Chem 112 Exam AID Course Pack

E sp2 combines the s orbital and two p orbitals

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Unformatted text preview: Order: LP- LP > LP- B> B- B General Rules for Electron Group Geometries: Electron Group Geometry ≠ Molecular Geometries i.e. OH2 and CH4 both have tetrahedral geometry but do not have the same molecular geometry Reason: Lone pairs. - Lone pairs decrease the angles between bonded atoms because of their stronger repulsive forces. Therefore OH2 has a much smaller bond length than CH4 In General: - - - - - Two electrons = linear Three electrons = trigonal planar Four electrons = tetrahedral Five electrons = trigonal bipyramidal Six electrons= octahedral In order to determine the molecular shape of a molecule 1. Draw the Lewis structure of the molecule, including all lone pairs and bonds 2. Position the molecule so that repulsion between groups is at a MINIMUM USE THE FOLLOWING CHART TO DETERMINE VSEPR SHAPES AND ARRANGEMENTS (yes you should memorize this & understand the corresponding theory) Polar Covalent Bonds Ionization Energy: Ability of an atom to give up an electron Electron Affinity: Ability of an atom to accept an electron Ethan Newton & Barry Zhang for SOS Winter 2012 11 Whichever atom has the highest electron affinity will be the atom which will pull the electron slightly towards it. This will result in this atom having a partial NEGATIVE charge (δ-) and as a result, the other atom in the molecule will have a partial POSITIVE charge (δ+). This can also be explained by electronegativities. In general, the most electronegative atom will bear the negative charge. Based on differences in electronegativities, it is possible to classify the type of bond which being formed. Large electronegativity differences relate to IONIC BONDS (>1.9). Medium electronegativity differences relate to POLAR COVALENT bonds (0.5- 1.9). Small electronegativity differences relate to NONPOLAR COVALENT bonds (<0.5). Note: Numbers are just a rough approximation. Predicting Dipoles in Molecules: *In general, symmetry in a molecule results in no Dipole. - It is always useful to start by drawing the Lewis structure of a molecule, and then assigning the appropriate molecular geometry. - By comparing the electronegativies of individual atoms, it is then possible to assign polarity to the molecule. (The arrow will point in the direction of increasing electronegativity) Be careful with molecular geometries like the BENT GEOMETRY. - > At first glance it may look like the O- H bond dipoles cancel out but since the geometry is not linear, the distance between bonds is less than 180˚ and therefore the molecule is not symmetrical and a dipole exists. Bond Order vs. Bond Length: Bond Order – The number...
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