Notes on Periodic Properties: Atomic and Ionic Radii

Introduction

• Today's topic: Periodic Properties focusing on Atomic Radii and Ionic Radii.
• Atomic radii = Atomic size

Atomic Radii

• Definition: Distance from the nucleus to the outermost electron of an atom.
• Example:
  • Carbon (C): Atomic number 6, Electronic configuration: 2, 4
  • Structure:
    • K shell: 2 electrons
    • L shell: 4 electrons
  • Atomic Radii is the distance from the nucleus to the outermost electron.

Ionic Radii

• Cation: Positively charged ion (e.g., Na⁺)
  • Sodium (Na): Atomic number 11, Loses 1 electron, Configuration: 2, 8
  • Ionic Radii: Distance from the nucleus to the outermost electron of a cation.
• Anion: Negatively charged ion (e.g., F⁻)
  • Fluoride (F): Atomic number 9, Gains 1 electron, Configuration: 2, 8
  • Ionic Radii: Distance from the nucleus to the outermost electron of an anion.

Types of Radii

• Covalent Radii:
  • Defined for nonmetals in covalently bonded molecules.
  • Calculated as half the bond distance between two bonded nuclei.
  • Example: For Cl₂, Bond length = 198 pm, Covalent Radii (Rₕ) = 198/2 = 99 pm.
• Van der Waals Radii:
  • Defined for noble gases and non-metallic elements.
  • Distance between two non-bonded adjacent atoms, measured in solid state.
  • Definition: Half the distance between the two nuclei of adjacent non-bonded atoms.
  • Example: For H₂, Covalent radii = 37 pm, Van der Waals radii = 120 pm.

Factors Affecting Covalent Radii

1. Number of Shells:
   • Directly proportional to atomic radii.
   • More shells = larger atomic radii.
2. Effective Nuclear Charge:
   • Inversely proportional to atomic radii.
   • Higher effective nuclear charge = smaller atomic radii.
3. Bond Multiplicity:
   • Inversely proportional to atomic radii.
   • Higher bond order (double/triple bonds) = smaller atomic radii.

Periodic Variation of Atomic Radii

• Across a Period: Atomic radii decrease from left to right due to increasing effective nuclear charge.
• Down a Group: Atomic radii increase due to addition of shells.

Ionic Radii

• Cations: Smaller than parent atoms (e.g., Na⁺ < Na).
• Anions: Larger than parent atoms (e.g., Cl⁻ > Cl).
• Periodic Variation:
  • Across a period: Ionic radii decrease.
  • Down a group: Ionic radii increase.

Conclusion

• Understanding atomic and ionic radii is essential for predicting behaviors of elements in chemical reactions.
• Key takeaways:
  • Relationship between atomic structure and size.
  • Differences between atomic and ionic radii.
  • Importance of effective nuclear charge and bonding in determining size.