(a) Merits of Mendeleev’s classification of elements:

(i) Mendeleev’s periodic law predicted the existence of some elements that had not been discovered at that time.

(ii) Mendeleev’s periodic table could predict the properties of several elements on the basis of their positions in the periodic table.

(iii) It could accommodate noble gases when they were discovered.

(b) Anomalies of Mendeleev’s classification of elements:

(i) The position of isotopes could not be explained: If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the periodic table. But, the isotopes were not given separate places in Mendeleev’s periodic table. They were placed at the same place in the table. This placing of the isotopes at same place could not be explained by Mendeleev’s periodic law.

(ii) Wrong order of atomic masses of some elements could not be explained: In Mendeleev’s periodic table, when certain elements were put in their correct group on the basis of their chemical properties, it was found that the element with higher atomic mass comes first and the element with lower atomic mass comes later. Mendeleev’s periodic law could not explain this abnormal situation of wrong order of atomic masses.