What are the defects of Mendeleev’s periodic table?

Mendeleev's periodic table was a groundbreaking achievement, but it did have some defects, mainly because it was based on the available data and knowledge of his time. Here are a few:

Gaps and inconsistencies: Mendeleev's table had gaps where elements were yet to be discovered. While he left spaces for these elements based on his predictions, some of these predictions didn't match the properties of the elements discovered later.

Position of isotopes: Mendeleev's periodic table did not account for isotopes, which are atoms of the same element with different numbers of neutrons. Isotopes have similar chemical properties but different atomic masses. Therefore, placing them in the same group as their parent elements can be misleading.

Position of hydrogen: Mendeleev placed hydrogen separately from the alkali metals, which didn't align with its chemical properties. Hydrogen shares similarities with both alkali metals and halogens, so its placement was a bit arbitrary.

Position of transition metals: Mendeleev's table didn't clearly differentiate between transition metals and other elements, leading to some ambiguity in the classification of elements in this region.

No explanation for the periodicity: While Mendeleev successfully organized the elements based on their properties and atomic masses, he didn't provide a theoretical explanation for why the periodicity occurred.

Despite these defects, Mendeleev's periodic table laid the foundation for the modern periodic table, which has been refined and expanded upon over time to better reflect the structure and properties of elements.