Assertion:Mendeleev's periodic table has some demerits. Reason:There was no place for all isotopes of the elements and some heavy elements were placed before lighter elements. A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion C. Assertion is correct but Reason is incorrect D. Both Assertion and Reason are Incorrect
Hint: In Mendeleev's periodic table elements were arranged on the basis of atomic mass. Of many periodic tables that were made, Mendeleev's periodic table is the most important one, as it predicted the non- discovered elements of that time, but the table had its own limitation even.
Complete answer: It is given to us, Assertion: - Mendeleev's periodic table has some demerits. Reason: - There was no place for all isotopes of the elements and some heavy elements were placed before lighter elements. Firstly, lets see what was Mendeleev's periodic table Mendeleev found that the properties of elements were related to atomic mass in a periodic way and stated his periodic law as- "the properties of elements are the periodic function of their atomic masses". We know that Mendeleev arranged the elements on the basis of atomic mass. But his major concern was that elements with the same properties should be placed together. Hence, to arrange elements with the same properties he ignored the concept of atomic mass. As a result, few heavier elements were placed before the light ones, so that elements with the same properties come in the same group. Now, when we talk of isotopes, they have the same atomic number and different mass number, and we know that chemical properties depend on the atomic number. Since the atomic number of isotopes were the same, and so they have the same chemical properties. Therefore, if Mendeleev would have arranged them then at one place elements and its isotopes would have been placed together. Hence, he did not arrange the isotopes. These were even the two demerits among others of the Mendeleev periodic table. Thus, analysing the above demerits and explanations we find that both assertion and reason are correct and reason in the correct explanation of the assertion.
Hence, Option (A) is the correct option
Note: Students should note that the main difference between Mendeleev's periodic table and modern periodic table is that Mendeleev's periodic table is based on atomic mass while current periodic table is based on atomic number however the position of hydrogen is still a debate.