It was found that the law of octaves was applicable only up to:

Two elements $M$ and $N$ belong to group $I$ and $II$ respectively and are in the same period of the periodic table. How do the following properties of $M$ and $N$ vary:
$(a)$ sizes of their atoms?
$(b)$ their metallic characters?
$(c)$ their valencies in forming oxides?
$(d)$ formulae of their chlorides?

Give reasons for the following:
$(a)$ Lithium atom is smaller than Sodium atom.
$(b)$ Chlorine (Atomic Number $17$) is more electronegative than Sulphur (Atomic Number $16$).

Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at Periodic Law?

The element with atomic number $14$ is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong?

An element which is an essential constituent of all organic compounds belongs to :