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Molecular Geometry and VSEPR Theory

Molecular Geometry and VSEPR Theory. 9/24/08. the properties of molecules depend not only on the bonding of atoms but also on the molecular geometry , or 3-D arrangement of a molecule’s atoms in space.

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Molecular Geometry and VSEPR Theory

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  1. Molecular Geometry and VSEPR Theory 9/24/08

  2. the properties of molecules depend not only on the bonding of atoms but also on the molecular geometry, or 3-D arrangement of a molecule’s atoms in space. • molecular polarity = the uneven distribution of molecular charge (NOT electrical charge, which refers to the charges within the atoms themselves) • the polarity of a molecule determines its chemical and physicalproperties • this is especially important for molecules of liquids and solids, because they are so close together (when compared to the molecules of gases) • unfortunately, the chemical formula of a compound does not tell you much about its molecular geometry, so we have a theory to help….

  3. VSEPR (Valence-Shell, Electron-Pair Repulsion) Theory = states that the repulsion between the sets of valence-level electrons in an atom causes these sets of electron pairs to be oriented as far apart as possible in all three dimensions. • rules of VSEPR theory: • all diatomic (2-atom) molecules Ex: HCl = are “linear”(duh). • 3-atom molecules can be one of 3 different shapes, depending on the number of unshared pairs the central atom has: • if it has no unshared pairs, then Ex: BeF2 = the shape is “linear.” (AB2) angle = 180 • if it has one unshared pair, then Ex: SnCl2 = the shape is “angular.” (AB2E) angle = 120 • if it has two unshared pairs, then Ex: H2O = the shape is “bent.” (AB2E2) angle = 105

  4. 4-atom molecules can be one of two different shapes, again, depending on the number of USPs the central atom has: • if it has no USPs, then the shape Ex: BF3 = is “trigonal-planar.” (AB3) angle = 120 • if it has one USPs, then the shape Ex: NH3 = is “trigonal-pyramidal.” (AB3E) angle = 107 • 5-atom molecules can be only one Ex: CH4 =shape: “tetrahedral.” (AB4) angle = 109.5 • 6-atom molecules can be only one Ex: PCl5 =shape: “trigonal-bipyramidal.” (AB5) angles = 90 onz-axis,120 on x & y-axis • 7-atom molecules can be only one Ex: SF6 = shape: “octahedral.” (AB6) angles = 90 on z-axis, 90 on x & y-axis

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