Shielding and Effective Nuclear Charge

Introduction to Concepts

• Shielding and effective nuclear charge are interrelated concepts.
• They help explain atomic properties based on interactions between positive protons and negative electrons in atoms.
• Focus is on the valence electron shell, where chemical reactions occur.

Forces in Atoms

• Attractive Force: Negative electrons and positive protons attract each other.
• Repulsive Force: Electrons repel each other.
• All atoms (except hydrogen) have multiple protons and electrons.

Atomic Models and Electron Configuration

• Lithium Example:
  • Protons: 3 (blue)
  • Core Electrons: 2 (violet)
  • Valence Electron: 1
  • Arrangement of electrons is key to understanding shielding and effective nuclear charge.
• Attractive Forces: Represented by inward blue arrows;
• Repulsive Forces: Represented by outward violet arrows.

Understanding Shielding and Effective Nuclear Charge

• Valence electrons feel less attraction due to core electron repulsion.
• The Effective Nuclear Charge (Z_eff):
  • It is the net attraction experienced by valence electrons.
  • Formula: Z_eff = Total Nuclear Charge (Z) - Shielding Constant (S)
  • Lithium Example: Z_eff = 3 protons - 2 core electrons = 1

Periodic Trends in the Second Period

• Beryllium:
  • Nuclear Charge: 4+
  • Core Electrons: 2
  • Z_eff = 4 - 2 = 2
• Fluorine:
  • Nuclear Charge: 9+
  • Z_eff = 9 - 2 = 7
• Atomic Size Trend:
  • Decreases across period due to increased Z_eff pulling electrons inward.

Moving to Period 3

• Neon:
  • Protons: 10, Core Electrons: 2
  • Z_eff = 10 - 2 = 8
• Sodium:
  • Valence Electron in higher energy shell (n=3), 10 core electrons increase shielding.
  • Z_eff = 11 - 10 = 1.
• Comparing Groups:
  • Sodium vs Lithium:
    • Sodium has a larger radius due to increased shielding and repulsion from core electrons.
    • Both have Z_eff of 1.
• Potassium:
  • Similar Z_eff pattern as sodium and lithium due to added energy levels.

Summary of Atomic Size Trends

• Across a Period: Atoms get smaller due to increased Z_eff.
• Down a Group: Atoms get larger due to increased shielding effects.

Ionization Energy Trends

• Energy required to remove valence electrons.
• Across Period: Ionization energy increases as electrons are held tighter.
• Down Group: Ionization energy decreases due to increased electron shielding.

Transition Metals

• Variability of stability based on electron addition in d orbitals is not covered in detail.
• Transition metals show different trends compared to main group elements.