Defects in the Mendeleef’s Periodic Table

Although Mendeleef’s periodic table was a great advancement over all the previous attempts to arrange elements yet it was not free from defects. The various defects of Mendeleef’s periodic table are as follows:-

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.