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Molecular structure and bonding Chemistry 481(01) Spring 2017 Lewis structures Instructor: Dr. Upali Siriwardane e-mail: [email protected] Office: CTH 311 Phone 257-4941 Office Hours: 2.1 The octet rule 2.2 Structure and bond properties 2.3 The VSEPR model M,W 8:00-9:00 & 11:00-12:00 am; Tu,Th, F 9:30 - 11:30 a.m. Valence-bond theory 2.4 The hydrogen molecule 2.5 Homonuclear diatomic molecules 2.6 Polyatomic molecules April 4 , 2017: Test 1 (Chapters 1, 2, 3, 4) April 27, 2017: Test 2 (Chapters (6 & 7) May 16, 2016: Test 3 (Chapters. 19 & 20) May 17, Make Up: Comprehensive covering all Chapters Chemistry 481, Spring 2017, LA Tech Molecular orbital theory 2.7 An introduction to the theory 2.8 Homonuclear diatomic molecules 2.9 Heteronuclear diatomic 2.10 Bond properties Chapter-2-1 What changes take place during this process of achieving closed shells? Chemistry 481, Spring 2017, LA Tech Chapter-2-2 How do you get the Lewis Structure from Molecular formula? • Add all valence electrons and get valence a) sharing leads to covalent bonds and molecules Covalent Bond: each atom gives one electron Coordinative bond: two electron comes from one atom b) gain/loss of electrons lead to ionic bond electron pairs • Pick the central atom: Largest atom normally or atom forming most bonds • Connect central atom to terminal atoms Cations and anions: Electrostatic attractions c) Sharing with many atoms lead to • Fill octet to all atoms (duet to hydrogen) metallic bonds: delocalization of electrons Chemistry 481, Spring 2017, LA Tech Chapter-2-3 Chemistry 481, Spring 2017, LA Tech Chapter-2-4 What is VSEPR Theory 1. Draw Lewis structure for SbF5, ClF3, and IF6+: Chemistry 481, Spring 2017, LA Tech Chapter-2-5 Valence Shell Electron Pair Repulsion This theory assumes that the molecular structure is determined by the lone pair and bond pair electron repulsion around the central atom Chemistry 481, Spring 2017, LA Tech Chapter-2-6 1 What Geometry is Possible around Central Atom? • What is Electronic or Basic Structure? • Arrangement of electron pairs around the central atom is called the electronic or basic structure • What is Molecular Structure? • Arrangement of atoms around the central atom is called the molecular structure Possible Molecular Geometry • Linear (180) • Trigonal Planar (120) • T-shape (90, 180) • Tetrahedral (109) • Square palnar ( 90, 180) • Sea-saw (90, 120, 180) • Trigonal bipyramid (90, 120, 180) • Octahedral (90, 180) Chemistry 481, Spring 2017, LA Tech Chapter-2-7 2. Predict geometry of central atom using VSEPR and the hybridization in problem 1. SbF5, ClF3, and IF6+: Chemistry 481, Spring 2017, LA Tech Chapter-2-8 Formal Charges Formal charge = valence electrons - assigned electrons If there are two possible Lewis structures for a molecule, each has the same number of bonds, we can determine which is better by determining which has the least formal charge. It takes energy to get a separation of charge in the molecule •(as indicated by the formal charge) so the structure with the least formal charge should be lower in energy and thereby be the better Lewis structure Chemistry 481, Spring 2017, LA Tech Chapter-2-9 Formal Charge Calculation Chemistry 481, Spring 2017, LA Tech Chapter-2-10 Electron counts" and" formal charges in NH4+ and BF4- An arithmetic formula for calculating formal charge. Formal charge = group number in periodic table Chemistry 481, Spring 2017, LA Tech – number of bonds – number of unshared electrons Chapter-2-11 Chemistry 481, Spring 2017, LA Tech Chapter-2-12 2 Resonance structures of CO32- ion Resonance structures of SO2 They both are! O -S=O O O =S- O S O This results in an average of 1.5 bonds between each S and O. Ave. Bond order= total pairs shared/ # bonds= 3/2=1.5 Chemistry 481, Spring 2017, LA Tech Chapter-2-13 Resonance structures of C6H6 Chemistry 481, Spring 2017, LA Tech Chapter-2-14 Exceptions to the octet rule • Benzene, C6H6, is another example of a compound for which resonance structure must be written. • All of the bonds are the same length. Not all compounds obey the octet rule. • Three types of exceptions • Species with more than eight electrons around an atom. • Species with fewer than eight electrons around an atom. or • Species with an odd total number of electrons. Chemistry 481, Spring 2017, LA Tech Chapter-2-15 Chemistry 481, Spring 2017, LA Tech Chapter-2-16 Linear Combination of Atomic Orbitals Valence-bond (VB) theory VB theory combines the concepts of atomic orbitals, hybrid orbitals, VSEPR, resonance structures, Lewis structures and octet rule to describe the shapes and structures of some common molecules. It uses the overlap of atomic orbitals or hybrid orbitals of the Symmetry Adapted Linear Combination of Atomic Orbitals –LCAO Atomic orbitals on single atom: Hybridization Atomic orbitals in a molecule with more than one atom: Molecular Orbital (MO) formation General rule Number of Hybrid Orbital produced = # hybridized Number of MO produced = # orbitals combined to from sigma (s) , pi (p) bonds and (d) bonds Chemistry 481, Spring 2017, LA Tech Chapter-2-17 Chemistry 481, Spring 2017, LA Tech Chapter-2-18 3 How do you tell the hybridization of a central atom? What is hybridization? Mixing of atomic orbitals on the central atom •Get the Lewis structure of the molecule Bonding •Look at the number of electron pairs on the central atom. Note: double, triple bonds are counted as a hybrid orbital could over lap with another ()atomic orbital or () hybrid orbital of another atom to make a covalent single electron pairs. •Follow the following chart bond. possible hybridizations: sp, sp2, sp3, sp3d, sp3d2 Chemistry 481, Spring 2017, LA Tech Chapter-2-19 Chemistry 481, Spring 2017, LA Tech Chapter-2-20 What is hybridization? Kinds of hybrid orbitals Mixing of atomic orbitals on the central atoms Hybrid sp sp2 geometry # of orbital linear 2 trigonal planar 3 sp3 sp3d sp3d2 tetrahedral trigonal bipyramid octahedral valence shell (highest n orbitals) Bonding: s 4 5 6 Px p Py Pz d dx2- dz2 2 y sp, sp2, sp3, sp3d, sp3d2 Chemistry 481, Spring 2017, LA Tech Chapter-2-21 Chemistry 481, Spring 2017, LA Tech Chapter-2-22 Possible hybridizations of s and p Possible hybridizations of s and p sp-hybridization: sp-hybridization: 1 = 1/2s - 1/2p 2 = 1/2s + 1/2p sp2-hybridization: 1 = 1/3s + 1/6px + 1/2py 2 = 1/3s + 1/6px - 1/2py 3 = 1/3s - 2/6px sp3-hybridization: 1 = 1/4s + 1/4px + 1/4py + 1/4pz 2 = 1/4s - 1/4px - 1/4py + 1/4pz 3 = 1/4s + 1/4px - 1/4py - 1/4pz 4 = 1/4s - 1/4px + 1/4py -1/4pz Chemistry 481, Spring 2017, LA Tech Chapter-2-23 Chemistry 481, Spring 2017, LA Tech Chapter-2-24 4 What are p and s bonds s bonds Atoms with more than eight electrons single bond resulting from head to head overlap of atomic orbital • Except for species that contain hydrogen, this is the most common type of exception. • For elements in the third period and beyond, the p bond double and triple bond resulting from lateral or side way overlap of p atomic orbitals d orbitals can become involved in bonding. Examples • 5 electron pairs around P in PF5 • 5 electron pairs around S in SF4 d bond double and triple bond resulting from lateral or side way overlap of d atomic orbitals Chemistry 481, Spring 2017, LA Tech Chapter-2-25 • 6 electron pairs around S in SF6 Chemistry 481, Spring 2017, LA Tech Chapter-2-26 An example: SO423. Why hypervalent compounds are formed by elements such as Si, P and S, but not by C,N and O? O 1. Write a possible arrangement. O S O O 2. Total the electrons. 6 from S, 4 x 6 from O add 2 for charge total = 32 O || 3. Spread the electrons Chapter-2-27 || O around. Chemistry 481, Spring 2017, LA Tech O - S- O Chemistry 481, Spring 2017, LA Tech Chapter-2-28 Atoms with fewer than eight electrons Atoms with fewer than eight electrons Beryllium and boron will both form compounds where they have less than 8 electrons around them. Electron deficient. Species other than hydrogen and helium that have fewer than 8 valence electrons. : :F:B:F: :F: F- : : : :Cl:Be:Cl: : : : : : : They are typically very reactive species. Chemistry 481, Spring 2017, LA Tech Chapter-2-29 F | B | F + Chemistry 481, Spring 2017, LA Tech H | :N – H | H F H | | F - B <- N - H | | F H Chapter-2-30 5 What is a Polar Molecule? How do you a Pick Polar Molecule? • Molecules with unbalanced electrical charges a) Get the molecular structure from VSEPR theory • Molecules with a dipole moment b) From c (electronegativity) difference of bonds • Molecules without a dipole moment are called non-polar molecules see whether they are polar-covalent. c) If the molecule have polar-covalent bond, check whether they cancel from a symmetric arrangement. d) If not molecule is polar Predicting symmetry of molecule and the polarity will be discussed in detail in Chapter 7. Chemistry 481, Spring 2017, LA Tech Chapter-2-31 Chemistry 481, Spring 2017, LA Tech Chapter-2-32 Linear Combination of Atomic Orbitals Symmetry Adapted Linear Combination of Atomic Orbitals –LCAO Atomic orbitals on single atom: Hybridization Atomic orbitals in a molecule with more than one atom: Molecular Orbital (MO) formation General rule Number of Hybrid Orbital produced = # hybridized Number of MO produced = # orbitals combined Chemistry 481, Spring 2017, LA Tech Chapter-2-33 What are p and s bonds s bonds Chemistry 481, Spring 2017, LA Tech Chapter-2-34 What are d bonds d bond double and triple bond resulting from lateral or side way overlap of d atomic orbitals p bond Chemistry 481, Spring 2017, LA Tech 6. Draw a diagram to illustrate each described overlap: a) s bonding overlap of two p orbitals b) d bonding overlap of two d orbitals c) p bonding overlap of a p orbital and a d orbital d) s antibonding overlap of a p and a d orbital e) d antibonding overlap of two d orbitals. Chapter-2-35 Chemistry 481, Spring 2017, LA Tech Chapter-2-36 6 Kinds of hybrid orbitals Hybrid sp sp2 geometry # of orbital linear 2 trigonal planar 3 sp3 sp3d sp3d2 tetrahedral trigonal bipyramid octahedral 5. Using valence-bond (VB) theory to explain the bonding in the coordination complex ion, Co(NH3)63+. 4 5 6 Chemistry 481, Spring 2017, LA Tech Chapter-2-37 Hybridization involving d orbitals •Co(NH3)6 3+ ion Co3+: [Ar] 3d6 Chemistry 481, Spring 2017, LA Tech Chapter-2-38 5. What is the oxidation state of metal in (a) Co(NH3)63+ ion (b) PtCl42- ion. a) [Co(NH3)6] 3+ •Co3+: [Ar] 3d6 4s0 4p0 Co3+ and NH3 is neutral •Concentrating the 3d electrons in the dxy, dxz, and dyz orbitals in this subshell gives the following electron configuration hybridization is sp3d2 Oxidation Sate of Co3+ is +3 and NH3 is 0 Therefore sum of the oxidation should be equal to +3 +3= Co(NH3)6 = (Co)3+6((NH3)0)= +3 Co is +3 in [Co(NH3)6]3+ b) Pt is +2 in [PtCl4]2- because Cl- is -1 Chemistry 481, Spring 2017, LA Tech Chapter-2-39 Linear Combination of Atomic Orbitals Symmetry Adapted Linear Combination of Atomic Orbitals –LCAO Atomic orbitals on single atom: Hybridization Atomic orbitals in a molecule with more than one atom: Molecular Orbital (MO) formation General rule Number of Hybrid Orbital produced = # hybridized Number of MO produced = # orbitals combined Chemistry 481, Spring 2017, LA Tech Chapter-2-41 Chemistry 481, Spring 2017, LA Tech Chapter-2-40 Basic Rules of Molecular Orbital Theory The MO Theory has five basic rules: • The number of molecular orbitals = the number of atomic orbitals combined • Of the two MO's, one is a bonding orbital (lower energy) and one is an anti-bonding orbital (higher energy) • Electrons enter the lowest orbital available • The maximum # of electrons in an orbital is 2 (Pauli Exclusion Principle) • Electrons spread out before pairing up (Hund's Rule) Chemistry 481, Spring 2017, LA Tech Chapter-2-42 7 Molecular Orbital Theory Bonding and Anti-bobding Molecular Orbital • Molecular orbitals are obtained by combining the atomic orbitals on the atoms in the molecule. Chemistry 481, Spring 2017, LA Tech Chapter-2-43 Bond Order Chemistry 481, Spring 2017, LA Tech Chapter-2-44 Homo Nuclear Diatomic Molecules • Calculating Bond Order Period 1 Diatomic Molecules: H2 and He2 Chemistry 481, Spring 2017, LA Tech Chapter-2-45 Homo Nuclear Diatomic Molecules Chemistry 481, Spring 2017, LA Tech Chapter-2-46 Homo Nuclear Diatomic Molecules Period 2 Diatomic Molecules and Li2 and Be2 Chemistry 481, Spring 2017, LA Tech Chapter-2-47 Chemistry 481, Spring 2017, LA Tech Chapter-2-48 8 Molecualr Orbital diagram for Molecualr Orbital diagram for B2, C2 and N2 O2, F2 and Ne2 Chemistry 481, Spring 2017, LA Tech Chapter-2-49 7. Using molecular orbital theory and diagrams, explain why, O2 is a paramagnetic whereas N2 is diamagnetic. Chemistry 481, Spring 2017, LA Tech Chapter-2-50 Electronic Configuration of molecules When writing the electron configuration of an atom, we usually list the orbitals in the order in which they fill. Pb: [Xe] 6s2 4f14 5d10 6p2 We can write the electron configuration of a molecule by doing the same thing. Concentrating only on the valence orbitals, we write the electron configuration of O2 as follows. O2: (2s) 2(2s*) 2 (2p) 4 (2p*) 2 Chemistry 481, Spring 2017, LA Tech Chapter-2-51 Electronic Configuration and bond order Chemistry 481, Spring 2017, LA Tech Chapter-2-53 Chemistry 481, Spring 2017, LA Tech Chapter-2-52 Electronic Configuration and bond order Chemistry 481, Spring 2017, LA Tech Chapter-2-54 9 Hetero Nuclear Diatomic Molecules Carbon monoxide CO Chemistry 481, Spring 2017, LA Tech Chapter-2-55 MO Correlation Diagrams ( Walsh Diagrams) 8. Draw molecular orbital diagrams for HF, CO, NO, NO+. Calculate their bond order and predict magnetic properties. Chemistry 481, Spring 2017, LA Tech Chapter-2-56 Walsh Diagram for H3: • The correlation diagram clearly indicates that the molecular orbital energy levels changes as the H3 changes from linear to cyclic (equilateral triangle) structure. In the case of • linear H3 the overlap between two terminal H is minimal, where as in the case of cyclic H3 the overlap is substantial. This will bring the lowest MO (bonding) and the highest MO (antibonding) down in energy. At the same time, the non-bonding MO (middle one) will • go up in energy, leading to a degenerate set of levels. Thus H3+ (two electrons) will be triangular. Chemistry 481, Spring 2017, LA Tech Chapter-2-57 9. Draw a molecular orbital diagram for triangular H3+ and describe the bonding. Chemistry 481, Spring 2017, LA Tech Chapter-2-59 Chemistry 481, Spring 2017, LA Tech Chapter-2-58 10. Draw a Walsh diagram (orbital correlation diagram) and show that triangular H3+ is more stable than linear H3+. Chemistry 481, Spring 2017, LA Tech Chapter-2-60 10 Conjugated and aromatic molecules trans-1,3-Butadiene • trans-1,3-Butadiene • Allyl radical • Cyclopropenium ion: C3H3+ • Cyclobutadiene • Cyclopentadiene • Benzene • C7H7+ (tropyllium) and C8H82+ Chemistry 481, Spring 2017, LA Tech Chapter-2-61 Allyl radical Chemistry 481, Spring 2017, LA Tech Chapter-2-62 Cyclopropenium ion: C3H3+ Chapter-2-63 Chemistry 481, Spring 2017, LA Tech Chapter-2-64 Benzene Cyclopentadiene Chemistry 481, Spring 2017, LA Tech Chemistry 481, Spring 2017, LA Tech Chapter-2-65 Chemistry 481, Spring 2017, LA Tech Chapter-2-66 11 Aromatic Rings 11. Using molecular orbital diagrams for pi (p) orbitals explain the relative stabilities of the following: (a) C3H3 and C3H3+ (b) C4H4 and C4H4+ (c) C5H5 and C5H5(d) C6H6 and C6H6+ (e) C7H7 and C7H7+ Chemistry 481, Spring 2017, LA Tech Chapter-2-67 Chemistry 481, Spring 2017, LA Tech Chapter-2-68 Chemistry 481, Spring 2017, LA Tech Chapter-2-70 The Isolobal Analogy • Different groups of atoms can give rise to similar shaped fragments. Chemistry 481, Spring 2017, LA Tech Chapter-2-69 12. Pick the isolobal fragments among the following: a) Co3(CO)9Co(CO) 3, Co3(CO)9PR, Metallic Bonding Co3(CO)9CH • Metals are held together by delocalized b) H3CCl, Mn(CO)5H, bonds formed from the atomic orbitals of all the atoms in the lattice. Re(CO) 5Cl • The idea that the molecular orbitals of the c) R2SiH2, Fe(CO)4H2, Chemistry 481, Spring 2017, LA Tech band of energy levels are spread or delocalized over the atoms of the piece of metal accounts for bonding in metallic solids. H2CH2 Chapter-2-71 Chemistry 481, Spring 2017, LA Tech Chapter-2-72 12 Linear Combination of Atomic Orbitals Bonding Models for Metals •Band Theory of Bonding in Solids •Bonding in solids such as metals, insulators and semiconductors may be understood most effectively by an expansion of simple MO theory to assemblages of scores of atoms Chemistry 481, Spring 2017, LA Tech Chapter-2-73 Chemistry 481, Spring 2017, LA Tech Chapter-2-74 Chemistry 481, Spring 2017, LA Tech Chapter-2-76 Linear Combination of Atomic Orbitals Chemistry 481, Spring 2017, LA Tech Chapter-2-75 Band Theory of Metals 13. Describe metallic bonding and properties in terms of: a) Electron-sea model of bonding: b) Band Theory: Chemistry 481, Spring 2017, LA Tech Chapter-2-77 Chemistry 481, Spring 2017, LA Tech Chapter-2-78 13 14. Draw the s band (molecular orbitals) for ten Na on a line (one dimensional) and show bonding and anti-bonding molecular orbitals and fill electrons. Chemistry 481, Spring 2017, LA Tech Chapter-2-79 15. Describe the metallic properties of sodium in terms of band theory. Chemistry 481, Spring 2017, LA Tech Chapter-2-80 Types of Materials 16. Using a band diagram, explain how magnesium can exhibit metallic behavior even though its 3s band is completely full. • A conductor (which is usually a metal) is a solid with a partially full band • An insulator is a solid with a full band and a large band gap • A semiconductor is a solid with a full band and a small band gap • Element C Si Ge Sn Chemistry 481, Spring 2017, LA Tech Chapter-2-81 Chemistry 481, Spring 2017, LA Tech Band Gap 5.47 eV 1.12 eV 0.66 eV 0 eV Chapter-2-82 17. Draw a Band diagram for carbon/silicon/germanium/tin, and label valence band, conduction band and band gap? Chemistry 481, Spring 2017, LA Tech Chapter-2-83 Chemistry 481, Spring 2017, LA Tech Chapter-2-84 14 18. Draw a band diagrams to show the difference between(Band gaps: C = 5.47, Si = 1.12, Ge = 0.66, Sn = 0) Conductor (Sn): 19. Draw a band diagram for thermal/photo (Intrinsic) and doped (Extrinsic) semiconductors and explain the origin of semicondictivity? Thermal/photo (Intrinsic) (Ge): Insulator (C): Doped (Extrinsic) (Si/As): Semiconductor (Ge): Chemistry 481, Spring 2017, LA Tech Chapter-2-85 20. Draw a band diagram for a p-type (Si/Ga) and ntype (Si/As) semiconductors and show holes and electrons that is responsible for semiconductivity. Chemistry 481, Spring 2017, LA Tech Chapter-2-86 22. What the difference between a transistor (semiconductor device) and vacuum tube? p-type(Si/Ga): n-type(Si/As): Chemistry 481, Spring 2017, LA Tech Chapter-2-87 What is a transistor? Chemistry 481, Spring 2017, LA Tech Chemistry 481, Spring 2017, LA Tech Chapter-2-88 21. What is a transistor with emitter (E), collector(C) and base (B), and how it works? Chapter-2-89 Chemistry 481, Spring 2017, LA Tech Chapter-2-90 15 23. Using the diagram explain how a diode work. Superconductors • When Onnes cooled mercury to 4.15K, the resistivity suddenly dropped to zero Chemistry 481, Spring 2017, LA Tech Chapter-2-91 The Meissner Effect Chemistry 481, Spring 2017, LA Tech Chapter-2-92 Theory of Superconduction •BCS theory was proposed by J. Bardeen, L. Cooper •Superconductors show perfect diamagnetism. •Meissner and Oschenfeld discovered that a and J. R. Schrieffer. BCS suggests the formation of so-called 'Cooper pairs' superconducting material cooled below its critical temperature in a magnetic field excluded the magnetic flux. Results in levitation of the magnet in a magnetic field. Chemistry 481, Spring 2017, LA Tech Chapter-2-93 Cooper pair formation - electronphonon interaction: the electron is attracted to the positive charge density (red glow) created by the first electron distorting the lattice around itself. Chemistry 481, Spring 2017, LA Tech Chapter-2-94 High Temperature Superconduction •BCS theory predicted a theoretical maximum to Tc of around 30-40K. Above this, thermal energy would cause electronphonon interactions of an energy too high to allow formation of or sustain Cooper pairs. • 1986 saw the discovery of high temperature superconductors which broke this limit (the highest known today is in excess of 150K) - it is in debate as to what mechanism prevails at higher temperatures, as BCS cannot account for this. Chemistry 481, Spring 2017, LA Tech Chapter-2-95 16