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 Tetrahedral and trigonal planar are non-polar. Pyramidal and bend are polar.

Shapes Tetrahedral – 4 bonding e- groups Pyramidal – 3 bonding e- groups, 1 lone pair Bent – 2 bonding e- groups, 2 lone pairs Trigonal Planar – 3 bonding e- groups Linear – 1 or 2 bonding e- groups Tetrahedral and trigonal planar are non-polar. Pyramidal and bend are polar.

Compounds with only 2 or 3 valence e- Can break the octet rule BF3 BeH2 •• •• • F B F • •• •• H Be H • F • •• Linear Trigonal Planar Exceptions: Hydrogen = 2 valence e- Beryllium = 4 valence e- Boron = 6 valence e-

Most compounds with double or triple bonds are linear C2H4 C C H H C2H2 H C 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 Angle of Bond Linear 180° Bent <109.5° Trigonal planar 120° Trigonal pyramidal Tetrahedral 109°

Intermolecular Forces From strongest to weakest: Covalent Bond Ionic Bond Metallic Bond Hydrogen Bonding Dipole-Dipole Forces Dispersion Forces Intramolecular 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)

(add these to your VSEPR shapes notes) Polarity (add these to your VSEPR shapes notes) Tetrahedral – nonpolar Pyramidal – polar Bent – polar Trigonal Planar – nonpolar Linear – nonpolar Tetrahedral and trigonal planar are non-polar. Pyramidal and bend are polar.

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.