Mendeleev arranged the elements in order of their relative atomic mass, and this gave him some problems. When ordered in this way, some elements didn't fit into the same group as other elements with similar properties.
The English physicist Henry Gwyn Jeffreys Moseley showed that Mendeleev's ordering generally places the elements in sequential order by nuclear charge, which is identical to proton count. By ordering the elements strictly according to nuclear charge, Moseley was able to resolve the anomolies in Mendeleev's table.
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