Mendeleev’s periodic table has the following defects:
1. Position of hydrogen
The position of hydrogen in the periodic table is anomalous. Hydrogen resembles alkali metals in certain properties. Hence, it is placed in group IA along with the alkali metals. Further, certain properties of hydrogen resemble the halogens. So, it may be placed in group VIIA with the halogens.
2. Position of lanthanides and actinides
The elements from atomic number 57 to 71 are collectively known as lanthanides. These element do not have proper place in the periodic table. They all have been placed at the same position in group III and period 6. Similarly, the actinides (atomic numbers 89-103) also have no proper place in the periodic table. These elements have also been placed in the same position in group III and period 6.
3. Separation in the same group
Some similar elements are separated In the periodic table. For example, copper (Cu) and mercury (Hg), silver (Ag) and thallium (Tl), and barium (Ba) and lead (Pb). On the other hand, some dissimilar elements have been placed together in the same group. For example, copper (Cu), silver (Ag) and gold (Au) have been placed in group I along with the alkali metals. Similarly, manganese (Mn) is placed in the group of the halogens.
4. Presence of some anomalous pairs of elements
In Mendeleev’s periodic table, the elements are arranged in the increasing order of their atomic masses. In some places, this order has been ignored.
(a) The atomic mass of argon is 40 and that of potassium is 39, But argon is placed before potassium in the periodic table.
(b) The positions of cobalt and nickel are not in proper order. Cobalt (at. wt. = 59.9) is placed before nickel (at. wt. = 58.6).
(c) Tellurium (at. wt. = 127.6) is placed before iodine (at. wt = 126.9).
(d) Thorium (at, wt. =232.12) is placed before protactinium (at. Wt = 231).
5. Position of isotopes
The isotopes of an element have no place in the periodic table.
The failure of Mendeleev’s periodic law to explain the wrong order of the atomic masses of some elements and the position of isotopes led scientists working in this field to conclude that atomic masses cannot be the basis for the classification of elements. There must be a more fundamental property of elements which can be the basis of classification.