Question

periodic classification of elements

Answer 1

is on the basis of atomic no. I.e no. of electrons and protons present.

Answer 2

when the three elements are arranged in the ascending of the atomic mass the mass of the middle element is nearly the same as average of atomic masses of other two elements
this statement is told by Triads




the physical and chemical properties of elements of the periodic functions of their atomic masses





this statement is told by Mendeleev