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Molecular Geometry

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Molecular Geometry
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Molecular Geometry

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  1. Molecular Geometry VSEPR Theory

  2. Molecular geometry • Area of chemistry that deals with the shape of molecules. • Shape determines many properties about a molecule: • Reactivity • Smell • Taste

  3. Molecular shape • Lewis Structures do not reveal anything about the 3-D arrangement of atoms in a molecules. • Could you have predicted the 3-D shape of CCl4 from the Lewis structure on the left?

  4. VSEPR Theory • Valence Shell Electron Pair Repulsion Theory • States that molecules will take on shape that is lowest in energy. • A low energy shape is one that minimizes the valence shell electron pair repulsion (VSEPR) between adjacent atoms. • Atoms in a molecule try to spread out from one another as much as possible to reduce the “like charge repulsion” between their outer electrons.

  5. Vsepr Example • Methane, CH4 • You might think this is the farthest that the hydrogens can get away from each other. • But if you think in 3 dimensions, this shape actually causes less repulsion between the bonding pairs of electrons.

  6. Steps for determining molecular geometry • 1. Draw proper Lewis structure. • 2. Count the # of bonded atoms and # of lone pairs around the central atom. • 3. Use CHART to determine name of shape. • 4. Redraw shape with proper geometry.

  7. 0 Geometry Chart

  8. Big picture on molecular geometry • These bonds (shared electrons) and lone pairs of electrons are all negatively charged so they want to spread out from each other as much as possible within a molecule. • This minimizes “like charge repulsion” between electrons and represents for the molecule.

  9. Difference between bonded electrons & Lone Pairs • Lone pairs take up more space than bonding pairs and thus push atoms farther away from each other. • This decreases the predicted bond angles.

  10. Example – nH3 • 1. Draw proper Lewis Structure. • 2. Count the # of bonded atoms and # of lone pairs around the central atom. • 3 Bonded Atoms • 1 Lone Pairs • 3. Use CHART to determine name of shape. • Trigonal Pyramidal • 4. Redraw shape with proper geometry.

  11. Example – CH3F • 1. Draw proper Lewis Structure. • 2. Count the # of bonded atoms and # of lone pairs around the central atom. • 4 Bonded Atoms • 0 Lone Pairs • 3. Use CHART to determine name of shape. • Tetrahedral • 4. Redraw shape with proper geometry.