What is the achievements and limitations …

Achievements of Mendeléev’s periodic table

• element with a slightly greater atomic mass were placed
  before an element with a slightly lower atomic mass with an inverted sequence so as to group the elements with similar properties. For example, cobalt with atomic mass 58.9 was placed before nickel with atomic mass 58.7.
• He left some gaps in the Periodic Table with the prediction of
  existence of some elements that were not discovered at that time.
• He named the future elements by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group.
• For instance, scandium, gallium and germanium, discovered later, have properties
  similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.
• Noble gases like helium (He), neon (Ne) and argon (Ar) were discovered
  later as they are present in exceptionally low concentrations in the atmosphere due to their inertness.

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.