Among the following the correct points regarding Mendeleev’s periodic table

Mendeleev have been classified the elements periods and groups on the basis of atomic weights of elements. Elements present in same group have similar properties due to common valency. Ex: Li, Na, K

Valency of oxygen is 2.

Thus, I column elements form${\text{R}}_{\text{2}}\text{O}$ . Ex: ${\text{Na}}_{\text{2}}{\text{O, K}}_{\text{2}}\text{O}$

II column elements form RO. Ex: $\text{MgO, CaO}$