Question

Why did Mendeleev leave gaps in his early periodic table, and what was the ultimate importance of the gaps?

Answer 1

Because he believed there were unknown elements and he left those 'holes' if someone discovered such elements

Answer 2

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Answer 3

elements are arrange according to the number of protons (atomic number) from left to right starting from the top. The "holes" refer to elements that have yet been discovered but predicted to exist because of the "gap" in the atomic number. i.e. oxygen have atomic number of 8 and Neon has an atomic number of 10, therefore we can be fairly certain without knowing that there's an element with atomic number 9. (Fluorine)