What are limitatoions and drawbacks of …

Two important limitations of dobreiner tridas are:
All known elements could not be classified into groups of triads on the basis of their properties .
Not all groups obeyed the Law of Triads. For example, nitrogen family does not obey the Law of Triads.

Limitations of Octaves:
(i) This law was applicable to the classification of elements up to calcium only. After calcium, every eighth element did not possess the properties similar to that of the first element.
(ii) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future.
(iii) In order to fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties. For example, the two elements cobalt (Co) and nickel (Ni) were put together in just one slot, and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.

Limitations of Mendeléev’s classification

• No fixed position could be assigned to hydrogen in the Periodic Table.
• Isotopes discovered long after Mendeléev periodic classification of
  elements imparted a challenge to Mendeleev’s Periodic Law.
• The atomic masses of elements did not increase in a steady manner while going from one element to other which made it difficult to predict the number of elements that could be discovered between two elements.