Lecture 30.2: Atomic Properties - Atomic Size and Radii

Introduction

• Focus on atomic size, also known as atomic radii.
• Two main trends:
  1. Increases going down a group.
  2. Decreases going across a period.

Atomic Radii: Definitions and Measurement

• Atomic radius is half the distance between adjacent nuclei in a molecule.
• Measured from nucleus to nucleus.

Trend 1: Increasing Atomic Radii Down a Group

• Reason: Addition of electron shells as you move down a group.
• Principal quantum number (n) increases, adding energy levels further from the nucleus.
• Example: As n increases from 1 to 2 to 3, the atomic size increases.

Trend 2: Decreasing Atomic Radii Across a Period

• Challenge: Understanding why radii decrease across a period.
• As atomic number increases, proton count increases.
• Positive nuclear charge increases, pulling electrons closer.
• Electrons added to the same energy level.

Effective Nuclear Charge

• Equation: Z-effective = Nuclear Charge (Protons) - Shielding Electrons (Core Electrons).
• Effect: As protons increase across a period, effective nuclear charge increases.
• Result: Stronger pull on electrons reduces atomic size.

Neutral Atoms to Ions

Formation of Positive Ions

• Loss of electrons from outermost shell, decreasing atomic size.
• Example: Aluminum becomes smaller when forming Al³⁺.
• Reason: Removal of an electron shell increases nuclear pull on remaining electrons.

Formation of Negative Ions

• Addition of electrons, increasing atomic size.
• Reason: Adding electrons decreases effective nuclear charge, increasing radius.

Ionic Radii Trends

• Positive Ions: Size decreases across a period as nuclear charge increases.
• Negative Ions: Size increases with additional electrons.
• Overall: Understanding ionic radii requires knowledge of electron configuration changes.

Summary of Trends

• Down a group: Atomic size increases due to added electron shells.
• Across a period: Size decreases due to increasing effective nuclear charge.
• Practical Insight: Understand why these trends occur, not just what the trends are.