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Chapter 9 Covalent Bonding: Orbitals the Localized Electron Model Valence Bond Theory Hybridization

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Chapter 9 Covalent Bonding: Orbitals the Localized Electron Model Valence Bond Theory Hybridization The Localized Electron Model Chapter 9 Covalent Bonding: Draw the Lewis structure(s) Orbitals Determine the arrangement of electron pairs (VSEPR model). Specify the necessary hybrid orbitals. Hybridization Valence Bond Theory • Valence bond theory or hybrid orbital theory is an approximate theory to explain the covalent bond from a quantum mechanical • The mixing of atomic orbitals to form special orbitals for bonding. view. • According to this theory, a bond forms between • The atoms are responding as needed to two atoms when the following conditions are met. (see Figures 10.21 and 10.22) give the minimum energy for the molecule. 1. Two atomic orbitals “overlap” 2. The total number of electrons in both orbitals is no more than two. 1 Figure 10.20: Formation of H2. Figure 10.21: Bonding in HCl. Bond formed between two s orbitals Bond formed between an s and p orbital Figure 9.1: (a) The Lewis structure of the methane molecule. (b) The Hybrid Orbitals tetrahedral molecular geometry of • One might expect the number of bonds formed the methane molecule. by an atom would equal its unpaired electrons. • Chlorine, for example, generally forms one bond and has one unpaired electron. • Oxygen, with two unpaired electrons, usually forms two bonds. • However, carbon, with only two unpaired electrons, generally forms four bonds. For example, methane, CH4, is well known. 2 Hybrid Orbitals • The bonding in carbon might be explained as 2p 2p follows: • Four unpaired electrons are formed as an electron from the 2s orbital is promoted 2s 2s (excited) to the vacant 2p orbital. • The following slide illustrates this excitation. (Recall that in the excited state for an element, a ground state electron is promoted to a higher Energy 1s 1s orbital) • More than enough energy is supplied for this C atom (ground state) C atom (promoted) promotion from the formation of two additional covalent bonds. Hybrid Orbitals Hybrid Orbitals • One bond on carbon would form using the 2s • Hybrid orbitals are orbitals used to describe orbital while the other three bonds would use bonding that are obtained by taking the 2p orbitals. combinations of atomic orbitals of an isolated atom. • This does not explain the fact that the four • In this case, a set of hybrids are constructed bonds in CH4 appear to be identical. from one “s” orbital and three “p” orbitals, so • Valence bond theory assumes that the four they are called sp3 hybrid orbitals. available atomic orbitals in carbon combine to 3 make four equivalent “hybrid” orbitals. • The four sp hybrid orbitals take the shape of a tetrahedron (see Figure 10.23). 3 Figure 9.2: The valence Figure 9.3: The "native" 2s and three 2p atomic orbitals characteristic of a free carbon atom are combined to form a new set of four sp 3 orbitals. The orbitals on a small lobes of the orbitals are usually omitted from diagrams for clarity. free carbon atom: 2s, 2px, 2py, and 2pz. Figure 9.4: Figure 9.6: The tetrahedral set of four sp3 orbitals of the carbon atom are used to share electron pairs with the four 1s Cross orbitals of the hydrogen atoms to form the four equivalent section of C—H bonds. This accounts for the known tetrahedral structure of the CH4 molecule. an sp3 orbital. 4 Figure 9.5: An energy-level Figure 9.7: The nitrogen atom in diagram showing the formation ammonia is sp3 hybridized. of four sp3 orbitals. You can represent the hybridization of carbon in CH4 as follows. A Problem to Consider • Describe the bonding in H2O according to 2p valence bond theory. Assume that the 3 3 sp sp molecular geometry is the same as given by C-H bonds the VSEPR model. 2s •From the Lewis formula for a molecule, determine its geometry about the central atom Energy using the VSEPR model. 1s 1s 1s C atom C atom C atom (ground state) (hybridized state) (in CH4) 5 A Problem to Consider A Problem to Consider • Describe the bonding in H2O according to • Describe the bonding in H2O according to valence bond theory. Assume that the valence bond theory. Assume that the molecular geometry is the same as given by molecular geometry is the same as given by the VSEPR model. the VSEPR model. • The Lewis formula for H2O is • From this geometry, determine the hybrid orbitals on this atom, assigning its valence electrons to these orbitals one at a time. A Problem to Consider A Problem to Consider • Describe the bonding in H2O according to • Describe the bonding in H2O according to valence bond theory. Assume that the valence bond theory. Assume that the molecular geometry is the same as given by molecular geometry is the same as given by the VSEPR model. the VSEPR model. • Note that there are four pairs of electrons about • Each O-H bond is formed by the overlap of a the oxygen atom. 1s orbital of a hydrogen atom with one of the • According to the VSEPR model, these are directed singly occupied sp3 hybrid orbitals of the tetrahedrally, and from the previous table you see oxygen atom. that you should use sp3 hybrid orbitals. 6 You can represent the bonding to the oxygen atom in H2O as follows: Figure 2p sp3 sp3 10.24: lone O-H Bonding 2s pairs bonds in H2O. Energy 1s 1s 1s O atom O atom O atom (ground state) (hybridized state) (in H2O) Figure 9.9: An orbital energy-level Figure 9.8: The hybridization of the s, px, and py atomic diagram for sp2 hybridization. Note orbitals results in the formation of three sp2 orbitals centered in the xy plane. The large lobes of the orbitals lie in the plane that one p orbital remains at angles of 120 degrees and point toward the corners of a unchanged. triangle. 7 Figure 9.21: A set of dsp3 hybrid orbitals on a Figure 9.14: When one s orbital and one phosphorus atom. Note that the set of five dsp3 p orbital are hybridized, a set of two sp orbitals has a trigonal bipyramidal arrangement. 3 orbitals oriented at 180 degrees results. (Each dsp orbital also has a small lobe that is not shown in this diagram.) 2 3 Figure 9.22: (a) The structure of the PCI5 molecule. (b) The Figure 9.23: An octahedral set of d sp orbitals used to form the bonds in PCl5. The phosphorus uses orbitals on a sulfur atom. The small lobe a set of five dsp3 orbitals to share electron pairs with sp3 orbitals on the five chlorine atoms. The other sp3 orbitals on of each hybrid orbital has been omitted each chlorine atom hold lone pairs. for clarity. 8 Figure 9.24: Hybrid Orbitals The relationship of the number • Note that there is a relationship between the of effective type of hybrid orbitals and the geometric pairs, their arrangement of those orbitals. spatial • Thus, if you know the geometric arrangement, arrangement, you know what hybrid orbitals to use in the and the hybrid bonding description. orbital set • Figure 9.24 summarizes the types of hybridization required. and their spatial arrangements. Hybrid Orbitals Hybrid Orbitals Hybrid Geometric Number of Example • To obtain the bonding description of any atom Orbitals Arrangements Orbitals in a molecule, you proceed as follows: sp Linear 2 Be in BeF 2 1. Write the Lewis electron-dot formula for the 2 sp Trigonal planar 3 B in BF3 molecule. 2. From the Lewis formula, use the VSEPR theory sp3 Tetrahedral 4 C in CH 4 to determine the arrangement of electron pairs 3 dsp Trigonal bipyramidal 5 P in PCl5 around the atom. 2 3 d sp Octahedral 6 S in SF6 9 Hybrid Orbitals Hybrid Orbitals • To obtain the bonding description of any atom • To obtain the bonding description of any atom in a molecule, you proceed as follows: in a molecule, you proceed as follows: 3. From the geometric arrangement of the electron 5. Form bonds to this atom by overlapping singly pairs, obtain the hybridization type. occupied orbitals of other atoms with the singly occupied hybrid orbitals of this atom. 4. Assign valence electrons to the hybrid orbitals of this atom one at a time, pairing only when necessary. A Problem to Consider A Problem to Consider • Describe the bonding in XeF4 using hybrid • Describe the bonding in XeF4 using hybrid orbitals. orbitals. • From this geometry, determine the hybrid • From the Lewis formula for a molecule, orbitals on this atom, assigning its valence determine its geometry about the central atom electrons to these orbitals one at a time. using the VSEPR model. 10 A Problem to Consider A Problem to Consider • Describe the bonding in XeF4 using hybrid • Describe the bonding in XeF4 using hybrid orbitals. orbitals. • The Lewis formula of XeF4 is • The xenon atom has four single bonds and two lone pairs. It will require six orbitals to describe the bonding. • This suggests that you use d2sp3 hybrid orbitals on xenon. A Problem to Consider 5d • Describe the bonding in XeF4 using hybrid orbitals. 5p • Each Xe-F bond is formed by the overlap of a xenon d2sp3 hybrid orbital with a singly occupied fluorine 2p orbital. • You can summarize this as follows: 5s Xe atom (ground state) 11 5d 5d d2sp3 d2sp3 lone pairs Xe-F bonds Xe atom (hybridized state) Xe atom (in XeF4) Multiple Bonding Multiple Bonding • According to valence bond theory, one hybrid • To describe the multiple bonding in ethene, orbital is needed for each bond (whether a we must first distinguish between two kinds of single or multiple) and for each lone pair.
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