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3.2 VSEPR Theory VSEPR Theory Valence Shell Electron Pair Repulsion •Predicts the 3-D geometry of molecules (covalent compounds) •The structure around a given atom is determined by minimizing electron-pair repulsions (getting pairs of electrons as far apart as possible) •Bonding geometry always starts out with 4 bonding areas in a tetrahedral shape. •Molecule geometry depends on bonding pairs. This can either bonding or lone pair 109.5o Shapes Shapes 1. Tetrahedral – 4 bonding e- groups 2. Pyramidal – 3 bonding e- groups, 1 lone pair 3. Bent – 2 bonding e- groups, 2 lone pairs 4. Trigonal Planar – 3 bonding e- groups 5. Linear – 1 or 2 bonding e- groups •Compounds with only 2 or 3 valence e- Can break the octet rule BeH2 H Be BF3 •• •• ••F B F •• •• •• •• F •• •• H Linear Trigonal Planar Exceptions: 1. Hydrogen = 2 valence e2. Beryllium = 4 valence •Most compounds with double or triple bonds are linear C2H4 C2H2 H H H C H C C H C H Bond Length •Bond length increases with atom size •Multiple bonds (double and triple) are shorter than single bonds Bond Angles Bond angles are determined by the number of non-bonding pairs of electrons which push the bonding pairs closer together Shape Linear Bent Trigonal planar Trigonal pyramidal Tetrahedral Angle of Bond 180° <109.5° 120° <109.5° 109° Intermolecular Forces From strongest to weakest: Intramolecular Forces •Covalent Bond •Ionic Bond •Metallic Bond •Hydrogen Bonding •Dipole-Dipole Forces •Dispersion Forces metallic bonding Intermolecular Forces Metallic Bond – positive nuclei in a SEA of electrons Dispersion Forces •Found between nonpolar covalent molecules •They occur when the electrons around a molecule become unevenly distributed, causing a slight dipole (positive end & negative end) Dipole-Dipole •Polar covalent molecules have permanent dipoles. •The attractions between the partial charges on the ends of the polar molecules are called dipole-dipole forces. Partial charge symbol (Lowercase sigma) Polarity (add these to your VSEPR shapes notes) 1. Tetrahedral – nonpolar 2. Pyramidal – polar 3. Bent – polar 4. Trigonal Planar – nonpolar 5. Linear – nonpolar Hydrogen Bonding •Special type of dipole-dipole forces. •It only occurs between molecules with H bonded to N, O, or F. •Hydrogen bonding is stronger than normal dipole-dipole forces.