Name the elements whose compounds formed the basis of classification in Mendeleev's periodic table. Why did Mendeleev choose these elements? How the formulae of these compounds had helped Mendeleev in deciding the position of an element in his periodic table?

The classification in Mendeleev's periodic table was based on the compounds formed by elements with hydrogen and oxygen.

1. Reason for choosing Hydrogen and Oxygen:

Mendeleev selected hydrogen and oxygen because they are highly reactive elements and form compounds (hydrides and oxides) with almost all other elements.

1. How the formulae helped in deciding the position:

The formulae of the hydrides and oxides formed by an element were treated as one of the basic chemical properties for its classification. Mendeleev wrote the formulae of these compounds at the top of each column (group) using general representations like $R_2O$, $RO$, $R_2O_3$ for oxides, and $RH$, $RH_2$, $RH_3$ for hydrides (where $R$ represents the element). Elements that formed oxides and hydrides with similar chemical formulae were placed in the same vertical column (group).