Which important property did Mendeleev use to classify the elements in his periodic table?

What do you understand by diagonal relationship?

The valence of representative elements is either equal to the number of valence electrons or eight minus the number. What is the basis of their rule?

Discuss briefly the anomalous behviour of elements of second period.
Or
The first element of each group differs significantly from rest of the elements of the same group. Give reasons.

Name the element in the periodic table with maximum I.E., value.