X and Y are the two elements having similar properties which obey Newland’s law of octaves. How many elements are there in-between X and Y ?

Name the scientists who gave the following laws in the early classification of elements:

(a) Law of octaves

(b) Law of triads

A, B and C are the elements of a Dobereiner’s triad. If the atomic mass of A is 7 and that of C is 39, what should be the atomic mass of B?

What was the Mendeleev’s basis for the classification of elements ?

In the classification of the then known elements, Mendeleev was guided by two factors. What are those two factors ?