Recording-2025-02-14T13_02_34.878Z

Lattice Energy and Ionic Compounds

• Definition of Lattice Energy: Energy released when oppositely charged ions come together to form a crystalline lattice.

• Importance of Charges: Lattice energy is directly proportional to the absolute value of the charges on the ions (q1, q2). The ratio of ions in a compound does not affect lattice energy.

  • Example: Beryllium oxide (BeO) is expected to have high lattice energy due to Be²⁺ and O²⁻ charges.

• Common Misconceptions: The ratio of ions (e.g., two iodines) does not play a role in lattice energy calculations.

• Comparative Analysis:

  • Calcium sulfide (CaS): Ca²⁺ and S²⁻.

  • Potassium bromide (KBr): K⁺ and Br⁻ (lowest charge).

  • Barium iodide (BaI₂): Ba²⁺ and I⁻.

  • Comparing size (radius) also affects energy calculations: larger radii lead to lower lattice energy.

Formation of Ionic Compounds

• Sodium and Phosphorus Reaction:

  • Charge on phosphorus in an ionic compound (with sodium) is -3 due to its position on the periodic table.

• Example of Naming Compounds:

  • Titanium dioxide (TiO₂) where titanium has a +4 charge to balance two -2 charges from oxygen.

Lewis Structures and Molecular Geometry

• Drawing Lewis Structures:

  • Important to differentiate bonding and lone pair electrons to correctly represent molecular structures.

  • Lewis structure for magnesium oxide (MgO) should not suggest covalent bonds due to significant electronegativity differences.

Electron Geometry and Molecular Shape

• Understanding Electron Clouds:

  • Electron clouds (lone pairs, bonding pairs) influence molecular geometry and shape.

  • Five primary geometries:

    • Linear (2 clouds)

    • Trigonal planar (3 clouds)

    • Tetrahedral (4 clouds)

    • Trigonal bipyramidal (5 clouds)

    • Octahedral (6 clouds)

• Practice with Electron Cloud Structures:

  • Example: Beryllium dihydride (linear structure). Adding lone pairs can alter bond angles from ideal values (e.g., tetrahedral angles).

Experimental Measurements and Molecular Structures

• Experimental Methods: Utilize techniques like X-ray diffraction to determine molecular arrangements and bond lengths.

• Competitive Electron Cloud Arrangements: Minimize electron-electron repulsion; lone pairs repel more than bonding pairs, which affects bond angles and molecular shapes.

  • Notably, the bonds get closer in the presence of lone pairs due to increased repulsion.

Steric Number and Electron Cloud Geometry

• Steric Number: Count of bonded atoms plus lone pairs determines geometry.

  • Example: Phosphorus in phosphorus trichloride (PCl₃) shows a steric number of 4 (3 bonds + 1 lone pair), leading to tetrahedral geometry.

• Application with Benzene and Other Molecules:

  • Analysis shows benzene has a trigonal planar geometry despite its double bonds.

Conclusion

• Key Points to Remember:

  • Electrons and lone pairs influence molecular shapes and bond angles significantly.

  • Practice identifying steric numbers and corresponding geometries to master molecular geometry insights.

  • Prepare for the upcoming exam by engaging with simulations and practical modeling using materials like toothpicks and gumdrops for spatial understanding.