Having understood the composition of matter, the problem now was to understand why atoms were of different sorts. This was the problem tackled by John Dalton. In 1805 he published his atomic

theory that every element consists of similar indivisible atoms which differ from other elements in their size and mass. But how were atoms to be classified by size and mass?
This was the problem finally solved by Dmitri Mendeleev. Devising the Periodic Table of elements in 1869, he listed the elements according to their mass. And in the greatest tradition of science, the table left spaces for elements not yet discovered. Hence, as well as true scientific classification, the table was also predictive.
We now know of 92 naturally existing elements on Earth. And since then, many manufactured elements have been added.

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