Lecture Notes: Structure of Ionic Solids

Learning Objective

• Goal: Represent ionic solids with a particulate model consistent with Coulomb's law and ion properties.

Key Concepts

Coulomb's Law

• Like charges repel; unlike charges attract.
• Essential in explaining ionic solid arrangements.

Arrangement of Ions

• Case A: Alternate arrangement of cations and anions; most stable due to minimum like-charge repulsion.
• Case B: Layered arrangement; less stable due to repulsion within layers.
• Case C: Two layers each of like charges; stability compromised by repulsion.
• Case D: Random arrangement; potential repulsion when like charges are adjacent.
• NaCl Example: Sodium donates an electron to chlorine, forming a stable ionic structure.

Periodic Trends and Ionic Size

• Atomic Size Influences:
  • Increases down a group (more electron shells).
  • Decreases across a period (effective nuclear charge increases).
• Comparison of Ionic Solids:
  • NaBr vs. RbBr: Sodium ions smaller than bromide; rubidium ions similar in size to bromide.
  • Periodic Table Position: Affects ionic sizes upon losing or gaining electrons.

Example Problems

• Identifying M2+ Ion: Based on periodic trends, Ba2+ is proposed due to its size relative to Sr2+.
• KCl Ionic Structure: Correct arrangement has smaller K+ ions compared to Cl- ions (Case D).

Properties of Ionic Solids

• Hardness & High Melting/Boiling Points: Due to strong electrostatic attractions.
• Brittleness:
  • Pressure disrupts ion layers causing repulsion and crystal cracking.
• Electrical Conductivity:
  • Non-conductive in solid state (ions fixed).
  • Conductive in liquid/aqueous states (ions free to move).

Conclusion

• Models of ionic solids must align with Coulomb's law and demonstrate properties of constituent ions.

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