Merits and Demerits of Mendeleev’s Periodic Table

Science is progressive, allowing revisions based on new discoveries, as seen in Mendeleev’s Periodic Table. While arranging elements by atomic mass, Mendeleev considered that knowledge might change over time.

• Atomic masses were corrected for better placement. Example: Beryllium’s atomic mass was changed from 14.09 to 9.4, placing it before boron.
• Vacant spaces were left for undiscovered elements, predicting their properties. Example:
  • Eka-boron → Scandium (Sc)
  • Eka-aluminum → Gallium (Ga)
  • Eka-silicon → Germanium (Ge)

    Property	Eka-aluminum (E) (Mendeleev’s prediction)	Gallium (Ga) (actual)
    1. Atomic mass	68	69.7
    2. Density (g/cm³)	5.9	5.94
    3. Melting point (°C)	Low	30.2
    4. Formula of chloride	ECl₃	GaCl₃
    5. Formula of oxide	E₂O₃	Ga₂O₃
    6. Nature of oxide	Amphoteric oxide	Amphoteric oxide

• Predicted properties matched actual properties, proving the periodic table’s accuracy.
• Noble gases were accommodated in a new ‘zero group’ without disturbing the table after their discovery.

• Cobalt (Co) and nickel (Ni) have nearly the same atomic mass, causing ambiguity in their placement.
• Isotopes posed a challenge since they have the same chemical properties but different atomic masses.
• Irregular atomic mass increases made it difficult to predict the number of elements between heavy elements.
• The position of hydrogen was unclear because it shares properties with both:
  • Alkali metals (Group I): Forms compounds like HCl (similar to NaCl) and H₂O (similar to Na₂O).
  • Halogens (Group VII): Forms compounds like NaH (similar to NaCl) and CH₄ (similar to CCl₄).

Similarity in hydrogen and alkali metals:

Similarity in hydrogen and halogens: