How was Mendeleev's assumption that not all of the elements were known at the time of his first publication of the Periodic Table astute?
Why? Because, it laid the basis for further discoveries that could be interpreted on a Periodic basis.
And within a few years of the first publication of the Periodic Table, several new elements had been discovered: (i)
These predictions were powerful evidence of the periodic nature of the elements. Later discoveries, notably Rutherford's discovery of