The answer is: Henry Moseley.
Henry Gwyn Jeffreys Moseley (1887–1915) proposed that the number of positive nuclear charges (protons) in atom is equal to its atomic number in the periodic table.
Mendeleev did not know about isotopes and he arranged the elements in order of increasing relative atomic mass.
For example, positions of iodine and tellurium were reversed in Mendeleev's table, because of different isotopes, but Moseley solved anomalies like this one.
