Question

Why did Mendeleev leave some gaps in his periodic table of elements? Name the three elements which were discovered later and placed in these gaps.

Answer 1

Some elements were not known at the time Mendeleev developed the periodic table.
He realised this fact and left gaps in his table for such elements and also predicted their properties.

Three elements that were discovered later for which positions were left vacant on the periodic table are scandium, gallium, technetium and germanium.