Dmitri Mendeleev developed the Periodic Table as we know it. What was his assumption while arranging the elements? Why is this assumption known to be Dmitri's biggest contribution?
He assumed that there were missing elements that would match properties of the existing elements above and below on his table
Dmitri Mendeleev made a hypothesis that the elements could be arranged by common properties. Instead of arranging the elements according to mass he arranged the elements according to their properties. Doing this left gaps in the Table. Mendeleev assumed that new elements would be found to fill in the gaps with similar properties as the elements above and below the gap. He even made predictions about the properties of the missing elements regarding their mass, boiling, melting points, the types of compounds the elements would form.
Good theories make good predictions. Mendeleev's theory of the periodicity of elements made excellent predictions. It was the accuracy of these predictions that persuaded other scientists of the value and truth of the Periodic Table.