(i) Position of hydrogen
The position of hydrogen is not correctly defined. It is still not certain whether to place hydrogen in group I A or VII A.
(ii) Anomalous pair
In certain pairs of elements like, Ar (40) and K (39); Co (58.9) and Ni (58.6); Te (127.6) and I (126.9) the arrangement was not justified. For e.g., argon was placed before potassium whereas its atomic mass is more than potassium.
(iii) Position of isotopes
Isotopes are atoms of the same element having different atomic mass but same atomic number. For e.g., there are three isotopes of hydrogen with atomic mass 1, 2, and 3. According to Mendeleev's periodic table these should be placed at three separate places. However isotopes have not been given separate places in the periodic table.
(iv) Grouping of elements
Some similar elements are separated and dissimilar elements are grouped together e.g., copper and mercury have similar properties but are 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.
(v) Cause of periodicity
Mendeleev's table was unable to explain the cause of periodicity among elements.
(vi) Position of lanthanides and actinides
Fourteen elements that follow lanthanum called lanthanides and fourteen elements following actinium called actinides were not given proper places in Mendeleev's periodic table.