He intentionally left gaps between the already discovered elements, so that later on those gaps would be filled with new elements that he knew existed, but haven’t been discovered yet. How did he know that?
In: Chemistry
The periodic table is called “periodic” because it has regular repeating patterns. Mendeleev recognized the patterns and assumed (correctly) that they ought to continue even in places where we hadn’t found the element that would fill that “slot” yet.
For example, if I tell you I have Heads-Tails-Heads-Tails-H-T-H-?-?-T-H-T-H-T you can probably guess there’s a missing “H-T” in there because of the pattern.
It turns out the patterns are caused by the number of electrons in different shells that quantum mechanics allows (kind of like orbits, but more complicated) but Mendeleev didn’t know that.
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