. Define the Modern Periodic law

Why was it necessary to change the basis of classification from the atomic mass to the atomic number?

Best Answer


  • Modern periodic law pronounces that the physical and chemical attributes (properties) of the elements are the periodic functions of their atomic number.
  • Modern periodic table comprises the classification of elements with similar properties in increasing order of their atomic numbers.
  • Altering the basis of classification from the atomic mass to the atomic number was necessary because of the following reasons:
  • Mendeleev periodic law claimed that “Element attributes (properties) are a periodic function of their atomic weight.” 
  • The Mendeleev's periodic law failed to explain the wrong order of the atomic masses of some elements and the location of isotopes that led to conclusion that atomic mass cannot be the foundation for the elements classification.
  • The limitations of Mendeleev's periodic table are: 
  • (i) Elements with large variations in their physical and chemical properties were placed in the same group of the periodic table. 
  • (ii) Mendeleev was unable to place Hydrogen at a definite place in the periodic table.
  • The chemical properties cannot be defined as per atomic mass.
  • Atomic mass does not show periodic repetition of  chemical properties at regular intervals among elements falling in the same group.
  • The chemical properties of the elements are dependent on the number of electrons (atomic number) instead of atomic mass.
  • Atomic number is equal to the number of electrons or number of protons in the atom.
  • Comparison of properties can be done if their atomic number is known.
  • So, an amendment of Mendleev's periodic table (classified elements based on atomic mass) was necessary.
  • To overcome the difficulties with atomic mass arrangement, elements were periodically arranged on the basis of atomic number in the modern periodic table.

Final Answer:

Modern periodic law claims that the properties of elements classified are dependent on periodicity of their atomic number.


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