how does chesmists of 1700s – 1900s knows how many protons the elements have?

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how does chesmists of 1700s – 1900s knows how many protons the elements have?

In: Chemistry

They didn’t. It was only in the early years of the 20th century that Rutherford showed that there was a small positively charged nucleus at the centre of an atom. That led to the development of modern pictures of the atom.

That doesn’t mean that clues were there. In the 18th century, Lavoisier and others had measured the masses of different elements that reacted together and found that there were fixed ratios and simple multiples. That led to valence and ideas of atomic weights. In turn that led to ordering the elements by mass and a number of tables to try and make sense of the data. Ultimately Mendeleev’s Periodic Table of 1869 and atomic physics discoveries from then on made sense of atomic structure and behaviour.