History and Classification of Elements

Initial Classification Attempts

• Johan Wolfgang Dobereiner (1829)
  • Discovered groups of three elements with similar properties, known as "Dobereiner Triads."
  • Found that the atomic mass of the middle element was the mean of the other two.
  • Limited success as not all elements fit into triads.

Newlands' Octaves

• John Newlands (1864)
  • Arranged 56 known elements by increasing atomic mass.
  • Observed that every eighth element had similar properties, termed "Newlands' Octaves."
  • Worked only up to calcium; failed for elements beyond that.

Mendeleev's Periodic Table

• Dmitri Mendeleev (1869)
  • Created the first periodic table, arranging elements by increasing atomic mass.
  • Established "Mendeleev's Law": Properties of elements are periodic functions of their atomic masses.
  • Arranged elements into 7 periods (rows) and 8 groups (columns).
  • Left spaces for undiscovered elements (e.g., eka-boron, eka-aluminum) and predicted some of their properties correctly.
  • Successfully incorporated noble gases without altering other element positions.
  • Demerits of Mendeleev's Table:
    • Hydrogen's position was unclear (resembles both alkali metals and halogens).
    • Some anomalies with atomic mass order (e.g., Cobalt before Nickel despite higher atomic mass).
    • Some elements grouped with dissimilar properties (e.g., manganese with halogens).

The Modern Periodic Table

• Henry Moseley (1913)
  • Established atomic number as the fundamental property, not atomic mass.
  • Led to the formulation of the Modern Periodic Law: Properties of elements are periodic functions of their atomic numbers.
  • Transitioned to the Modern Periodic Table, most commonly used in its long form.

Summary

• Transition from classifying elements by atomic mass to using atomic number corrected previous inconsistencies.
• Led to better understanding and organization of elements based on periodic trends.
• Modern Periodic Table has evolved but is rooted in the principles discovered by Moseley.