Question

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?

Answer 1

Mendeleev's notion that not all the elements were known was indeed astute............

Explanation:

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) `"gallium"` in 1875, predesignated by Mendeleev as `"eka-aluminum"`; (ii) `"scandium"` in 1879, anticipated by Mendeleev as `"eka-boron"`, and (iii) `"germanium"` in 1886, anticipated by Mendeleev as `"eka-silicon"`.

These predictions were powerful evidence of the periodic nature of the elements. Later discoveries, notably Rutherford's discovery of `alpha-"particles"`, reinforced the idea of periodicity. And course, the discovery of the Noble gases, which was powerful new evidence for periodicity, led to the development of electronic structure as the driving force behind these periodic phenomena.