The main drawbacks of Mendeleev's periodic table are
(i) Some elements having similar properties were placed in different groups whereas some elements having dissimilar properties were placed in the same group.
e.g., alkali metals such as Li, Na, K, etc., (IA group) are grouped together with coinage metals such as Cu, Ag, Au (IB group) though their properties are quite different. Chemically similar elements such as Cu (IB group) and Hg (IIB group) have been placed in different groups.
(ii) Some elements with higher atomic weights are placed before the elements with lower atomic weights in order to maintain the similar chemical nature of elements.
i.e.,
(iii) Isotopes did not find any place in the periodic table. However, according to Mendeleev's classification, these should be placed at different places in the periodic table.
(All the above three defects were however removed when modern periodic law based on atomic number was given).
(iv) Position of hydrogen in the periodic table is not fixed but is controversial.
(v) Position of elements of group VIII could not be made clear which have been arranged in three triads without any justification.
(vi) It could not explain the even and odd series in IV, V and VI long periods.
(vii) Lanthanides and actinides which were discovered later on have not been given proper positions in the main frame of periodic table.
