(i) Position of isotopes: Isotopes of various elements cannot be filled into Mendeleev’s table. Isotopes are the elements having similar chemical properties but different atomic masses.
(ii) Anomalies of atomic weights and properties of elements: If arranged according to gradation in chemical properties, elements of higher atomic weights are to be placed before those of lower atomic weights at some places. This is contrary to Mendeleev’s law.
For example, argon with atomic mass of 40 comes before potassium with atomic mass 39. Iodine (at. mass =127) comes after tellurium (at. mass = 127.8).
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(iii) Falling of dissimilar elements in same group: Some chemically similar elements fall in different groups, while some chemically dissimilar elements are placed together.
For example, copper, silver and gold are placed in the same group with alkali metals. Mercury, lead and barium which possess similar properties fall into different groups.
(iv) Hydrogen resembles both alkali metals (group I) and halogens (group VII) and thus no fixed position could be given to hydrogen in the periodic table.
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