What were the limitations of Mendeleev's periodic table?

1. Position of hydrogen: Resembles both alkali metals and halogens not correctly placed.
2. Anomalous pairs: Some elements arranged incorrectly (e.g., iodine placed after tellurium despite lower atomic mass).
3. No room for isotopes: Different atomic masses would require separate positions.
4. Group VIII anomaly: Fe, Co, Ni placed in the same group — unexplained.
5. No theoretical basis for why properties repeat periodically.

Mendeleev's table had flaws in hydrogen's placement, anomalous element pairs, no accommodation for isotopes, and lacked a scientific basis for periodicity.