Question

Why did mendeleev leave some gaps in the periodic table of elemants with eg?

Answer 1

There were only 63 elements known when mendeleev make his periodic table of element. But he thought that these elements would be discovered later on even predicted the properties of these then unknown elements by studing the properties of the neighbouring elements. The missing elements of Mendeleev's periodic table were discovered later on and their properties were found to be very close to those predicted by mendeleev.