CBSE Class 10 Answered

Mendeleev arranged elements on the basis of the atomic massin the periodic table (1 mark)

  Achievements of Mendeleev’s Periodic Table:

a. Through this table, it was very easy to study the physical and chemical properties of various elements.(1/2 mark)

b. Mendeleev adjusted few elements with a slightly greater atomic mass before the elements with slightly lower atomic mass, so that elements with similar properties could be grouped together. For example, aluminum appeared before silicon, cobalt appeared before nickel.(1/2 mark)

c. Mendeleev left some gaps in his periodic table. He predicted the existence of some elements that had not been discovered at that time. His predictions were quite true as elements like scandium; gallium and germanium were discovered later(1/2 mark)

d. The gases like helium, neon and argon, which were discovered later, were placed in a new group without disturbing the existing order(1/2 mark)

·               Limitations of Mendeleev’s Periodic Table :

a. He could not assign a correct position to hydrogen in the periodic table    (1/2 mark)

b. Positions of isotopes of all elements was not certain according to Mendeleev’s periodic table(1/2 mark)

c. Atomic masses did not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements(1 mark)