GEOMETRY AND POLARITY OF MOLECULES

GEOMETRY – arrangement in space of atoms in a molecule

VALENCE SHELL ELECTRON-PAIR REPULSION (VSEPR) model Accounts for the geometrical arrangement of shared and unshared pairs around a central atom in terms of repulsions between electron pairs.

Number of groups Molecular structure Bond angles 2 linear 180o 3 In determining the geometry of the molecule, bonds and unshared electron pairs are considered as groups. Single bonds, double bonds and triple bonds are basically treated as if they are all the same. Number of groups Molecular structure Bond angles 2 linear 180o 3 Trigonal planar 120o 4 tetrahedral 109.3o 5 Trigonal bipyramid 90o, 120o, 180o 6 octahedral 90o

H2O Shape = bent or v-shaped Bond angle = 104.5 o

NH3 Shape = trigonal pyramid Bond angle = 107o

CCl4 Shape = tetrahedral Bond angle = 109.3o

SF6 Shape = octahedral Bond angle = 90o

# regions of high electron density molecule Lewis structure # regions of high electron density BeCl2                                 2 BF3 3 HCN SO3                                  CO2                                NO2                               CH4 4 NH3             PCl5 5 SF6 6

BOND POLARITY Due to the differences in the electronegativity of the elements bonded covalently. the electrons are concentrated at the more electronegative end of the bond, thus giving it a partial negative charge (δ-) and the less electronegative element, a partial positive charge (δ+). Non polar since electronegativity is the same H H δ+ δ- Polar since Cl is more electronegative than H H Cl C H C-H bond is considered as non-polar

POLARITY OF THE MOLECULE - Considers the net dipole moment (net polarity) of all the bonds present in the molecule. NET DIPOLE MOMENT = 0 = = O C O δ- δ- δ+ δ- NET DIPOLE MOMENT = O H H δ+ δ+

H C H H H H H C Cl H CH4 CH3Cl NET DIPOLE MOMENT = 0 δ- δ+

Cl Cl C H H H Cl C Cl H CH2Cl2 CH2Cl2 δ- δ+ δ- δ+ NET DIPOLE MOMENT =