Molecular Geometries And Bonding Theories PDF

Molecular Geometries and Bonding Theories Chemistry, The Central Science, 10th edition Theodore L. Brown, H. Eugene LeMay, Molecular Jr., and Bruce E. Bursten Geometries 1 and Bonding...

Molecular Geometries and Bonding Theories Chemistry, The Central Science, 10th edition Theodore L. Brown, H. Eugene LeMay, Molecular Jr., and Bruce E. Bursten Geometries 1 and Bonding  Outline Molecular Geometries and Bonding  Molecular Shapes Lewis structures help us understand the compositions of molecules and their covalent bonds. However, Lewis structures do not show one of the most important aspects of molecules—their overall shapes. The shape and size of molecules—sometimes referred to as molecular architecture—are defined by the angles and Molecular Geometries 3 distances between the nuclei of the component atoms. and Bonding  Molecular Shapes The shape of a molecule plays an important role in its reactivity. By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of Molecular the molecule. Geometries 4 and Bonding  What Determines the Shape of a Molecule? Simply put, electron pairs, whether they be bonding or nonbonding, repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the Molecular molecule. Geometries 5 and Bonding  Electron Domains We can refer to the electron pairs as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count This molecule has four as one electron domain. electron domains. Molecular Geometries 6 and Bonding  Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.” Molecular Geometries 7 and Bonding  Electron-Domain Geometries BeCl2, HgCl2 BF3 These are the electron- domain geometries for two through six electron CH4, NH4+ domains around a central atom. PCl5 Molecular Geometries SF6 8 and Bonding  Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to that number of electron domains. Molecular Geometries 9 and Bonding  Molecular Geometries The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding Molecular Geometries pairs. and Bonding 10  Molecular Geometries Within each electron domain, then, there might be more than one molecular geometry. Molecular Geometries 11 and Bonding  Linear Electron Domain In this domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron Molecular domain is. Geometries H–F 12 and Bonding  Trigonal Planar Electron Domain There are two molecular geometries: Trigonal planar, if all the electron domains are bonding Bent, if one of the domains is a nonbonding pair. Molecular Geometries 13 and Bonding  Tetrahedral Electron Domain There are three molecular geometries: Tetrahedral, if all are bonding pairs Molecular Trigonal pyramidal if one is a nonbonding pair Geometries 14 and Bonding Bent if there are two nonbonding pairs  Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: Axial Equatorial Molecular Geometries 15 and Bonding  Trigonal Bipyramidal Electron There are four distinctDomain molecular geometries in this domain: Trigonal bipyramidal, if all are bonding pairs Seesaw (distorded tetrahedron) if one is a nonbonding pair T-shaped if there are two nonbonding pairs Linear if there are Molecular three Geometries 16 nonbonding pairs and Bonding  Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. Molecular Geometries 17 and Bonding  Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: Octahedral if all are bonding pairs Square pyramidal if one is a nonbonding pair Square planar there are Molecular Geometries two nonbonding pairs 18 and Bonding  Molecular Geometries 19 and Bonding  SAMPLE EXERCISE Using the VSEPR Model Use the VSEPR model to predict the molecular geometry of O3 and SnCl3–. (a) The Lewis structure for the O3 ion is Resonance structures Arrangement of electron pair: trigonal planar Molecular geometry: Bent Molecular Geometries 20 and Bonding  SAMPLE EXERCISE Using the VSEPR Model Use the VSEPR model to predict the molecular geometry of O3 and SnCl3–. (b) The Lewis structure for the SnCl3– ion is The central Sn atom is bonded to the three Cl atoms and has one nonbonding pair. Therefore, the Sn atom has four electron domains around it. The resulting electron-domain geometry is tetrahedral with one of the corners occupied by a nonbonding pair of electrons. The molecular geometry is thus trigonal pyramidal, like that of NH3. Molecular Geometries 21 and Bonding  Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule. Molecular Geometries 22 and Bonding  Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds. Therefore, they also affect bond angles. Molecular Geometries 23 and Bonding  Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole. Molecular Geometries 24 and Bonding  Larger Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule. Molecular Geometries 25 and Bonding  EX: Molecular Geometries 26 and Bonding  Molecular Geometries 27 and Bonding  Polarity In Chemical bonding chapter, bond dipoles was discussed. But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. Molecular Geometries 28 and Bonding  Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. Molecular Geometries 29 and Bonding  Polarity Molecular Geometries 30 and Bonding  SAMPLE EXERCISE Polarity of Molecules Predict whether the following molecules are polar or nonpolar: (a) BrCl (Bromine monochloride), (b) SO2 (Sulfur dioxide), (c) SF6 (Sulfur hexafluoride) Solution Analyze: We are given the molecular formulas of several substances and asked to predict whether the molecules are polar. Solve: (a) Chlorine is more electronegative than bromine. All diatomic molecules with polar bonds are polar molecules. Consequently, BrCl will be polar, with chlorine carrying the partial negative charge: The actual dipole moment of BrCl, as determined by experimental measurement, is µ = 0.57 D. (b) Because oxygen is more electronegative than sulfur, SO2 has polar bonds. Three resonance forms can be written for SO2: Molecular Geometries 31 and Bonding  SAMPLE EXERCISE continued For each of these, the VSEPR model predicts a bent geometry. Because the molecule is bent, the bond dipoles do not cancel and the molecule is polar: Experimentally, the dipole moment of SO2 is µ = 1.63 D. (c) Fluorine is more electronegative than sulfur, so the bond dipoles point toward fluorine. The six S—F bonds are arranged octahedrally around the central sulfur: Because the octahedral geometry is symmetrical, the bond dipoles cancel, and the molecule is Molecular nonpolar, meaning that µ = 0. Geometries 32 and Bonding  Overlap and Bonding We think of covalent bonds forming through the sharing of electrons by adjacent atoms. This can only occur when orbitals on the two atoms overlap. Molecular Geometries 33 and Bonding  Overlap and Bonding Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion. However, if atoms get too close, the internuclear repulsion greatly raises the energy. Valence bond (VB) theory assumes that the electrons in molecule occupy atomic orbitals of the individual atoms. In VB theory, bonds are considered to form from the overlapping of two atomic orbitals on different atoms, each orbital containing a single electron. Molecular Geometries 34 and Bonding  Valence Bond Theory Although Lewis and VSEPR structures contain localized electron-pair bonds, neither description uses an atomic orbital approach to predict the stability of the bond. Doing so forms the basis for a description of chemical bonding known as valence bond theory, which is built on two assumptions: * The strength of a covalent bond is proportional to the amount of overlap between atomic orbitals; that is, the greater the overlap, the more stable the bond. * An atom can use different combinations of atomic orbitals to maximize the overlap of orbitals used by bonded atoms. Molecular Geometries 35 and Bonding  Valence Bond Theory Figure : Three Different Ways to Form an Electron-Pair Bond. An electron-pair bond can be formed by the overlap of any of the following combinations of two singly occupied atomic orbitals: two ns atomic orbitals (a), an ns and an np atomic orbital (b), and two np atomic orbitals (c) where n = 2. The positive lobe is indicated in yellow, and the negative lobe is in blue. Molecular Geometries 36 and Bonding  Hybrid Orbitals VB theory uses the concept of hybridization. Hybridization is the mixing of atomic orbitals in an atom (usually a central atom) to generate a set of new atomic orbitals, called hybrid orbitals. Hybrid orbitals which are atomic orbitals obtained when two or more nonequivalent orbitals of the same atom combine, are used to form covalent bond. Molecular Geometries 37 and Bonding  Hybrid Orbitals Consider beryllium: In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals. Molecular Geometries 38 and Bonding  Hybrid Orbitals But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds. Molecular Geometries 39 and Bonding  Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. These sp hybrid orbitals have two lobes like a p orbital. One of the lobes is larger and more rounded as is the s orbital. Molecular Geometries 40 and Bonding  Hybrid Orbitals These two degenerate orbitals would align themselves 180 from each other. This is consistent with the observed geometry of beryllium compounds: linear. Molecular Geometries 41 and Bonding  Hybrid Orbitals With hybrid orbitals the orbital diagram for beryllium would look like this. The sp orbitals are higher in energy than the 1s Molecular Geometries orbital but lower than the 2p. and Bonding 42  Hybrid Orbitals Using a similar model for boron leads to… Molecular Geometries 43 and Bonding  Hybrid Orbitals …three degenerate sp2 orbitals. Molecular Geometries 44 and Bonding  Hybrid Orbitals With carbon we get… Molecular Geometries 45 and Bonding  Hybrid Orbitals …four degenerate sp3 orbitals. Molecular Geometries 46 and Bonding  Hybrid Orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids. Molecular Geometries 47 and Bonding  Hybrid Orbitals This leads to five degenerate sp3d orbitals… …or six degenerate sp3d2 orbitals. Molecular Geometries 48 and Bonding Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom. Molecular Geometries 49 and Bonding  Hybridization in molecules containing multiple bonds Hybridization is a major player in this approach to bonding. There are two ways orbitals can overlap to form bonds between atoms. Molecular Geometries 50 and Bonding  Sigma () Bonds Sigma bonds are characterized by Head-to-head overlap. Cylindrical symmetry of electron density about the Molecular Geometries internuclear axis. 51 and Bonding  Pi () Bonds Pi bonds are characterized by Side-to-side overlap. Electron density above and below the internuclear axis. Molecular Geometries 52 and Bonding  Single Bonds Single bonds are always  bonds. Molecular Geometries 53 and Bonding  Multiple Bonds In a multiple bond one of the bonds is a  bond and the rest are  bonds. Molecular Geometries 54 and Bonding  Multiple Bonds In a molecule like formaldehyde (shown at left) an sp2 orbital on carbon overlaps in  fashion with the corresponding orbital on the oxygen. The unhybridized p orbitals overlap in  fashion. Molecular Geometries 55 and Bonding  Multiple Bonds In triple bonds, as in acetylene, two sp orbitals form a  bond between the carbons, and two pairs of p orbitals overlap in  fashion to form the two  bonds. Molecular Geometries 56 and Bonding  Multiple bonds each carbon forms a set of ethylene (C2H4) three σ bonds: two C–H (sp2 + s) and one C–C (sp2 + sp2) Molecular Geometries 57 and Bonding Delocalized Electrons: Resonance When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion. Molecular Geometries 58 and Bonding Delocalized Electrons: Resonance In reality, each of the four atoms in the nitrate ion has a p orbital. The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen. Molecular Geometries 59 and Bonding Delocalized Electrons: Resonance This means the  electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion. Molecular Geometries 60 and Bonding  Resonance The organic molecule benzene has six  bonds and a p orbital on each carbon atom. Molecular Geometries 61 and Bonding  Resonance In reality the  electrons in benzene are not localized, but delocalized. The even distribution of the  electrons in benzene makes the molecule unusually stable. Molecular Geometries 62 and Bonding  Molecular Orbital (MO) Theory Though valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals. Molecular Geometries 63 and Bonding  Molecular Orbital (MO) Theory MO theory describes covalent bonds in terms of molecular orbitals which results from interaction of the atomic orbitals of the bonding atoms and are associated with the entire molecule. Molecular Geometries 64 and Bonding  Molecular Orbital (MO) Theory Whenever two atomic orbials overlaps, two molecular orbitals form. 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 Molecularorbitals from which it was formed. Geometries 65 and Bonding  MO Theory In H2 , the two electrons go into the bonding molecular orbital. The bond order is one half the difference between the number of bonding and antibonding electrons. It gives us measure of the strength of the bond. Molecular Geometries 66 and Bonding  MO Theory For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is Figure. Energy levels of bonding and antibonding MO in H2. 1 (2 - 0) = 1 2 Molecular Geometries 67 and Bonding  MO Theory In the case of He2, the bond order would be 1 (2 - 2) = 0 2 Therefore, He2 does not exist. Molecular Geometries 68 and Bonding  MO Theory Two possible interaction between two equivalent p orbitals and the corresponding molecular orbitals. Molecular Geometries 69 and Bonding  MO Theory The resulting MO diagram looks like this. There are both  and  bonding molecular orbitals and * and * antibonding molecular orbitals. Molecular Geometries 70 and Bonding  MO Theory The smaller p-block elements in the second period have a sizeable interaction between the s and p orbitals. This flips the order of the s and p molecular orbitals in these elements. Molecular Geometries 71 and Bonding  Second-Row MO Diagrams Molecular Geometries 72 and Bonding  How many valence-shell electrons are there in NO? FigureThe energy-level diagram for atomic Molecular and molecular orbitals in NO. Geometries 73 and Bonding  Example: Molecular Geometries 74 and Bonding  Exercise: Write the groud-state electron configuration for N2+, find bond order, magnetic characterand bond length relative to the bond length of N2 ( is it longer or shorter?). (σ1s)2(σ*1s)2(σ2s)2(σ*2s)2(π2p)4(σ2p)1 Bond order: ½ (9-4) = 2.5 Paramagnetic Bond order of N2 = ½ (10-4) = 3 Bond length of N2+ is 112 pm, 110 pm for N2 Molecular Geometries 75 and Bonding