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Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals Chapter 10 1 Molecular Geometry and Bond Theory • In this chapter we will discuss the geometries of molecules in terms of their electronic structure –. We will also explore two theories of chemical bonding: valence bond theory and molecular orbital theory. Molecular geometry is the general shape of a molecule, as determined by the relative positions of the atomic nuclei- three dimensional arrangement of atoms in a molecule The Valence-Shell Electron Pair Repulsion Model • The valence-shell electron pair repulsion (VSEPR) model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible. To predict the relative positions of atoms around a given atom using the VSEPR model, you first note the arrangement of the electron pairs around that central atom. Predicting Molecular Geometry • The following rules and figures will help discern electron pair arrangements. 1. Draw the Lewis structure 2. Determine how many electrons pairs are around the central atom. Count a multiple bond as one pair. 3. Arrange the electrons pairs. Arrangement of Electron Pairs About an Atom 2 pairs Linear 3 pairs Trigonal planar 5 pairs Trigonal bipyramidal 4 pairs Tetrahedral 6 pairs Octahedral Valence shell electron pair repulsion (VSEPR) model: Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs. Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 0 linear linear B B 0 lone pairs on central atom Cl Be Cl 2 atoms bonded to central atom VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 0 linear linear 0 trigonal planar trigonal planar AB3 3 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 0 linear linear trigonal planar tetrahedral AB3 3 0 trigonal planar AB4 4 0 tetrahedral VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 0 linear linear trigonal planar AB3 3 0 trigonal planar AB4 4 0 tetrahedral tetrahedral AB5 5 0 trigonal bipyramidal trigonal bipyramidal VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 0 linear linear trigonal planar AB3 3 0 trigonal planar AB4 4 0 tetrahedral tetrahedral AB5 5 0 trigonal bipyramidal trigonal bipyramidal AB6 6 0 octahedral octahedral a b c bonding-pair vs. bonding pair repulsion (a) ‹ lone-pair vs. bonding pair repulsion (b) lone-pair vs. lone pair ‹ Repulsion (c) VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB3 3 0 AB2E 2 1 Arrangement of electron pairs Molecular Geometry trigonal planar trigonal planar trigonal planar bent 18 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB4 4 0 AB3E 3 1 Arrangement of electron pairs Molecular Geometry tetrahedral tetrahedral tetrahedral trigonal pyramidal 19 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB4 4 0 Arrangement of electron pairs Molecular Geometry tetrahedral tetrahedral AB3E 3 1 tetrahedral trigonal pyramidal AB2E2 2 2 tetrahedral bent 20 VSEPR Class AB5 AB4E # of atoms bonded to central atom 5 4 # lone pairs on central atom Arrangement of electron pairs Molecular Geometry 0 trigonal bipyramidal trigonal bipyramidal 1 trigonal bipyramidal distorted tetrahedron 21 VSEPR Class AB5 # of atoms bonded to central atom 5 # lone pairs on central atom 0 AB4E 4 1 AB3E2 3 2 Arrangement of electron pairs Molecular Geometry trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal distorted tetrahedron T-shaped 22 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB5 5 0 AB4E 4 1 AB3E2 3 2 AB2E3 2 3 Arrangement of electron pairs Molecular Geometry trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal distorted tetrahedron trigonal bipyramidal linear T-shaped 23 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB6 6 0 octahedral octahedral AB5E 5 1 octahedral square pyramidal Arrangement of electron pairs Molecular Geometry 24 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB6 6 0 octahedral octahedral AB5E 5 1 octahedral AB4E2 4 2 octahedral square pyramidal square planar Arrangement of electron pairs Molecular Geometry 25 26 Dipole Moment and Molecular Geometry • The dipole moment is a measure of the degree of charge separation in a molecule. We can view the polarity of individual bonds within a molecule as vector quantities. Thus, molecules that are perfectly symmetric have a zero dipole moment. These molecules are considered nonpolar. d+ C Odd- O Dipole Moments and Polar Molecules electron poor region electron rich region H F d+ d- m=Qxr Q is the charge, only the magnitude, not to its sign. r is the distance between charges 1 D (D=debye unit) = 3.36 x 10-30 C m (C is coulomb, m is meter No electric field Electric field Dipole Moment and Molecular Geometry : Molecules that exhibit any asymmetry in the arrangement of electron pairs would have a nonzero dipole moment. These molecules are considered polar. dd- N H H H d+ d+ Bond moment (B.M) is the individual dipole moment (D.M) which is a vector quantity, has both magnitude and direction. D.M = sum of vector B.M The arrow shows the shift of electron density from less electronegative atom to the more electronegative atom Which of the following molecules have a dipole moment? H2O, CO2, SO2, and CH4 O S dipole moment polar molecule dipole moment polar molecule H O C O no dipole moment nonpolar molecule H C H H no dipole moment nonpolar molecule Does BF3 have a dipole moment? The trigonal planar shape means that the three bond moments exactly cancel one another, makes it a nonpolar molecule. Does CH2Cl2 have a dipole moment? 10.2 Because chlorine is more electronegative than carbon, which is more electronegative than hydrogen, the bond moments do not cancel and the molecule possesses a dipole moment: Thus, CH2Cl2 is a polar molecule. Predicting Molecular Geometry 1. Draw Lewis structure for molecule. 2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3. Use VSEPR to predict the geometry of the molecule. What are the molecular geometries of SO2 and SF4? O S AB2E bent F O F S F AB4E F distorted tetrahedron Predicting Molecular Geometry • Two electron pairs (linear arrangement). C : : : : O O You have two double bonds, or two electron groups about the carbon atom. Thus, according to the VSEPR model, the bonds are arranged linearly, and the molecular shape of carbon dioxide is linear. Bond angle is 180o. Predicting Molecular Geometry • Three electron pairs (trigonal planar arrangement). :O : C : : : : :Cl Cl: The three groups of electron pairs are arranged in a trigonal plane. Thus, the molecular shape of COCl2 is trigonal planar. Bond angle is 120o. Predicting Molecular Geometry : • Three electron pairs (trigonal planar arrangement). O : : : : O O: Ozone has three electron groups about the central oxygen. One group is a lone pair. These groups have a trigonal planar arrangement. Predicting Molecular Geometry : • Three electron pairs (trigonal planar arrangement). O : : : : O O: Since one of the groups is a lone pair, the molecular geometry is described as bent or angular. Predicting Molecular Geometry : • Three electron pairs (trigonal planar arrangement). O : : : : O O: Note that the electron pair arrangement includes the lone pairs, but the molecular geometry refers to the spatial arrangement of just the atoms. Predicting Molecular Geometry • Four electron pairs (tetrahedral arrangement). : H :Cl: C Cl: :N H :O : : :Cl: : : : : :Cl H H H Four electron pairs about the central atom lead to three different molecular geometries. Predicting Molecular Geometry • Four electron pairs (tetrahedral arrangement). : H C :N Cl: : :Cl: tetrahedral H H :O : : : : : :Cl :Cl: H H Predicting Molecular Geometry • Four electron pairs (tetrahedral arrangement). : : C N H Cl: H H : :Cl: H tetrahedral trigonal pyramid :O : : : : : :Cl :Cl: H Predicting Molecular Geometry • Four electron pairs (tetrahedral arrangement). : C N O : : : : : : :Cl :Cl: Cl: H H : H : :Cl: H tetrahedral trigonal pyramid bent H Predicting Molecular Geometry : • Five electron pairs (trigonal bipyramidal arrangement). : P : : : :F: :F F: : F: : :F: : This structure results in both 90o and 120o bond angles. Predicting Molecular Geometry • Other molecular geometries are possible when one or more of the electron pairs is a lone pair. SF4 ClF3 Let’s try their Lewis structures. XeF2 Predicting Molecular Geometry • Other molecular geometries are possible when one or more of the electron pairs is a lone pair. F F F S : F see-saw ClF3 XeF2 Predicting Molecular Geometry • Other molecular geometries are possible when one or more of the electron pairs is a lone pair. F F F F S : : : Cl F F F see-saw T-shape XeF2 Predicting Molecular Geometry • Other molecular geometries are possible when one or more of the electron pairs is a lone pair. F F F F : Xe : : F see-saw T-shape linear F F S : : : F Cl F Predicting Molecular Geometry • Six electron pairs (octahedral arrangement). :F: : : S : : : : :F : : : :F :F: F: F: : This octahedral arrangement results in 90o bond angles. Predicting Molecular Geometry • Six electron pairs (octahedral arrangement). IF5 XeF4 Six electron pairs also lead to other molecular geometries. Predicting Molecular Geometry • Six electron pairs (octahedral arrangement). F I F F : F F square pyramid XeF4 Predicting Molecular Geometry : • Six electron pairs (octahedral arrangement). I F F : F F square pyramid F F Xe F F : F square planar Valence Bond Theory • Atomic orbitals take part in bond formation by the overlap of atomic orbitals. • Eg. H2 and F2 molecules by the overlap of s orbitals of individual atoms of H and p orbitals of F atoms. Overlap of orbitals in bond formation How does Lewis theory explain the bonds in H2 and F2? Sharing of two electrons between the two atoms. Overlap Of Bond Dissociation Energy Bond Length H2 436.4 kJ/mole 74 pm 2 1s F2 150.6 kJ/mole 142 pm 2 2p Valence bond theory – bonds are formed by sharing of e- from overlapping atomic orbitals and accounts for difference in bond enthalpy, bond length etc. Change in Potential Energy of Two Hydrogen Atoms as a Function of Their Distance of Separation Electrons repels each other, nucleus as well when too close. Most stable, minimum energy, maximum attraction of nucleus and electrons Change in electron density as two hydrogen atoms approach each other. Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals 1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals. 2. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process. 3. Covalent bonds are formed by: a. Overlap of hybrid orbitals with atomic orbitals b. Overlap of hybrid orbitals with other hybrid orbitals 61 Hybrid Orbitals • Hybrid orbitals are orbitals used to describe bonding that are obtained by taking combinations of atomic orbitals of an isolated atom. In this case, a set of hybrids are constructed from one “s” orbital and three “p” orbitals, so they are called sp3 hybrid orbitals. The four sp3 hybrid orbitals take the shape of a tetrahedron. sp3 Hybridization Ground state Carbon atom ↑↓ ↑ 2s ↑ 2p Excited state Carbon atom ↑ 2s ↑ ↑ 2p ↑ Formation of sp3 Hybrid Orbitals Methane, CH4 Formation of sp Hybrid Orbitals Formation of sp2 Hybrid Orbitals sp2 Hybridization of Carbon 69 Unhybridized 2pz orbital (gray), which is perpendicular to the plane of the hybrid (green) orbitals. 70 Bonding in Ethylene, C2H4 Sigma bond (s) – electron density between the 2 atoms Pi bond (p) – electron density above and below plane of nuclei of the bonding atoms 71 Sigma (s) and Pi Bonds (p) Single bond 1 sigma bond Double bond 1 sigma bond and 1 pi bond Triple bond 1 sigma bond and 2 pi bonds 72 Another View of p Bonding in Ethylene, C2H4 73 sp Hybridization of Carbon 74 Experiments show O2 is paramagnetic O O No unpaired eShould be diamagnetic Molecular orbital theory – bonds are formed from interaction of atomic orbitals to form molecular orbitals. 75 Energy levels of bonding and antibonding molecular orbitals in hydrogen (H2). A bonding molecular orbital has lower energy and greater stability than the atomic orbitals from which it was formed. An antibonding molecular orbital has higher energy and lower stability than the atomic orbitals from which it was formed. 76 Two Possible Interactions Between Two Equivalent p Orbitals 77 Molecular Orbital (MO) Configurations 1. The number of molecular orbitals (MOs) formed is always equal to the number of atomic orbitals combined. 2. The more stable the bonding MO, the less stable the corresponding antibonding MO. 3. The filling of MOs proceeds from low to high energies. 4. Each MO can accommodate up to two electrons. 5. Use Hund’s rule when adding electrons to MOs of the same energy. 6. The number of electrons in the MOs is equal to the sum of all the electrons on the bonding atoms. 78 1 bond order = 2 bond order ½ ( 1 Number of electrons in bonding MOs - Number of electrons in antibonding MOs ½ 0 ) 79 General molecular orbital energy level diagram for the second-period homonuclear diatomic molecules Li2, Be2, B2, C2, and N2. 80 81 Delocalized molecular orbitals are not confined between two adjacent bonding atoms, but actually extend over three or more atoms. Example: Benzene, C6H6 Delocalized p orbitals 82 Homework-Ch-10 10.2,10.8,10.9,10.20,10.24,10.30, 10.32,10.42,10.46,10.49,10.53,10.56,10.58