Why Mendeleev called hydrogen rogue element?

Mendeleev referred to hydrogen as a "rogue element" because its properties did not fit neatly into his Periodic Table, which was organized by Atomic Mass and chemical properties. Hydrogen has unique characteristics that resemble both alkali metals and nonmetals, allowing it to bond in diverse ways. This inconsistency made it difficult to categorize hydrogen alongside other elements, leading Mendeleev to see it as an outlier or a "rogue" in the periodic system.