What are the defects of mendeleev's periodic table?

Mendeleev’s periodic table was a good attempt to classify sixty elements known at this time. There some noteworthy aberrations in the periodic table and no explanations were, given for the deviations.

i) Elements not obeying increasing atomic mass

Elements like, Argon-potassium, cobalt-nickel, tellurium-iodine and thorium and protactinium, are arranged in the decreasing mass order than increasing mass order.

ii) Properties and uneven grouping.

The periodic table contains elements of dissimilar properties together and separates elements of same properties. For example, copper resemble mercury and so of silver & thallium, barium& lead. But they are, placed in different column. Copper is in first column while mercury is in second grouping. Silver is, present in first group while thallium is in third group. Similarly barium and lead are, separated in second and fourth groups. Coinage metals copper, silver, and gold elements are, grouped together with alkali metals. Manganese metal is, grouped with totally different halogens in the seventh group.

iii) Position of hydrogen

Hydrogen resembles both alkali metals in some respect and halogens in some other respects. But it is placed, along with alkali metals only.

iv) Position of isotopes

All elements, more or less have isotopes. The isotopes cannot be accommodated din the table.

v) Position of Lanthanides and actinides

Elements of Lanthanides and actinides, which were, discovered later, cannot be, arranged in the table without disturbing it.

vi) Eighth group elements

4th, 5th and 6th periods have three elements in their eighth group. Reasons for such triad in eight column was not given.

vii) Source of periodicity

The relationship between the properties and atomic weight is a fact. But no explanation was, given for this relationship.

viii) Mendeleev’s periodic table may be better for the sixty-three elements known at his time. But it cannot accommodate the presently known 118 elements.

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