Define the modern periodic law. Why was it necessary to change the basis of classification?

Modern periodic law:

• This law was given by Moseley.
• According to this law, the physical and chemical properties of elements are the periodic functions of their atomic number.
• Elements are arranged according to their increasing atomic number not according to their atomic masses.

It is necessary to change the basis of classification from the atomic mass to the atomic number because:

• The atomic mass cannot define the chemical properties of elements.
• The dependency of the chemical properties of elements is on the number of electrons and not the atomic mass.