Question

Answer why Mendeleev's choice of formulae of compounds as the basis for deciding the position of an element in his table was a good one and how would this lead to elements with similar chemical properties being placed in the same group?

Answer 1

Mendeleev developed a periodic table of elements where in the elements were arranged on the basis of their atomic mass and also on the similarity on chemical properties.
Among chemical properties , he took the formula of the hybrides and oxide formed by an element as one of the basic properties of an element for classification.
He selected oxygen and hydrogen as they are very reactive and formed compounds with most elements.
He took 63 cards and wrote the names  of each element with their properties.
He arranged them in the increasing order of their atomic mass and observed periodic repetition of their properties.
He also formulated a law based on their properties:
"The properties of elements are the periodic function of their atomic masses"
Hence Mendeleev choice of formula of chemical compounds as the basis for deciding the position of element in his table was appreciable. This lead to the new discoveries of elements with similar properties placed in same group.
For example he predicted Eka Boron, Eka Aluminum , Eka Silicon later they were discovered as Scandium, Gallium and Germanium.
Mendeleev period table laid the foundation for the modern periodic table.
Mendeleev placed an element with greater atomic mass before element of lower atomic mass so that elements with similar properties could be grouped together.

Answer 2

He chose hydrogen and oxygen to form compounds with element as these elements react with lots of elements.
His experipent prove to be benificial as he came to know that certain group of elements react with these elements giving rise to same chemical formulae