ICSE Class 9 Answered

Defects in Mendeleev Periodic Table

1.  Anomalous pairs

The following pairs of elements did not follow Mendeleev’s Principles:

Cobalt (58.9) with higher atomic mass precedes Nickel (58.6) in the periodic table.

Tellurium (127.6) with higher atomic mass than Iodine (126.9) is also placed before it in the periodic table.

2.  Position of Isotopes

The isotopes of an element are atoms of that element having similar chemical properties but different atomic masses. According to Mendeleev’s Periodic Law, isotopes of an element must be given separate places in the periodic table because they have different atomic masses.

3.  Grouping of chemically dissimilar elements

Elements such as Copper and Silver bear no resemblance with the alkali metals such as Lithium and Sodium etc., but they have been placed together in the first group.

4.  Separation of chemically similar elements

Elements which are chemically similar such as Gold and Platinum have been placed in separate groups.

5.  Electronic arrangement

It does not explain the electronic arrangement of elements.

6.  The position of hydrogen

Hydrogen was not given a fixed position. It was considered in Group IA as well as in Group VIIA, because it forms a positive ion as in HCl but also a negative ion as in NaH.