Lecture 30.2: Atomic Properties - Atomic Size

Introduction

• Continuation of discussion on atomic properties, focusing on atomic size.
• Atomic size/atomic radii: Measured as half the distance between adjacent nuclei in a molecule.

Trends in Atomic Radii

1. Down a Group

• Trend: Atomic radii increase.
• Reason:
  • Adding electron shells (Bohr model) as principal quantum number (energy level) increases.
  • Electrons move further from the nucleus, resulting in larger atomic size.

2. Across a Period

• Trend: Atomic radii decrease.
• Reason:
  • Increase in atomic number (more protons) increases nuclear charge.
  • Electrons added to the same energy level; no increase in shielding electrons.
  • Effective nuclear charge increases, pulling electrons closer to the nucleus.
  • Effective Nuclear Charge (Z_eff) = Nuclear charge (protons) - Core electrons (shielding electrons).

Understanding Effective Nuclear Charge

• Importance of understanding how shielding impacts electron configuration.
• As electrons are added across a period, core electrons remain the same, increasing effective nuclear charge.

Ionization and Ionic Radii

Positive Ions

• Formation: Loss of outermost electrons, leading to a smaller size.
• Trend: Become smaller across a period due to increased effective nuclear charge.

Negative Ions

• Formation: Gain of electrons, leading to a larger size.
• Trend: Increase in size due to the addition of electrons, reducing the effective nuclear charge.

Detailed Comparison

• Positive Ions: Decrease in size as effective nuclear charge increases.
• Negative Ions: Increase in size as additional electrons are added.

Summary of Trends

• Down a Group: Size increases (addition of shells).
• Across a Period:
  • Positive ions: Size decreases (increased effective nuclear charge).
  • Transition from metals to nonmetals influences ionic size.
  • Negative ions: Size increases (more electrons).

Conclusion

• Understanding the trends and reasons behind atomic and ionic size changes is crucial.
• Importance of reviewing and understanding the underlying principles and effective nuclear charge calculations for better comprehension of periodic trends.