Question

5 defects of Mendeleev's periodic table

Answer 1

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1. POSITION OF HYDROGEN
Hydrogen shows similarity in certain chemical properties with halogens and alkali metals and Mendeleef could not decide whether it should be placed at the top of group I (Alkali metals) or on the top on group VII (Halogens).

2. POSITION OF ISOTOPES
Isotopes of an element have similar chemical properties but different atomic masses. As the isotopes have similar chemical properties and hence occupy same group. But on the basis of atomic masses they should occupy different groups. So, Mendeleef could not explain the positions of isotopes.

3. UNDUE EMPHASIS ON VALENCY
Mendeleef’s classification lays undue emphasis on one valency of the element (valency indicated by group in which element is placed). Often other valencies are ignored which are more important than one indicated by the group in which element is placed. For example, the important valency of copper is 2 not 1.

4. CAUSE OF PERIODICITY
Mendeleef could not explain why elements exhibit a periodicity in their properties when arranged in order of increasing atomic masses.

5. POSITION OF LANTHANIDES & ACTINIDES
Both these families have fourteen elements each, but they could not find proper positions in the main set up of the Mendeleef’s periodic table. Instead, these elements have been placed in two separate rows at the bottom of periodic table without assigning a proper reason.
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Answer 2

In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses. Thus, the position of these elements was not justified e.g. cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6).