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Molecular Geometry in 3-Dimensions

Molecular Geometry in 3-Dimensions. VSEPR Theory: “Valence-shell electron-pair repulsion” Theory States that: electron pairs – both bond pairs and lone pairs – repel each other ; therefore, electron pairs assume orientations around an atom in such a way to minimize repulsions.

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Molecular Geometry in 3-Dimensions

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  1. Molecular Geometry in 3-Dimensions • VSEPR Theory: • “Valence-shell electron-pair repulsion” Theory • States that: • electron pairs – both bond pairs and lone pairs – repel each other; therefore, electron pairs assume orientations around an atom in such a way to minimize repulsions

  2. Electron-Group Geometry: • The VSEPR Theory uses the central atom as a reference point • Electron-groups are then considered in reference to the central atom • Electron-groups may be: • 1) Bond pairs of electrons (bonded to terminal atoms) • 2) Lone pairs of electrons

  3. Electron-Group Geometry: • We will encounter situations in which central atoms have 5 possible electron-groups distributed around them, each with a corresponding shape: • 2 electron groups: linear • 3 electron groups: trigonal-planar • 4 electron groups: tetrahedral • 5 electron groups: trigonal-bipyramidal • 6 electron groups: octahedral

  4. VSEPR Notation • Known as “AXE notation” • Represents: A – central atom X – bond pairs of electrons E – Lone pairs of electrons • In addition to electron-group shape, this helps determine specific molecular geometry

  5. Examples • a) CH4 • 4 electron groups • Electron group geometry: tetrahedral • 4 bond pairs, 0 lone pairs • VSEPR notation: “AX4” • Molecular geometry: tetrahedral

  6. Examples • b) H2O • 4 electron groups • Electron group geometry: tetrahedral • 2 bond pairs, 2 lone pairs • VSEPR notation: “AX2E2” • Molecular geometry: angular or bent

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