Big Idea

• Periodic trends = predictable patterns in element properties
• Driven by:
  • Electron configuration
  • Distance from nucleus (n)
  • Effective nuclear charge (Zeff)

Effective Nuclear Charge (Zeff)

• Across a period:
  • Z increases
  • Shielding ~ constant
  • Zeff increases
• Down a group:
  • Shielding increases
  • Zeff roughly constant

Electronegativity

1. Atomic Radius

• Definition: Half the distance between nuclei in a bond
• Trend:
  • Decreases across a period →
  • Increases down a group ↓
• Why:
  • Across: Zeff increases → pulls electrons closer
  • Down: Higher n → electrons farther away
• Memory: Largest = bottom left, smallest = top right

2. Ionic Radius

• Cations (+): smaller than parent atom
• Anions (−): larger than parent atom
• Higher positive charge → smaller ion
• Isoelectronic series:
  • Same electrons, different protons
  • More protons → smaller size

3. Ionization Energy (IE)

• Energy required to remove an electron
• Always positive (endothermic)
• Trend:
  • Increases across →
  • Decreases down ↓
• Why:
  • Across: higher Zeff
  • Down: electrons farther from nucleus
• Key patterns:
  • Alkali metals = low IE
  • Noble gases = high IE
• Exceptions:
  • Be > B (s vs p)
  • N > O (half-filled stability)
• Successive IE:
  • IE₁ < IE₂ < IE₃
  • Large jump = core electron removal

4. Electron Affinity (EA)

• Energy change when adding an electron
• Trend:
  • More negative across →
  • Less negative down ↓
• Key ideas:
  • Negative EA = favorable (energy released)
  • Halogens = most negative EA
• Exceptions:
  • Group 2 (filled s)
  • Group 15 (half-filled p)
  • Group 18 (full shell)
• Note: Cl has more negative EA than F (less repulsion)

5. Metallic Character

• Increases down a group
• Decreases across a period
• Metals lose electrons easily (low IE)

Summary Table

Quick Analogies

• Zeff = nuclear “pull”
• Shielding = inner electron blocking
• Atomic size = electron distance
• Ionization energy = difficulty removing electron

Allied Health Connection

• Ions (Na⁺, K⁺, Cl⁻) → nerve signaling
• Reactivity → drug interactions
• Ion size/charge → membrane transport