Question

Why Mendeleev`s focus only oxide and hydride by these chemical?​

Answer 1

Answer:

because on that time there were only 63 elements were known therefore he blanked the space because he knew that in future many more elements were classify

and also it had by modern periodic table

I hope u will understand that

Answer 2

Answer:

Mendeleev has focused on oxides and hydrides because , the elements which would form compounds with other elements in the same ratio could be classified into similar elements hence used this criterion and classified them into same groups.