Limitations of Mendeleev's Periodic Table

Mendeleev could not assign a fixed, correct position for Hydrogen in his table.

• Hydrogen resembles Alkali Metals (Group 1) because it forms H+ ions and combines with halogens.
• However, it also strongly resembles Halogens (Group 17) because it exists as a diatomic gas (H2) and forms covalent bonds. Because of this dual nature, placing it only in Group 1 was scientifically debatable.

Long after Mendeleev published his table, scientists discovered 'Isotopes' (atoms of the same element that have the exact same chemical properties but different atomic masses—like Carbon-12 and Carbon-14).

• Since Mendeleev's entire table was strictly based on increasing Atomic Mass, isotopes of the same element should theoretically be placed in different slots.
• But placing Carbon in two different boxes made no chemical sense, breaking his foundational rule.

To force elements with similar properties into the same column, Mendeleev had to cheat his own rule of 'increasing atomic mass'.

• Example (Cobalt and Nickel): Cobalt has a higher atomic mass (58.9) than Nickel (58.7). According to his rule, Nickel should come first. But Mendeleev placed Cobalt before Nickel so that Cobalt could sit in the same column as Rhodium (which it resembles chemically).