Chapter 7 Notes Molecular Geometry and Bonding Theories lewis structures do not tell what the shapes of the molecules are shapes are important to know because some of the physical and chemical properties of molecules depend of their shapes molecular structure takes into account bond length distance between two atoms bond angles angle between bonds around an atom in the molecule experimentally these parameters can be determined by x ray diffraction method the simplest method to predict the molecular structure is to use the valance shell electron pair repulsion VESPR model In this method bonding is directional and the model predicts the shapes of molecules and ions in which the valance shell electron pairs are arranged about each atom so that electron pairs are kept as far away from one another as possible thus mini mizing electron pair repulsion Molecular Geometry takes into account the direction of the bonding electron pairs between the nuclei and the bond angles generated between the bonding electron pairs in the molecule Electron Domain Geometries takes into account all the valence electrons bonding and non bonding and arranges them around the central atom in such a geometry as to minimize the repul sions between these electrons 2 electron pairs linear 3 electron pairs trigonal planar 4 electron pairs tetrahedral 5 electron pairs trigonal bipyramidal 6 electron pairs octahedral 7 electron pairs pentagonal bipyramidal Rules double or triple bonds count as one valence shell electron pair bond angles that include multiple bonds are generally longer than predicted the non bonding electron pairs occupy more space than the bonding electron pairs the relative repulsion of electron pairs are as follows lone pair lone pair bonding pair repelling repelling repelling lone pair bonding pair bonding pair MEMORIZE THE PAGE IN THE BOOK I TOOK A PICTURE OF for molecules with more than one central atom treat each central atom individually and combine the overall description Molecular Geometry and Polarity consider H Cl electronegativity H 2 1 Cl 3 0 since Cl is more electronegative than H it will pull the electron cloud distort towards it self thus generating an uneven distribution of the electron cloud Cl will acquire a partial negative charge and H will acquire a partial positive charge gen erating a dipole therefore the bond between H and Cl is a polar covalent bond the extent of polarity or the magnitude of the dipole is given by the electric dipole mo ment mu which has the unit of debye mu magnitude of charge distance Molecular Polarity a molecule is polar if its net dipole moment is not zero a molecule is non polar if its net dipole moment is zero Bonds Molecule Non polar Non Polar Polar Polar Non Polar Valance Bond Theory is a theory stemming out from quantum mechanics to explain bonding It assumes the involvement of the valance orbitals in bonding How 1 An orbital of one atom comes to occupy a portion of the same region of space as an or bital on the other atom These two orbitals are said to overlap The total number of orbitals that overlap are no more than two 2 Hybridization is the mixing of atomic orbitals in an atom usually the central atom to generate a set of new orbitals called hybrid orbital These hybrid orbitals are used to form covalent bonds rather than the atomic orbitals Note 1 2 the hybridization concept is not applied to an isolated atom It is used to explain bonding Hybridization is the mixing of at least two non equivalent atomic orbitals ex s and p The resulting orbital s is no longer a pure atomic orbital Instead it is called a hybrid orbital 3 Hybrid orbitals have different shapes than pure atomic orbitals