Periodic Properties of Elements - In Depth Notes

Chapter 4: Periodic Properties of Elements

4.1 The Periodic Table

  • Development of Periodic Table

    • Proposed by Dmitri Mendeleev and Lothar Meyer in 1869
    • Organized elements by similar properties and reactivities
    • Currently accounts for 118 discovered elements

  • Periodic Classification

    • Elements arranged in increasing energy levels: s, p, d, f blocks
    • 18 groups (columns) and 7 periods (rows)
    • Same column = Same valence electron count
    • Same period = Same outermost shell

  • Classification of Elements

    • Metals (92 elements):

    • Good conductors

    • Malleable and shiny, solid (except Hg)

    • Form basic metal oxides

    • Tend to lose electrons (form cations)

    • Non-metals (19 elements):

    • Poor conductors, brittle, and not lustrous

    • Can be gases or solids (Br₂ is liquid)

    • Form acidic nonmetal oxides

    • Tend to gain electrons (form anions)

    • Metalloids (7 elements):

    • Intermediate properties between metals and nonmetals

    • Semiconductors (Si, Ge)

4.2 Effective Nuclear Charge

  • Definition:

    • Total positive charge from protons (Z)
    • Acts as the electrostatic force attracting electrons

  • Effective Nuclear Charge (Z_eff):

    • Actual charge an electron feels, accounting for shielding from inner electrons
    • Formula: Z_eff = Z - σ (σ = shielding constant)
    • Increases across periods, remains constant down groups

  • Trends:

    • Across a period: Z_eff increases because Z increases
    • Down a group: Z_eff stays relatively constant; distance increases

4.3 Sizes of Atoms and Ions

  • Atomic Size Trends:

    • Decreases across a period and increases down a group
    • Factors affecting atomic size: Zeff, shielding, n values
    • Example: Helium (smallest) to Cesium (largest)

  • Ionic Size Trends:

    • Cations (smaller than parent atoms): Electron removal leads to higher Zeff
    • Anions (larger than parent atoms): Electron addition increases electron repulsion
    • Isoelectronic Series: Ions with the same electron number but different Z will have different radii

4.4 Ionization Energy and Electron Affinity

  • Definitions:

    • Ionization Energy (IE): Energy required to remove an electron from an atom

    • Successive IEs are higher due to increased Zeff on remaining electrons

    • Electron Affinity (EA): Energy change when an electron is added to an atom

    • Typically negative values, indicating energy release

  • Trends:

    • IE increases across a period and decreases down a group
    • EA generally becomes more negative across periods and less negative down groups
    • Exceptions in noble gases (positive EA values)

4.5 Electronegativity

  • Definition: Measure of an atom’s ability to attract electrons in a bond
  • General Trend: Increases across a period and decreases down a group
  • Highest Electronegativity: Fluorine (3.98)

4.6 Metallic Behavior of Elements

  • Metallic Behavior: Ability to lose electrons easily
    • Higher metallic character indicates more metallic properties
  • Trends: Metallic behavior decreases across a period and increases down a group
  • Non-metallic behavior shows the opposite trend

Summary of Periodic Trends

  • Atomic Radius:
    • Decreases across a period, increases down a group
  • Ionization Energy:
    • Increases across a period, decreases down a group
  • Electron Affinity:
    • Increases across a period, decreases down a group
  • Electronegativity:
    • Increases across a period, decreases down a group
  • Metallic Behavior:
    • Decreases across a period, increases down a group