Question

Define the Modern Periodic law. Why was it necessary to change the basis of classification from the atomic mass to the atomic number?

Answer 1

Modern Periodic Law:

The physical and chemical properties of elements are the periodic functions o of their atomic number.

In the modern periodic table, the elements are arranged in the increasing order of their atomic number.

Change in the basis of classification from atomic mass to the atomic number was required because :

1) The atomic mass can in no way determine the chemical properties of elements because it does not vary regularly with gradation in the chemical properties of elements.

2) The chemical properties of the elements depend upon the number of electrons in the valence shell which is related to the atomic number.

3) Thus the properties of different elements can be compared if we know their atomic numbers.

so the adjustments were made to the Mendeleev's periodic table (which was classified based on the atomic mass of the elements) and the modern periodic table(which is classified based on the atomic numbers of the elements ) was prepared.