Chapter 9b Molecular Geometry and Bonding Theories

Chapter 9b Molecular Geometry and Bonding Theories - 9.4...

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Unformatted text preview: 9.4 Covalent Bonding and Orbital Overlap q How can we explain bonding and account for the geometries of molecules in terms of atomic orbitals? Formation of a Covalent Sigma ( ) Bond q A covalent bond is formed by the overlap of two half-filled valence shell orbitals. Hybrid Orbitals q There are some problems for polyatomic molecules: q Must unpair electrons in some cases to form all the bonds. q Equivalent bonds are predicted to be different because electrons are in different types of orbitals. q Measured bond angles are different from predicted bond angles. Some incorrect predictions q Wrong bond angles predicted for water and ammonia Hybridization q The presence of 2 nuclei changes the behavior of electrons in atoms, so the orbitals in molecules should be expected to be different from those in atoms. To predict the orbitals in molecules: q We make orbitals equivalent in energy and position by mixing the orbitals to make new orbitals, called hybrid orbitals. Hybridizaton q Hybridization q The number of hybrid orbitals is equal to the number of atomic orbitals that it is made from. qs qs qs + + + p two sp hybrid orbitals p + p three sp2 hybrid orbitals p + p + p four sp3 hybrid orbitals sp Hybridization BeCl2 qWe can obtain new orbitals by allowing the 2s and one 2p orbital to mix or form hybrid orbitals (process called hybridization). qThe hybrid orbitals come from an s and a p orbital and are called sp hybrid orbitals. sp2 Hybridization BCl3 qsp2 hybrid orbitals are formed with one s and two p orbitals (leaving one unhybridized p orbital). qThe large lobes of sp2 hybrids lie in a trigonal plane. qAll molecules with trigonal planar electron domain geometries have sp2 orbitals on the central atom. sp Hybridization 2 Hybridization for Boron qs + p + p three sp2 orbitals, which can overlap with a p orbital on each Cl atom 1 sp Hybridization 3 qsp3 hybrid orbitals are formed from one s and three p orbitals. Therefore, there are four large lobes. qEach lobe points towards the vertex of a tetrahedron. qThe angle between the large lobes is 109.5 qAll molecules with tetrahedral electron domain geometries are sp3 hybridized. 1 sp3 Hybridization CH4 1 Hybridization Involving d Orbitals qSince there are only three p-orbitals, trigonal bipyramidal and octahedral electron domain geometries must involve d-orbitals. qTrigonal bipyramidal electron domain geometries require sp3d hybridization. qOctahedral electron domain geometries require sp3d2 hybridization. 1 Geometry and Hybrid Orbitals (Table 9.4) 1 Geometry and Hybrid Orbitals (Table 9.4) 1 Use hybrid orbitals for the sigma bonding and unshared electrons. q What orbitals are used in methane, water, and ammonia? q What bond angles? 1 Which of the following molecules has a C atom with sp2 hybridization? 25% 25% 25% 25% 1. CO2 2. C2H2 3. C2H4 4. C2H6 1 2 3 4 1 9.6 Multiple Bonds q In the covalent bonds considered so far, the electron density is concentrated symmetrically about the axis between the nuclei. q -Bonds: electron density lies on the axis between the nuclei. q All single bonds are sigma ( ) bonds. q Pi ( ) Bonds: electron density lies above and below the plane of the nuclei. 1 Formation of a covalent bond from p + p orbitals to give a pi ( ) bond q Sideways overlap of p orbitals gives a pi bond, with a high probability of finding the electrons above and below the bond axis. 1 Multiple Bonds q Often, the p-orbitals involved in -bonding come from unhybridized orbitals. q A double bond consists of one -bond and one -bond. q A triple bond has one -bond and two bonds. 2 What hybridization for a double bond? q Use hybrid orbitals only for the sigma bonds, use p orbitals for the pi bond. q sp2 + p Figure 9.24 2 Hybrid Orbitals in a Double Bond 2 What hybridization for a triple bond? q Use hybrid orbitals only for the sigma bond, use p orbitals for the pi bonds. q sp + p + p Figure 8.20 2 Hybrid Orbitals in a Triple Bond 2 Multiple Bonds q Use hybrid orbitals to form sigma bonds. q Use p orbitals to form pi bonds. q Describe the bonding in CO2. 2 How many sigma and pi bonds are in this molecule? O H - C - O -H 25% 25% 25% 25% 1. 2. 3. 4. 3 sigma, 1 pi 3 sigma, 2 pi 4 sigma, 1 pi 4 sigma, 2 pi 1 2 3 4 2 Delocalized Bonding qHow do we explain the bonding in molecules that have delocalized double bonds? qStructure from sigma framework 2 Delocalization q Pi bonds can be used to accommodate the double bonds, representing the two resonance forms 2 Delocalization q Pi bonds are delocalized because of continuous overlap of adjacent p orbitals. 2 Delocalization q Describe the bonding in NO3- 3 Delocalization in Benzene q Benzene, C6H6, has a cyclic structure. q What hybridization is used? 3 Delocalization in Benzene 3 Why does graphite conduct electricity? Figure 8.27 3 Electron Microscope Image of Graphite 3 ...
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This note was uploaded on 04/06/2011 for the course CHM 113 taught by Professor Anbar during the Spring '06 term at ASU.

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