
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    $\begingroup$ "Dalton showed" is quite a bit of an overstatement. Atomic theory was around for a long time before that, and was not firmly proven for a long time after. $\endgroup$ Jan 20 at 10:31
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As elements were considered such substances that could not be dissembled to more substances.

Before oxygen and hydrogen discovery and observation they form water together, water was considered as an element.

Before realising air major composition, air was considered as an element.

Before praseodymium and neodymium were separated, their mixture was erroneously considered as an element didymium.

At some point, most substances being considered as elements were verified to be real chemical elements at atomic level.

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