Question

while preparing periodic table why did mendeleev concentrated on the compounds formed by the elements with o₂ and H₂ ? WHY NOT WITH OTHER ELEMENTS?

Answer 1

Hydrogen and oxygen are present in large amounts in nature and are reactive enough to form compounds with a variety of elements.Thus, hydrides and oxides of elements were easily found in nature and their properties were widely studied and compared.
Hence, Mendeleev considered hydrides and oxides in categorizing elements.