Answer: Position of hydrogen, isotopes, irregular order of atomic masses, and the unexplained cause of periodicity.
- Position of hydrogen: hydrogen resembles the alkali metals in forming H+ and the halogens in forming H–, so it could not be given a fixed place in the table.
- Isotopes: isotopes of an element have different atomic masses but the same chemical properties; Mendeleev’s table had no place for them.
- Irregular increase of atomic mass: to keep elements of similar properties together, some elements of higher atomic mass were placed before those of lower atomic mass (e.g. Co before Ni, Ar before K), which went against the basis of the table.
- Lanthanides and actinides were not given a proper place, and the table could not explain the cause of periodicity — that is, why properties repeat.