Unformatted text preview: Meeting 14 ‐ Chapter 10 Bonding Theory and Molecular Structure We want to answer such questions as: • Why CO2 is linear, while H2O is bent? • Why are CCl3 molecules polar while CCl4 molecules are non‐polar? • Why is there is only one Butane isomer and two isomers of 2‐butene? • To understand the structure and properties of molecules we will need two theories of bonding. These are Valence‐Bond (VB) theory (VB) and Molecular Orbital (MO) theory. • We begin by examining the structure of molecules using VSEPR, valence‐shell electron‐ pair repulsion to determine the shapes of molecules. 1 Shapes of molecules
Lewis structures only indicate the connectivity of bonded atoms and account for all the valence electrons. Lewis structures tell us nothing about the shape of the molecule VSEPR will give us information regarding molecular shape.
VSEPR assumes that the main determinant of molecular shape is the repulsive force among negatively charged valence shell electrons. It also assumes that valence‐shell electron pairs move as far apart as possible. A third assumption is that lone pairs occupy more space than bonded pairs. 2 Examples of VSEPR: some common shapes of molecules (a nifty use of balloons) : Also Linear as with CO2 which has 2 electron groups about the central C atom. Q: what are the bond angles for the ideal molecular shapes above? 3 Bond angles of the basic shapes are Linear ‐ 180° Trigonal planar ‐ 120° Tetrahedral ‐ 109.5° Trigonal bipyramidal ‐ 90°, 120° Octahedral ‐ 90° Note: other factors, such as the existence of lone pairs, will lead to variation from these ideal angles. Electron Groups: any vE’s localized in a region around a central atom, including • An unpaired e–, a lone pair of e–s, and 1, 2, or 3 bonding pairs of e–s • Ex: acetylene: the ,o;ecule. 3 e– groups in 4 VSEPR Notation AXnEm A = central atom; X = # atoms bonded to central atom; E = # lone pairs of unpaired electron groups about the central atom • Example; water is an Ax2E2 molecule. Molecular geometry vs. e– pair geometry • Molecular geometry = geometry of the nuclei – • e– pair geometry = geometry of the e groups surrounding the central atom. 5 Examples: what are the molecular and electron pair geometries of CO2 and H2O? Draw Lewis structures, look at e– group and nuclei positions. AX2 Bonding pairs and nuclei are linear Water 6 Water: ideal electron groups are tetrahedral, nuclei are bent (angular). The molecule has a bent (angular) geometry. Get to know table 10.1 below: VESPR Examples 7 8 Note: SO2 example above: expect resonance structures, trigonal‐planar electron groups, bent geometry, with O‐S‐O angle < 1200. Observed value =1190. Class: determine geometry of BrF5 Predicting shapes 9 1. Determine the central atom and draw a Lewis structure. 2. Count electron groups to determine the group geometry 3. Examine the nuclei andcount the lone pairs to determine the molecular geometry and alter the bond angles. Trigonal‐bipyramidal electron groups • In the case of trigonal bipyramidal electron groups, there are 2 kinds of positions • Axial are like poles • equatorial around the middle like a belt • Lone pairs want to be far away from other electron pairs, so they tend to occupy equatorial positions. Example: Apply the VSEPR method to determine the geometry of XeF2. (22 Ve’s) Lewis structure: 10 F follows octet rule, Xe has expanded valence shell. Electron group geometry is trigonal bipyramidal . Three possibilities for the structure: 11 • F atoms ea, ee, or aa: which is correct? • The third structure (AA for F) places the lone pairs the farthest apart and is the correct structure. More than one “central” atom? Draw Lewis structure and apply VSEPR to each “central” atom. Example: nitric acid, HNO3 N is less electronegative than O, so it is the central atom. We expect the H atom to be bonded to O, as with HCLO4, etc., so we obtain the following skeleton structure (1) and by completing the octet (duet), the Lewis structure (2): Notice that N has a formal charge of +1 and one O has a formal charge of ‐1. We could write resonance structures here. 12 The VSEPR notation for N is AX3 and the geometry is trigonal planar. (The VSEPR notation for O is AX2E2. The electron groups are tetrahedral and the structure is bent or angular. In this way, we could analyze and understand the structures of complex molecules. 13 ...
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