Question

Why Mendeleev used the formula of oxides and hydrides as one of the basis of classifying his periodic table?

Answer 1

Because the formulae of many simple elemental hydrides and oxides were known at the time.

Explanation:

While it may have been initially an `ad` `hoc` rule of thumb, these observations of hydride/oxide formula allowed development of the idea of valence and oxidation number. Mendeleef was able to incorporate these ideas into Periodic Table. Later researchers were able to use the Periodic Table to develop modern ideas of electronic structure.