Why did Mendeleev leave gaps in his periodic table with an example?

Answer 1

There were gaps in the Periodic Table a hundred years ago because investigators had not discovered some elements yet, though there was a "place" for them in the array. Recall that through the genius of Mendeleyev, the elements had been given an "order" owing to their chemical properties. By creating a "chart" (the first periodic table), Mendeleyev could see where elements "would be" though none had yet been discovered. He could predict that eventually an element would appear "here" on the table and would have a "these" properties because of the arrangements in the array or table of element.

Answer 2

Mendeleyev left gaps in his periodic table because there weren't as many elements back in that time, and so he had to leave gaps for the elements that weren't discovered yet. He predicted that scientists would find out more elements in the future, and his predictions were right.