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Molecular Structure Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space. Physical and chemical properties depend on the geometry of a molecule.

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Molecular Structures 3-D Model 3-D Drawing

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3 VSEPR Model The Valence Shell Electron Pair Repulsion model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible to minimize the repulsion between them.

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VSEPR Model Electron Pair Geometry – is determined by the number and arrangement of all electron pairs (bonding and lone) around the central atom. Molecular geometry – is determined by the arrangement of atoms (or bonding electron pairs only) around the central atom. In molecules with no lone pairs, Electron Pair Geometry = Molecular Geometry N H H : H

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AXE shorthand notation: A - central atom X - terminal atoms E - lone pair electrons

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6 Only five basic shapes. When a lone pair replaces an atom, the molecular geometry changes as well as the angles. Predicting Molecular Geometry: VSEPR # e- pairs2 3 4 56

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1.Draw the Lewis structure. 2.Determine how many electron pairs (bonded and non-bonded) are around the central atom. **Treat a multiple bond like a single bond when determining a shape. 3.(Write the AXE shorthand notation.) 4.Determine the electron pair geometry (**one of the five basic shapes). 5.If the molecule has lone pairs around the central atom, then determine the molecular geometry. (This is a subset of the electron geometry.) Predicting Molecular Geometry: VSEPR

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(electron pair geometry = molecular geometry) Geometry is LINEAR. Bond angle is 180°. Example 1: BeCl 2 Cl Be Cl 1. Draw the Lewis structure 2. Two electron pairs around the central atom. Two bonded and Zero lone pairs. Predicting Molecular Geometry

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(electron geometry = molecular geometry) Geometry is TRIGONAL PLANAR. Bond angle is 120°... F: :F B :F:.. Example 2: BF 3 Three electron pairs around the central atom. Three bonded and Zero lone pairs.

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Predicting Molecular Geometry AX 3 E 0 AX 2 E 1 (electron geometry = trigonal planar) Molecular geometry = BENT Bond angle <120 ⁰. Example 3: SO 2 O S O S OO Three electron pairs around the central atom. Two bonded and One lone pairs.

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bond lone pairs 4 0 tetrahedral.. 3 1 triangular pyramidal 2 2 angular (bent).. Tetrahedral (Electron Geometry) Four e- pairs about central atom Model

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Predicting Molecular Geometry CH 4 (electron pair geometry = molecular geometry) Molecular geometry = TETRAHEDRAL Bond angle is 109.5 ⁰. AX 4 E 0 Example 4: H H H C H Four electron pairs around the central atom. Zero lone pairs.

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Predicting Molecular Geometry NH 3 AX 4 AX 3 E 1 (electron geometry = tetrahedral) Molecular geometry = TRIGONAL PYRAMIDAL Example 5: H H N H H N H H Four electron pairs around the central atom. Three bonded and One lone pair.

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Predicting Molecular Geometry H2OH2O AX 4 AX 2 E 2 (electron geometry = tetrahedral) Molecular geometry = BENT Example 6: H O H Four electron pairs around the central atom. Two bonded and Two lone pairs. O H H

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Predicting Molecular Geometry Tetrahedral - bond angles Order of increasing repulsion : bonding pair-bonding pair < bonding pair-lone pair < lone pair-lone pair

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Predicting Molecular Geometry : F : : : F : : : : F : : P F : : : Example 7: PF 5 electron and molecular geometry= TRIGONAL BIPYRAMIDAL Five electron pairs around the central atom. Zero lone pairs.

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Predicting Molecular Geometry F: : : :F : : S F: : : :F : : :F: : : Example 8: SF 6 (electron geometry = octahedral) Molecular geometry = OCTAHEDRAL Six electron pairs around the central atom. Six bonded and Zero lone pairs.

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dipole moment, μ = 1.85 D.. H O + Net dipole dipole moment, μ = 0 D Molecular Geometry Dipole Moment and Polarity nonpolar, bp=-79 C polar, bp=100 C

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polar In general, a molecule is polar if: it is not a basic VSEPR shape (has lone pairs of electrons) Ex: (polar) Ex: H 2 O, bent (polar) or if the terminal atoms/groups in a basic VSEPR shape differ. Ex: (polar) Ex: CH 2 C l 2, tetrahedral (polar) Molecular Geometry Dipole Moment and Polarity

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Dipole Moment and Molecular Geometry Molecules that exhibit any asymmetry in the distribution of electrons would have a nonzero net dipole moment. These molecules are considered polar. Non polar VSEPR shape identical atomsPolar VSEPR shape atoms differ

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Dipole Moment and Molecular Geometry

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Non polar VSEPR shape identical atoms PF 5 PF 4 C l Non polar Atoms differ. BUT can be divided into nonpolar VSEPR shapes: linear + triangular planar linear + triangular planar PF 3 C l 2 Polar VSEPR shape atoms differ + Polar Atoms differ. Doesn’t divide into nonpolar VSEPR shapes + Molecular Geometry Dipole Moment and Polarity

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SF 4 ClF 3 Cl : : F F F F F : F F S Xe F F F F Xe F F : : : XeF 4 XeF 2 T-shaped No symmetry → polar SeeSaw No symmetry → polar Square Planar Symmetric → non polar Linear Symmetric → non polar Dipole Moment and Molecular Geometry

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Which compound is the most polar? Which compounds on the list are non-polar? Molecular Geometry Dipole Moment and Polarity CO, PCl 3, BCl 3, GeH 4, CF 4

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