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VSEPR Theory and Molecular Geometries

VSEPR Theory and Molecular Geometries. VSEPR Theory. VSEPR (pronounced “vesper”) stands for: V alence S hell E lectron P air R epulsion It attempts to explain the 3-D shapes of molecules. How do we know what molecules look like? X-Ray Crystallography

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VSEPR Theory and Molecular Geometries

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  1. VSEPR Theory and Molecular Geometries

  2. VSEPR Theory • VSEPR (pronounced “vesper”) stands for: • Valence Shell Electron Pair Repulsion • It attempts to explain the 3-D shapes of molecules. • How do we know what molecules look like? • X-Ray Crystallography • X-rays are fired through a crystallized sample of a substance and are diffracted in many directions. • By carefully studying the directions and intensity of the diffracted X-rays, a crystallographer can tell a lot about the locations of atoms within the crystal. • Remember, theories explain observations! • VSEPR Theory explains the observed shapes of molecules.

  3. X-Ray Crystallography

  4. VSEPR Theory • The basic principles of VSEPR Theory: • Electron pairs repel each other. • Electron pairs in molecules tend to arrange themselves so as to minimize the repulsion between them. • In other words, get as far apart as possible.

  5. The Geometry of CH4 • Based on the Lewis structure of methane, you might expect the geometry on the left. • VSEPR theory predicts the geometry on the right. • The geometry on the right is confirmed by observations. H 90º 109.5º H C H H The spheres represent the centers of the carbon and hydrogen atoms.

  6. VSEPR Formulas • Lewis structures do not show geometry, only electron pair placement. • However, the 3-D shape (geometry) of a molecule can be determined from a properly-drawn Lewis structure. • All monocentric molecules can be represented by a VSEPR formula: • AXE • A = central atom • X = outer atoms (doesn’t matter what they actually are or how many bonds they are held by) • E = lone pairs of electrons on the central atom only.

  7. VSEPR Formulas • What VSEPR formula corresponds to the chlorate ion, ClO3-1? • First draw a proper Lewis structure: • Needed = 32 • Available = 26 • Shared = 6 • One central atom, three outer atoms, one lone pair: • AX3E -1 O Cl O O

  8. VSEPR Formulas • What VSEPR formula corresponds to sulfur trioxide, SO3? • Draw a Lewis structure. • Needed = 32 • Available = 24 • Shared = 8 • 1 central atom, 3 outer atoms, no lone pairs: • AX3 O S O O

  9. Molecular Geometries • Each VSEPR formula corresponds to a certain molecular geometry. • There are six possible geometries for molecules whose central atoms obey the octet rule.

  10. Molecular Geometries • VSEPR Formula: AX2 • Geometry: Linear • Bond Angle: 180º • Example: CO2 O C O

  11. Molecular Geometries • VSEPR Formula: AX3 • Geometry: Trigonal Planar • Bond Angle: 120º • Example: SO3 O O S O

  12. Molecular Geometries • VSEPR Formula: AX4 • Geometry: Tetrahedral • Bond Angle: 109.5º • Example: CCl4 Cl Cl C Cl Cl

  13. Molecular Geometries • VSEPR Formula: AX2E • Geometry: Bent (Angular) • Bond Angle: Less than 120º • Example: SO2 O S O

  14. Molecular Geometries • VSEPR Formula: AX3E • Geometry: Trigonal Pyramidal • Bond Angle: Less than 109.5º • Example: NH3 H H N H

  15. Molecular Geometries • VSEPR Formula: AX2E2 • Geometry: Bent (Angular) • Bond Angle: Less than 109.5º • Example: H2O H O H

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