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. Molecular Structures 3-D Model 3-D Drawing 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. 3 VSEPR Model H N : H H 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 AXE shorthand notation: • A - central atom • X - terminal atoms • E - lone pair electrons Predicting Molecular Geometry: VSEPR # e- pairs Only five basic shapes. When a lone pair replaces an atom, the molecular geometry changes as well as the angles. 2 3 4 5 6 6 Predicting Molecular Geometry: VSEPR 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 Example 1: BeCl2 1. Draw the Lewis structure Cl Be Cl 2. Two electron pairs around the central atom. Two bonded and Zero lone pairs. (electron pair geometry = molecular geometry) Geometry is LINEAR. Bond angle is 180°. Predicting Molecular Geometry Example 2: BF3 .. :F: .. :F .. B .. F: .. Three electron pairs around the central atom. Three bonded and Zero lone pairs. (electron geometry = molecular geometry) Geometry is TRIGONAL PLANAR. Bond angle is 120°. Predicting Molecular Geometry Example 3: SO2 AX3E0 O S AX2E1 O Three electron pairs around the central atom. Two bonded and One lone pairs. S (electron geometry = trigonal planar) Molecular geometry = BENT Bond angle <120⁰. O O Tetrahedral (Electron Geometry) Four e- pairs about central atom bond pairs lone pairs 4 0 3 1 2 2 tetrahedral .. triangular pyramidal .. angular (bent) Model Predicting Molecular Geometry Example 4: CH4 AX4E0 H H C H H Four electron pairs around the central atom. Zero lone (electron pair geometry = molecular geometry) Molecular geometry = TETRAHEDRAL pairs. Bond angle is 109.5⁰. Predicting Molecular Geometry Example 5: NH3 H N H AX4 AX3E1 H Four electron pairs around the central atom. Three bonded and One lone pair. (electron geometry = tetrahedral) N H H H Molecular geometry = TRIGONAL PYRAMIDAL Predicting Molecular Geometry Example 6: H2O AX4 H O AX2E2 H Four electron pairs around the central atom. Two bonded and Two lone pairs. O (electron geometry = tetrahedral) Molecular geometry = BENT H H Predicting Molecular Geometry Tetrahedral - bond angles Order of increasing repulsion: bonding pair-bonding pair < bonding pair-lone pair < lone pair-lone pair Predicting Molecular Geometry Example 7: PF5 : : : :F P F: F: : :F: : : : : :F: Five electron pairs around the central atom. electron and molecular geometry= Zero lone pairs. TRIGONAL BIPYRAMIDAL Predicting Molecular Geometry Example 8: SF6 : S : : : : : : :F :F: : F: : :F F: : :F: Six electron pairs around the central atom. Six bonded and Zero lone pairs. (electron geometry = octahedral) Molecular geometry = OCTAHEDRAL Molecular Geometry Dipole Moment and Polarity d- d+ d- O C O nonpolar, bp=-79C dipole moment, μ = 0 D d- O polar, bp=100C d+ H H d+ + Net dipole dipole moment, μ = 1.85 D Molecular Geometry Dipole Moment and Polarity In general, a molecule is polar if: it is not a basic VSEPR shape (has lone pairs of electrons) Ex: H2O, bent (polar) or if the terminal atoms/groups in a basic VSEPR shape differ. Ex: CH2Cl2, tetrahedral (polar) 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 Polar VSEPR shape identical atoms VSEPR shape atoms differ Dipole Moment and Molecular Geometry Molecular Geometry Dipole Moment and Polarity Non polar + PF3Cl2 PF5 Non polar VSEPR shape identical atoms Atoms differ. BUT can be divided into nonpolar VSEPR shapes: linear + triangular planar PF4Cl Polar Polar Atoms differ. Doesn’t divide into nonpolar VSEPR shapes VSEPR shape atoms differ PF3Cl2 Dipole Moment and Molecular Geometry F Cl ClF3 F T-shaped No symmetry → polar F F XeF4 F .. Xe .. F F S: F F F F F Xe: Square Planar Symmetric → non polar F SF4 SeeSaw No symmetry → polar XeF2 Linear Symmetric → non polar Molecular Geometry Dipole Moment and Polarity CO, PCl3, BCl3, GeH4, CF4 Which compound is the most polar? Which compounds on the list are non-polar?