Question

Which of the following are the demerits of the Mendeleev periodic table?
(a) No proper position of hydrogen is given
(b) Grouping of chemically dissimilar elements
(c) Position given to isotopes
(d) Presence of anomalous pairs

Answer 1

Hint: The demerits arise as Mendeleev placed elements in order of increasing atomic masses so that they could fit into the proper group.

Complete step by step answer:
Mendeleev’s periodic table helped in correcting the atomic masses of some of the elements based on their positions in the periodic table. For example, the atomic mass of Beryllium was corrected from 13.5 to 9. Atomic masses of indium, gold and platinum were also corrected. But As we know, the demerits are always associated with the merits side by side ,so, now we will discuss about the defect of Mendeleev's periodic table:
Position of Hydrogen
The correct position for hydrogen could not be assigned in the periodic table. Hydrogen could be placed in the alkali metal group (groupIA) as well as in the halogens group (group VIIA). It is still not certain whether to place H-atom in group IA or VIIA.
Anomalous positions of some elements
Although the elements on Mendeleev’s periodic table had been arranged in order of increasing atomic masses, in some cases, the element with higher atomic mass preceded the element with lower atomic mass. Some examples are:
Argon (39.9) is placed before potassium (39.1)
Tellurium (127.6) is placed before iodine(126.9)
Position of isotopes
There was no place for isotopes of various elements, which were discovered later. If we arrange the elements in order of their increasing atomic masses, then, it becomes difficult to assign the proper place for a large number of isotopes. For example, hydrogen has 3 isotopes but is assigned only one position.
Grouping of elements
Some similar elements were separated and dissimilar elements are grouped together. For instance, the pairs such as copper and mercury, silver and thallium, barium and lead have similar properties but these were placed in different groups. On the other hand, elements of group IA such as lithium, sodium, and potassium were grouped with dissimilar elements such as copper, silver and gold.
Position of lanthanides and actinide
Fourteen elements that follow lanthanum are called lanthanides and fourteen elements that follow actinium are called actinides. These were not given proper places in Mendeleev’s periodic table
Cause of periodicity
No proper explanation has been offered for the fact why the elements placed in a group show resemblance in their properties.
From the above discussion, we can conclude that all the demerits of Mendeleev’s periodic table that are given in the options are correct.
Therefore, all the options are correct in this question.

Note: There is one more important point which a student should know. There was also no position given to the noble gases. Original Mendeleev’s periodic table did not indicate any of the noble gases, which were discovered later. These were added by Sir William Ramsay as Group Zero, without the disturbance to the basic concept of the periodic table.