(a) Which two criteria did Mendeleev use to classify the elements in his periodic Table?
(b) State Mendeleev's periodic law.
( c) Why could no fixed position be given to hydrogen in Mendeleev's Periodic Table ?
(d) How and why does the atomic size vary as you go
(i) from left to right along a period
(ii) down a group ?

Explanation

(a) (i) Increasing order of atomic mass as physical property and similarities in chemical properties of elements.
(ii) The formulae and nature of hydrides and oxides formed by elements was treated as basic chemical property for its classification.
(b) Mendeleev.s Periodic Law: Properties of elements are the periodic functions of their atomic masses.
( c) It is because it resembles both with alkali metals as well as halogens.
(d) (i) Atomic size goes on decreasing from left to right because one proton and one electron is being added successively therefore, force of attraction between valence electrons and nucleus increases, therefore, atomic size decreases.
(ii) The atomic size goes on increasing from top to bottom in a group because number of shells keep on increasing therefore, distance between nucleus and valence electrons increases.