Following are the main limitations (defects) of Mendeleev’s periodic table:
(i) the location of hydrogen-In this table, hydrogen is placed in two places with alkali metals in the first group because of their similar electro-electrochemical properties and with halogens in group seventh because of their similar electro-electrochemical properties, but hydrogen is placed in both groups (first). and VII) is faulty.
(ii) To put elements with dissimilar properties in the same group-The elements in this table are grouped together on the basis of similarity of properties, yet there are some elements which have dissimilarities in their properties. In other words, some elements with different properties are placed in a group, such as elements of IA (alkali metals) and elements of IB (coin metals) in the same group.
have been placed in, although their properties differ.
(iii) To put elements with similar properties in different groups-
Somewhere in Mendeleev’s periodic table, elements with similar properties are placed at different places, such as Pt (195.09) and Au (196.97) have similar properties, yet they are placed in the eighth and first group. Apart from this, despite having similar properties of copper, mercury, barium lead, etc., they have been kept in different groups.
(iv) placing heavier elements before lighter elementsIn Mendeleev’s periodic table, some heavy elements are placed before the lighter elements like-
(a) Cobalt (atomic mass = 58.93), is placed before nickel (atomic mass = 58.71).
(b) Tellurium (atomic mass = 127.6) is placed before iodine (atomic mass = 126.9).
Such changes in the increasing order of atomic masses in Mendeleev’s original periodic table are contrary to Mendeleev’s basic periodic law.
(v) Location of rare earth elements-Rare earth elements have similar chemical properties, but they have different atomic weights. Yet these elements 14,14 are placed together in the third subgroup B (sixth period) which is not correct.
,vi) Location of isotopes-The discovery of isotopes and isotopes made it clear that the basic characteristic of elements is not their atomic mass. The atomic weights of isotopes are different, but they have similar properties. Isobars have the same atomic mass, but their properties are different. Hence the position of isotopes in Mendeleev’s original periodic table is fixed.
(vii) Elements of group VIII to be placed in three vertical columns.
Read in Hindi: Limitations of Mendeleev’s periodic table