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Molecular Geometry and Bonding Theories Chapter 9 Part 1 November 11th, 2004 1 In this chapter • Molecular shapes • Valence Shell Electron Pair Repulsion model. • Molecular shapes and polarity. • Valence bond theory. • Hybridization. • Molecular orbitals, multiple bonds. 2 1 Molecular Shapes • Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which. • The shape of a molecule is determined by its bond angles. • Consider CCl4: experimentally we find all Cl-C-Cl bond angles are 109.5°. – Therefore, the molecule cannot be planar. – All Cl atoms are located at the vertices of a tetrahedron with the C at its center. 3 Molecular Shapes 4 2 Molecular Shapes • The Valence Shell Electron Pair Repulsion (VSEPR) theory proposed by Gillespie and Nyholm is a reliable method for predicting shapes of covalent molecules and polyatomic ions. • The VSEPR theory is based on the idea that bond and lone pair electrons in the valence shell of an element repel each other and move as far apart as possible. • VSEPR is extremely successful in predicting the structures of molecules and ions of the main group elements, but is generally useless for transition metal containing compounds. 5 There are five fundamental geometries for molecular shape: AB2 AB3 AB5 AB4 AB6 6 3 Predicting the Shape • Predict the shape of CH2Cl2 • CH2Cl2 has one central atom and 4 other atoms around it. Draw the Lewis structure. • Referring to the possible arrangements we can have for such a molecule the shape can be predicted. 7 Molecular Shapes • When considering the geometry about the central atom, we consider all electrons (lone pairs and bonding pairs). • When naming the molecular geometry, we focus only on the positions of the atoms. 8 4 Molecular Shapes • To determine the shape of a molecule, need to distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms). • The electron domain geometry is defined by the positions in 3D space of ALL electron pairs (bonding + non-bonding). This becomes especially important when the central atom has lone pairs. • The electrons adopt an arrangement in space to minimize e--e- repulsion. 9 10 5 11 Lone pairs on Central Atom • Consider NH3: The Lewis structure of NH3 is: • The molecular geometry of NH3 is described as trigonal pyramidal with the N atom at the apex and the 3 H atoms forming the base of the triangle. • The electron-pair geometry is described as tetrahedral. 12 6 13 Lone Pairs & Bond Angles • The electron pair geometry is decided only by looking at the positions of the electrons. • The molecular geometry is decided only by the positions of atoms, lone pairs are ignored in the molecular geometry. • Lone pairs are considered to be “fat”, while bonding pairs are considered “skinny”. • The increased volume of lone pairs makes bond pairs squeeze together, reducing bond angles. 14 7 Lone Pairs & Bond Angles 15 Lone Pairs & Bond Angles • Experimentally, the H-X-H bond angle decreases on moving from C to N to O: H H C H H 109.5O H N H H 107O O H H 104.5O • Since the electrons in a bonding pair are attracted by two nuclei they do not repel as much as lone pairs. • The bond angle decreases as the number of lone pairs on the central atom increases. • Typically the relative strengths of repulsion is: Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair 16 8 Multiple Bonds & Bond Angles • Similarly, electrons in multiple bonds repel more than electrons in single bonds. • A multiple bond is counted as one electron domain. • The Lewis structure of COCl2 (carbonyl chloride) indicates the molecular and the electron pair geometry to be trigonal-planar. Cl 111.4o Cl C O 124.3o 17 Molecules with Expanded Valence Shells • Atoms that have expanded octets have AB5 (trigonal bipyramidal) or AB6 (octahedral) electron pair geometries. • For trigonal bipyramidal structures there is a plane containing three electrons pairs. The fourth and fifth electron pairs are located above and below this plane. • For octahedral structures, there is a plane containing four electron pairs. Similarly, the fifth and sixth electron pairs are located above and below this plane. 18 9 19 20 10