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Molecular Geometry and Bonding Theories

Molecular Geometry and Bonding Theories. Brown, LeMay Ch 9 AP Chemistry Monta Vista High School. Ch 9 PS #8, 10, 16, 20 (and draw overall dipole moments), 30, 34, 36, 41, 59, 73; Recommended #9, 11, 13, 19, 25, 31, 40, 55, 60. Valence Shell Electron Pair Repulsion Theroy.

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Molecular Geometry and Bonding Theories

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  1. Molecular Geometry andBonding Theories Brown, LeMay Ch 9 AP Chemistry Monta Vista High School Ch 9 PS #8, 10, 16, 20 (and draw overall dipole moments), 30, 34, 36, 41, 59, 73; Recommended #9, 11, 13, 19, 25, 31, 40, 55, 60

  2. Valence Shell Electron Pair Repulsion Theroy • Lewis structure is a flat drawing showing the relative placement of atoms, bonds etc in a molecule, but does not tell anything about the shape of the molecule. • VSEPR theory helps construct molecular shape (3-D) from the Lewis structures, which are 2-D structures. • The basis principal of VSEPR is the each group of valence electrons (electron domain) around a central atom tend to be as far as possible from each other to minimize repulsions. • Electron domain may be a single bond, a double bond, , a triple bond or a lone pair. (= is treated same as a – in VSEPR). • Repulsions between two e domains are : lone pair-lonepair>lone pair-bond pair> bond pair-bond pair

  3. Valence-shell electron-pair repulsion theory • Because e- pairs repel, molecular shape adjusts so the valence e- pairs are as far apart as possible around the central atom. • Electron domains: areas of valence e- density around the central atom; result in different molecular shapes • Includes bonding e- pairs and nonbonding e- pairs • A single, double, or triple bond counts as one domain http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/carbshpe.htm#shp1http://www.grandinetti.org/Teaching/Chem121/Lectures/VSEPR http://www.slideshare.net/XuanTruc/ap-chemistry-vsepr Summary of LmABn (Tables 9.1 - 9.3): L = lone or non-bonding pairs A = central atom B = bonded atoms Bond angles notation used here: < xº means ~2-3º less than predicted << xº means ~4-6º less than predicted

  4. X B |X |B Tables http://intro.chem.okstate.edu/1314F97/Chapter9/VSEPR.html A A

  5. X B B X B : |X |B |B A A A

  6. 109.5º 109.5º 109.5º 109.5º Example: CH4 Molecular shape = tetrahedral Bond angle = 109.5º H | H—C—H | H

  7. B B X B X : : B X A A A A X B X B : B B B X X A X

  8. : : :Cl: :Cl: \ / :Cl—P—Cl: | :Cl: : : : : : 90º 90º 120º 90º 120º PCl5 Molecular shape = trigonal bipyramidal Bond angles equatorial = 120º axial = 90º

  9. X B X B A A X B | X | B : X B B - A - B B B

  10. X B B X : : A A A X B : | B | X | B X : :

  11. X X X B B X B B B X B X A A A A X |.. | B | X | X X B B X X B B

  12. B B B .. .. .. .. .. B .. B B A A A A B B |.. |.. | B | B B B B .. .. B

  13. : : H-Cl: H-Cl: : : 9.3: Molecular Polarity • A molecule is polar if its centers of (+) and (-) charge do not coincide. • A bond’s polarity is determined by the difference of EN between atoms in bond. • Partial (+) and partial (-) charges on atoms in a polar bond can be represented as d+ and d-. d+d- • Bond polarity is most often represented by an arrow that points toward the d- (most EN atom), showing the shift in e- density.

  14. The dipole moment (m) is a vector (i.e., has a specific direction) measuring the polarity of a bond which contains partial charges (Q) that are separated by a distance (r). The sum of the bond dipole moments in a molecule determines the overall polarity of the molecule. • Draw the true molecular geometry. • Draw each bond dipole as an arrow (not lone pairs) • Add the vectors, and draw the overall dipole moment. If none, then m = 0. m = Q r

  15. Ex: Draw molecular geometires, bond dipole moments, and overall dipole moments. Also, name the e- domain geometry and the molecular geometry. CO2 BF3 H2O CCl4 NH3 PH3

  16. Covalent Bonding and Orbital Overlap: Valence Bond Theory • The basic principle of VB theory is that a covalent bond forms when the orbitals of two atoms overlap.Three central themes of VB theory derive from this principle: • 1. Opposing spins of e pairs: In accordance with Pauli’s exclusion principle, an orbital can have max of two e with opposite spins. • 2. Maximum overlap of bonding orbitals: The bond strength depends upon the attraction of nuclei for the shared e, so the greater the overlap, the stronger the bond. • 3. End to end overlap of the atomic orbitals form a sigma bond and allows the free rotation of the parts of the molecule. Side-to-side overlap forms a pi bond, which restricts rotation. A multiple bond consists of one sigma bond and rest pi bonds.

  17. Sigma and Pi bonds Sigma (s) bond: • Covalent bond that results from axial overlap of orbitals between atoms in a molecule • Lie directly on internuclear axis • “Single” bonds, could form between s-s orbital or s-p orbital or p-p orbital by axial overlapping Ex: F2 Pi (p) bond: • Covalent bond that results from side-by-side overlap of orbitals between atoms in a molecule. • Are “above & below” and “left & right” of the internuclear axis and therefore have less total orbital overlap, so they are weaker than s bonds. Forms between two p orbitals (py or pz) • Make up the 2nd and 3rd bonds in double & triple bonds. Ex: O2 N2

  18. 9.4: Covalent Bonding and Orbital Overlap • Valence-bond theory: overlap of orbitals between atoms results in a shared valence e- pair (i.e., bonding pair) • As 2 H atoms approach, the 2 valence e- in the 1s orbitals begin to overlap, becoming more stable. • As H-H distance approaches 0.74 Å, energy lowers b/c of electrostatic attraction between the nuclei & the incoming e-. • When H-H distance = 0.74 Å, energy is at its lowest because electrostatic attractions & repulsions are balanced. (This is the actual H-H bond distance. • When H-H distance < 0.74 Å, energy increases b/c of electrostatic repulsion between 2 nuclei & between the 2 e-. Figure 9.13: Formation of bond in H2 Energy (kJ/mol) 0  -436 d a b c 0.74 Å H-H distance

  19. 9.5: Hybrid Orbital Theoryhttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/hybrv18.swf • Explains the molecular geometries in terms of s,p,d,f orbitals. • VSEPR explains that e domains must be farthest from each other around central atom, but fails to explain these in terms of orbitals as defined in wave mechanical model of atom. • Hybrid orbital theory of Linus Pauling proposed that the valence atomic orbitals in the molecule are very different from those in the isolated atoms. • The process of orbital mixing is called hybridization, and the new atomic orbitals are called hybrid orbitals. http://chemmovies.unl.edu/ChemAnime/SP11ORBD/SP11ORBD.htmlhttp://sponholtzproductions.com/Hybridization_Theory.html

  20. Hybrid Orbital Theory • Two key points about the number and types of hybrid orbitals are that • 1. The number of hybrid orbitals obtained equals the number of atomic orbitals mixed. • 2. The type of hybrid orbitals obtained varies with the types of atomic orbitals mixed.

  21. “sp” hybrid orbitals • BeF2 (g): observed as a linear molecule with 2 equal-length Be-F bonds. Valence bond theory predicts that each bond is an overlap of one Be 2s e- and one 2p e- of F. However, Be’s 2s e- are already paired. So… To form 2 equal bonds with 2 F atoms: • In Be, one 2s e- is promoted to an empty 2p orbital. • The occupied s and p orbitals are hybridized (“mixed”), producing two equivalent “sp” orbitals. • As the two “sp” hybrid orbitals of Be overlap with two p orbitals of F, stronger bonds result than would be expected from a normal Be s and F p overlap. (This makes up for energy needed to promote the Be e- originally.)

  22. F F Be (ground state) → Be (promoted) → Be (sp hybrid) 2p 2s Energy → • Orbital “shapes”One s + one p → Two sp orbitals (to bond with 2 F’s) • A central atom in a Lewis structure with exactly 2 e- domains has sp hybrid orbitals.

  23. F F F “sp2” hybrid orbitals • BF3 (g): observed as trigonal planar molecule with 3 equal-length B-F bonds. However, 2 valence e- in B are paired, and are the s and p e- not at the observed 120º angle. B (ground) → B (promoted) → B (sp2 hybrid) 2p 2s • One s + two p → Three sp2 orbitals(to bond with 3 F’s) • A central atom with exactly 3 e- domains has sp2 hybrid orbitals.

  24. H H H H “sp3” hybrid orbitals • CH4 (g): observed as tetrahedral C (ground) → C (promoted) → C (sp3 hybrid) 2p 2s • One s + three p → Four sp3 orbitals(to bond with 4 H’s) • A central atom with exactly 4 e- domains has sp3 hybrid orbitals.

  25. Cl Cl Cl Cl Cl “sp3d” hybrid orbitals (or dsp3) • PCl5 (g): observed as trigonal bipyramidal; forms 5 bonds of equal energy (* but not equal length: equatorial are slightly longer) P (ground) → P (promoted) → P (sp3d hybrid) 3d 3p 3s • One s + three p + one d → Five sp3d orbitals(to bond with 5 Cl’s) • A central atom with exactly 5 e- domains has sp3d hybrid orbitals.

  26. “sp3d2” hybrid orbitals (or d2sp3) • SF6 (g): observed as octahedral; forms 6 equal-length bonds • One s + three p + two d → Six sp3d2 orbitals • A central atom with exactly 6 e- domains has sp3d2 hybrids.

  27. 2 bonding pairs 2 non-bonding pairs(lone pairs) H H Non-bonding e- pairs • Lone pairs occupy hybrid orbitals, too Ex: H2O (g): observed as bent; but e- domain is tetrahedral O (ground) → O (sp3 hybrid) 2p 2s • Four sp3 orbitals (2 bonding, 2 non-bonding)

  28. 9.6: Multiple Bonds Draw Lewis structures. For C’s: label hybridization, molecular geometry, and unique bond angles C2H6 C2H4 C2H2 C6H6

  29. Sigma (s) bonds in C2H4 Ex: ethene; C-C s-bonds and C-H s-bonds result from axial overlap of H s-orbitals and C sp2-orbitals

  30. Pi (p) bonds in C2H4 • Each C has 4 valence e-: • 3 e- for 3 s-bonds • 1 e- for 1 p-bond, which results from side-by-side overlap of one non-hybridized p-orbital from each C p orbital bonds side-by-side = p bond 2p C 2s sp2 hybrids bond axially = s bonds

  31. Sigma (s) bonds in C2H2 Ex: ethyne (a.k.a. acetylene) C-C s-bond and C-H s-bonds result from axial overlap of H s-orbitals and C sp-orbital

  32. Pi (p) bonds in C2H2 p orbital bonds side-by-side = p bonds • Each C has 4 valence e-: • 2 e- for 2 s-bonds • 2 e- for 2 p-bonds, which result from side-by-side overlap of two non-hybridized p-orbitals from each carbon 2p C 2s sp hybrids bond axially = s bonds p

  33. Sigma (s) bonds in C6H6 Ex: benzene; C-C s-bonds and C-H s-bonds result from axial overlap of H s-orbitals and C sp2-orbitals http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/intro3.htm#strc8c

  34. Localized vs. Delocalized p Bonds (localized) (delocalized – MINIMUM OF 4 c’S)

  35. Delocalized p bonds in C6H6 • C-C p-bonds result from overlap of one non-hybridized p-orbitals from each C • Delocalization of e- in p-bonds results in a “double-donut” shaped e- cloud above and below the molecular carbon plane.

  36. 9.7: Molecular Orbital (MO) theory • So far we have used valence-bond theory (covalent bonds form from overlapping orbitals between atoms) with hybrid orbital theory and VSEPR theory to connect Lewis structures to observed molecular geometries. However, VB theory does not explain the magnetic or spectral properties of a molecule. • MO theory is similar to atomic orbital (AO) theory (s, p, d, f orbitals) and helps to further explain some observed phenomena, like unpredicted magnetic properties in molecules like those in O2. • AO are associated with the individual atoms, but MO are associated with the whole molecule.

  37. *AO & MO in H2 Atomic orbitals • Combination of two 1s AO from each H forms two MO in H2 molecule. • Bonding MO: form between nuclei and are stable • Antibonding MO: marked with *; form “behind” nuclei and are less stable. Anti-bonding orbital s*1s Molecular orbitals E 1s 1s Bonding orbital s1s

  38. *Types of MO • Sigma (s) MO: form from combinations of: • Two 1s or 2s orbitals from different atoms; written as s1s or s2s. • Two 2pz orbitals from different atoms (axial overlap); written as s2pz.(Some sources say 2 px orbitals?) • Pi (p) MO: form from combinations of: • Two 2px or 2py orbitals from different atoms; written as p2px or s2py. • Do not appear until B2 molecule

  39. MO s from Atomic p-Orbital Combinations • P orbitals can interact with each other forming either sigma molecular orbitals, s2p , in a end-to-end overlap or pi molecular obrbitals, p2p, in a side-to-side overlap. • The order of energy for MO s derived from 2p orbitals is s2p < p2p < p2p*< s2p* • There are three perpendicular p orbitals, so two sigma p orbitals (one bonding and one antibonding) and four pi p orbitals (two bonding and two antibonding) are formed. • This energy order gives the expected MO diagram for most of the p-block elements for homonuclear diatomic molecules.

  40. MO s for B, C and N • The energy order of p orbitals results from the assumption that since s and p orbitals have differences in energy, they do not interact with each other. (or mix) • However, when 2p atomic orbitals are half filled, such as in B, C and N, the repulsions between e are little and the energy of these p orbitals is not much different than the s atomic orbital, which leads to s and p orbital mixing. This mixing lowers the energy of the 2s bonding and antibonding orbitals and increases the energy of sigma 2p (bonding and antibonding) orbitals.The pi 2p orbitals are not affected. This mixing gives a different energy order: • s2s < s2s*<p2p<s2p < p2p*< s2p

  41. *MO diagrams for “< O2” • Resulting MO for diatomic molecules with < 16 e- (B2, C2, N2, etc.) • Bond order = ½ (# bonding e- - # antibonding e-) B.O. (N2) = ½ (10 – 4) =6 / 2 = 3 (triple bond) • N2 has no unpaired electrons which makes it diamagnetic. N atom N atom

  42. *MO diagrams for “≥ O2” • Resulting MO for diatomic molecules with ≥ 16 e- (like O2, F2, Ne2, etc.) • Bond order = ½ (# bonding e- - # antibonding e-) B.O. (O2) = ½ (10 – 6) == 2 (double bond) • O2 has unpaired electrons which makes it paramagnetic. O atom O atom

  43. Liquid N2 and liquid O2 • From U. Illinois: http://www.chem.uiuc.edu/clcwebsite/liquido2.html N2 O2

  44. Magnetism In an element or compound: • Diamagnetism: all e-paired; no magnetic properties • Paramagnetism: at least 1 unpaired e- • * Drawn into exterior magnetic field since spins of atoms become aligned; unlikely to retain alignment when field is removed Ex: N O Sc Mn O2 • * Ferromagnetism: occurs primarily in Fe, Co, Ni • Drawn into exterior magnetic field since spins of atoms become aligned; very likely to retain alignment when field is removed (i.e., “a permanent magnet”) • Nd2Fe14B is very ferromagnetic

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