Chapter 10: Chemical Bonding II – Molecular Geometry and Hybridization of Atomic OrbitalsStructure Determines Properties- Molecules = 3D objects- The properties of molecular substances depend on the structure of the moleculeoSkeletal arrangement of the atomsoType of bonds between the atoms Ionic, polar covalent, or covalentoShape of moleculeVSEPR (Valence Shell Electron Pair Repulsion) Theory- VSEPR Theory: Electron groups around the central atom will be the most stable when they are as far apart as possibleoBecause they’re negatively charged- The resulting geometric arrangement will allow us to predict the shapes and bond angles in the moleculeElectron Groups- The Lewis structure predicts the arrangement of valence electrons around the central atoms- Each lone pair of electrons constitutes one electron group on a central atom- Each bond constitutes ONE electron group on a central atom!oRegardless of whether it is single, double, or tripleMolecular Geometries- 5 basic arrangements of electron groups around a central atomoBased on a maximum of 6 bonding electron groups1oMay be more than 6 on very large atoms, but it’s very rare- Each of these 5 basic arrangements results in 5 basic molecular shapeso“Perfect” geometric figure = all electron groups must be bonds & all bonds must be equivalent- For molecules that exhibit resonance, it doesn’t matter which resonance for you use, the molecular geometry will be the sameLinear Geometry- When there are 2 electron groups around the central atom, they will occupy positions opposite each other around the central atom- Bond angle = 180°Trigonal Planar Geometry- When there are 3 electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom- Bond angle = 120°Not Quite Perfect Geometry- Because the bonds are not identical, the observed angles are slightly different from the ideal onesTetrahedral Geometry- When there are 4 electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom2- Bond angle = 109.5°Trigonal Bipyramidal Geometry- When there are 5 electron groups around the central atom, they will occupy positions in the shape of two tetrahedrons that are base-to-base with the central atom in the center of the shared bases- Bond angle (equatorial positions) = 120°- Bond angle (axial & equatorial positions) = 90°- Equatorial positions = in the same base plane as the central atom- Axial positions = above and below the central atomOctahedral Geometry- When there are 6 electron groups around the central atom, they will occupy positions in the shape of 2 square-base pyramids that are base-to-base with the central atom in the center of the shared bases3- All positions = equivalent- Bond angle = 90°- Called octahedral because the geometric figure has 8 facesThe Effect of Lone Pairs- Lone pair groups “occupy more space” on the central atomoTheir electron density is exclusively on the central atom rather than shared like bonding electron groups Area of negative charge takes more space- Relative sizes of repulsive force interactions:oLone pair – lone pair > lone pair – bonding pair > bonding pair – bonding pair- This affects the bond angles, making them smaller than expectedoMolecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengthsoThe shape will be a derivative of one of the basic shapesDerivative of Trigonal Geometry- Trigonal Planar – Bento3 electron groups around the central atomo1 lone pairoBond angle = < 120°4Derivatives of Tetrahedral Geometry- Tetrahedral – Pyramidalo4 electron groups around central atomo1 lone pairoBond angle = < 109.5°- Tetrahedral – Bento4 electron groups around central atomo2 lone pairsoBond angles = < 109.5°oPlanar – looks similar to the trigonal planar-bent shape, except with smaller anglesDerivatives of the Trigonal Bipyramidal Geometry- When there are 5 electron groups around the central atom and some are lone pairs, they will occupy the equatorial positions because there is more room- Trigonal Bipyramidal See-SawoDistorted tetrahedrono5 electron groups around central atomo1 lone pair5oBond angles (equatorial) = < 120°oBond angles (axial & equatorial) = < 90°- Trigonal Bipyramidal T-Shapedo5 electron groups around central atomo2 lone pairsoBond angles = 90°o- Trigonal Bipyramidal Linearo5 electron groups around central atomo3 lone pairsoLinear shape = 180° (axial to axial)6Derivatives of Octahedral Geometry- When there are 6 electron groups around the central atom, and some are lone pairs, each even number lone pair will take a position opposite of the previous lone pair- Octahedral – Square Pyramid Shapeo6 electron groups around central atomo1 lone pairoBond angles (axial & equatorial) = < 90°- Octahedral – Square Planar Shapeo6 electron groups around central atomo2 lone pairsoBond angles (equatorial) = 90°78Predicting the Shapes around Central Atoms1. Draw the Lewis structure2. Determine the number of electron gorups around the central atom3. Classify each electron group as bonding or lone pair, and count each timea. Remember that multiple bonds count as one group (single, double, etc.)4. Determine the shape & bond angles- Practice Problem #10.1oDetermine the shape of the following (derivative shapes)o-------------------- FFAsF Pyramidalo1 OClO---------------- Tetrahedral bento T-shape- Practice Problem #10.2oPredict the molecular geometry and bond angles in SiF51-9F Cl FF••••••••••••••••••- Practice Problem #10.3oPredict the molecular geometry and bond angles in ClO2FMultiple Central Atoms- Many molecules have larger structures with many interior atoms10- Practice Problem #10.4oPredict the molecular geometries in H3BO3Polarity of Molecules- For a molecule to be polar, it must have a net dipole momentoHave polar bonds Electronegativity difference along the bond (non-zero dipole moment along the bond)oUnsymmetrical shape- Polarity affects the intermolecular forces of attractionoIntermolecular forces of attraction between molecules of the same substance determines boiling/melting point11oIntermolecular forces of attraction between molecules of different substances determines solubility- Nonbonding pairs affect molecular polarityoStrong pull in its direction- H-Cl bond =
View Full Document
Unlocking...