Limitations of Mendeleeff s periodic table :
1) Position of hydrogen : The position of hydrogen in table is not certain because it can be placed in group IA as well as in group VIIA as it resembles both with alkali metals of IA group and halogens of VIIA group.
2) Anomalous pair of elements : Certain elements of higher atomic mass precede those with lower atomic mass. Eg: Tellurium preceeds iodine, potassium placed after Argon. (Atomic mass 40)
3) Dissimilar elements placed together : Elements with dissimilar properties were placed in same groups as sub-group A and sub - group B.
Eg: Li, Na, K of IA group have little resemblance with Cu, Ag, Au of IB group.
4) Some similar elements are separated : Some similar elements like copper
and mercury', 'silicon and thallium' etc., are placed in different groups of the periodic table.
5) Position of Isotopes : Isotopes of elements are placed in the same position in the table.
Modern periodic table removes various anomalous of Mendeleeffs table in the following way :
1) Hydrogen is placed in IA group according to its electronic configuration. But it is not included in IA group elements.
2) Basing on their electronic configuration anomalous pairs (K and Ar) are put in order according to their atomic number.
3) Dissimilar elements were placed in different groups.
4) Copper and mercury come in different groups according to their electron configuration.
5) All the isotopes of same element have same atomic number. So all the isotopes are put in same group as a single element in the modern periodic table.