Understanding Periodic Trends in Chemistry

Oct 18, 2024

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Lecture Notes: Periodic Trends

In this lecture, we discuss four important periodic trends: atomic radius, ionization energy, electronegativity, and electron affinity.

1. Atomic Radius

  • Definition: The distance from the center of the nucleus to the outermost electron shell.
  • Trend Across a Period:
    • Decreases from left to right (e.g., from Lithium to Fluorine).
    • Reason: Increase in the number of protons increases effective nuclear charge, pulling electrons closer to the nucleus according to Coulomb's Law.
  • Trend Down a Group:
    • Increases from top to bottom (e.g., from Lithium to Potassium).
    • Reason: Addition of new energy levels (or shells) increases the size of the atom.

2. Ionization Energy

  • Definition: The energy required to remove an electron from an atom.
  • First Ionization Energy: Energy needed to remove the first electron.
  • Second Ionization Energy: Energy needed to remove the second electron.
  • Trend Across a Period:
    • Increases from left to right.
    • Reason: Higher effective nuclear charge and smaller atomic radius increase the force, requiring more energy to remove an electron.
  • Trend Down a Group:
    • Decreases from top to bottom.
    • Reason: Larger atomic radius increases the distance between nucleus and outer electrons, reducing the force and energy required.

3. Electronegativity

  • Definition: The ability of an atom to attract electrons.
  • Trend Across a Period:
    • Increases from left to right.
    • Reason: Higher effective nuclear charge leads to stronger attraction.
    • Highest: Fluorine (most electronegative) with a value of 4.0 on the Pauling scale.
  • Trend Down a Group:
    • Decreases from top to bottom.
    • Reason: Increased atomic radius decreases the attractive force.
  • Note on Noble Gases: Electronegativity is undefined because their electron configuration is full and cannot accept more electrons.

4. Electron Affinity

  • Definition: The amount of energy released when an atom gains an electron.
  • Trend Across a Period:
    • Increases from left to right.
    • Reason: Atoms become more stable and release energy when gaining electrons, particularly fluorine.
  • Trend Down a Group:
    • Decreases from top to bottom.
    • Reason: Larger atomic radius leads to less stable negative ions, making reactions more endothermic.

Summary: The periodic trends describe how atomic properties change across periods and groups in the periodic table, influenced by atomic structure and Coulombic forces.