Periodic Classification of Elements

• Need for Classification

  • Organize vast chemical information
  • Simplify study of elements and compounds
  • Discover regular patterns in properties

• Early Attempts at Classification

  • Dobereiner's Triads
    • Grouped elements in threes
    • Atomic mass of middle element is the mean of the other two
    • Limitation: Could only identify a few triads
  • Newlands' Law of Octaves
    • Arranged by increasing atomic mass
    • Properties of every eighth element resemble the first
    • Compared to musical notes
    • Limitation: Applicable only up to Calcium

• Mendeleev's Periodic Table

  • Periodic Law: Properties are a periodic function of atomic mass
  • Features
    • 8 vertical columns (Groups)
    • 7 horizontal rows (Periods)
  • Merits
    • Left gaps for undiscovered elements (e.g., Eka-boron, Eka-aluminium)
    • Predicted properties of new elements
    • Accommodated noble gases later without disturbing the table
  • Demerits
    • Position of isotopes was uncertain
    • Anomalous pair of atomic masses (e.g., Cobalt and Nickel)
    • Position of hydrogen was ambiguous

• Modern Periodic Table

  • Henry Moseley's Discovery (1913)
    • Atomic number ($Z$) is a more fundamental property than atomic mass
  • Modern Periodic Law
    • Properties are a periodic function of their atomic number
  • Structure
    • 18 vertical columns (Groups / Families)
    • 7 horizontal rows (Periods / Shells)
  • Position of Elements
    • Elements in a group have the same number of valence electrons
    • Elements in a period have the same number of shells

• Trends in the Modern Periodic Table

  • Valency
    • Combining capacity of an element
    • Remains same down a group; varies across a period
  • Atomic Size (Atomic Radius)
    • Distance from center of nucleus to outermost shell
    • Decreases across a period (left to right) due to increased nuclear charge
    • Increases down a group due to addition of new shells
  • Metallic and Non-Metallic Properties
    • Metallic character decreases across a period, increases down a group
    • Non-metallic character increases across a period, decreases down a group
    • Metalloids (Semimetals) lie along the zig-zag line separating metals and non-metals