Question

What were the major defects of Mendeleev's classification?

Answer 1

Defects of Mendeleev's periodic table

Mendeleev's periodic table was introduced in 1869 and arranged the elements in the increasing order of their atomic mass. He also put the elements with same chemical properties in a group and predicted the properties of undiscovered elements using the properties of elements in the same group. There were 63 elements discovered at the time of introduction of Mendeleev's work.

The defects of Mendeleev's periodic table include

• Anomalous pairs- There were elements which were not arranged in the order of increasing atomic mass. The positions were switched to make the properties of the elements of a group same. For example, Cobalt (Co) has a higher atomic mass than Nickel (Ni). But Copper comes before Nickel in Mendeleev's periodic table.
• Dissimilar elements in same group- In Mendeleev's periodic table, Copper (Cu) and Silver (Ag) are placed in same group (alkali metals) although they have no similar properties.
• Similar elements in different groups- Gold (Au) and Platinum (Pt) are placed under different groups in Mendeleev's periodic table although they have very similar properties.
• Electronic arrangement- Mendeleev arranged the elements based on atomic mass and failed to explain the electronic arrangement of atoms.
• Hydrogen- The position of Hydrogen (H) was not fixed in Mendeleev's periodic table.
• Isotopes- The isotopes of an elements have different atomic mass and therefore was given separate position in his periodic table.