Mendeleev’s Periodic Law could not explain the position of ________ in the periodic table. 

The correct answer is 'Isotopes'.

Key Points

• Mendeleev arranges the elements in the increasing order of their atomic masses. According to Mendeleev, the physical and chemical properties are the periodic function of their atomic mass.
• The limitations of Mendeleev's periodic tables are as under:
  • Position of hydrogen: The position of a hydrogen atom is uncertain because it shows a resemblance with alkali metals and halogens. But in Mendeleev's periodic table, Hydrogen is placed with alkali metals in group 1A.
  • Position of isotopes: There is no separate position given to the isotopes of an element.
  • Cause of periodicity: Mendeleev's periodic table does not explain successfully the cause of periodicity. Some dissimilar elements are placed together. Some elements having higher atomic mass are placed before the elements with lower atomic mass.

Additional Information

• Isobars: 
  • Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons.
  • Correspondingly, isobars differ in atomic number (or number of protons) but have the same mass number.
  • An example of a series of isobars is ⁴⁰S, ⁴⁰Cl, ⁴⁰Ar, ⁴⁰K, and ⁴⁰Ca.
• Isodiaphers :
  • Isodiaphers are a pair of nuclides with the identical number of protons and neutrons in the atom's nucleus.
  • Although the nuclei of both nuclides have different numbers of protons and neutrons, the difference between them is the same.
  • An example of a series of isodiaphers is Thorium-234 and Uranium-238.
• Isotones:
  • Two nuclides to be isotones if they have the same neutron number N, but different proton number Z.
  • For example, boron-12 and carbon-13 nuclei both contain 7 neutrons, 
  • Similarly, ³⁶S, ³⁷Cl, ³⁸Ar, ³⁹K, and ⁴⁰Ca nuclei are all isotones of 20 because they all contain 20 neutrons.