What event helped the development of the periodic table?
The discoveries of hitherto unknown elements that filled in predicted gaps on the then Periodic Table..........
Old Mendeleev was astute enuff to realize that when he first published his Periodic Table, NOT ALL of the naturally occurring elements had been isolated and characterized. Using his principle of Periodicity he was able to ANTICIPATE the discovery of new elements, the which would fill gaps in his then Table.
And thus when he first published the table he proposed