(a) What was the basis of Mendeleev's Classification of elements ?
(b) List two achievements of Mendeleev's periodic tables.
(c) List any two observations which posed a challenge to Mendeleev's periodic law.

Explanation

(a) The basis of Mendeleev's periodic law, The properties of elments are a periodic funcation of their atomic masses.
(b) Achievements of Mendelev's periodic tables :
(i) Mendeleev left a number of gaps in his table to accommodate the new elements which would be discovered later on. So mendeleev boldly perdicated the existence of some more elements. He even predicted the properties of some of these elements and named themn as Eka-boron, Eka-aluminium and Eka-silicon respectively. Later on the elements were discovered and Gallium replaced Eka-aluminum as it showed properties similar to that of Eka- aluminium.
(c) Limitations
(i) The position of isotopes could not be explained because isotopes have same chemical properties but different atomic masses. If the elements are arranged according to atomic masses, the isotopes should be palced in different groups of the periodic table.
(ii) He could not assign a correct postion to hydrogen in his table because hydrogen has some properties similar to alkali metals and some properties similar to halogens. the atomic masses of elements did not increase in a steady manner while going from one element to the other wihch made it difficult to perdicty the undiscovered elements between two elements.