Lecture Notes: Classification of Elements

Introduction to Element Classification

• 19th Century Developments: Beginnings of systematic classification of known elements.

Johann Wolfgang Dobereiner's Triads (early 1800s)

• Concept: Classified elements into groups of three (triads).
• Method:
  • Elements in each triad were arranged such that the atomic mass of the second element is the average of the first and third.
  • Example:
    • Triad: Lithium (Li), Sodium (Na), Potassium (K)
    • Atomic Masses:
      • Li: 6.9
      • K: 39
      • Average: (6.9 + 39) / 2 = 23.45 → Na: 23
• Limitations:
  • Not all elements fit into this triadic classification.
  • Initial classification was based on limited known elements.

John Newland’s Laws of Octaves (1866)

• Concept: Grouped elements in series of eight based on increasing atomic mass.
• Observation: Every eighth element resembled the first, analogous to musical octaves.
• Example:
  • Series: Lithium (Li) → Sodium (Na) is eighth after lithium with similar properties.
  • Other Example: Beryllium (Be) → Magnesium (Mg) also follows the trend.
• Limitations:
  • Valid only up to Calcium, not applicable beyond.
  • Newland did not anticipate discovery of more elements after 56.
  • Placement issues: Cobalt and Nickel placed in incorrect positions among halogens (F, Cl, Br).

Dmitri Mendeleev’s Periodic Classification

• Breakthrough: Provided a systematic method for classifying elements.
• Impact: Laid foundations for the modern periodic table, which is still in use today.

Conclusion

• Transition from Dobereiner's triads and Newland's octaves to Mendeleev's periodic classification.
• Overview of Mendeleev’s system to be covered in the next lecture.