What are the limitation of mendeleev periodic table?

Mendeleev's Periodic Table was a major achievement, but it had several limitations:

• Position of hydrogen was uncertain because hydrogen shows properties similar to both alkali metals and halogens.

• Isotopes could not be accommodated since the table was based on atomic mass, while isotopes have different masses but similar chemical properties.

• Anomalous pairs existed, such as argon-potassium and cobalt-nickel, where elements were not arranged strictly in increasing atomic mass.

• No proper place for rare earth elements (lanthanides and actinides) was provided.

These limitations were later resolved by the Modern Periodic Table, which is based on atomic number.