GHW#3 Louisiana Tech University, Chemistry 281. POGIL exercise on Chapter 2. Covalent Bonding: VSEPR, VB and MO Theories. How and Why? How is Valence Shell Electron Pair Repulsion Theory developed from the Lewis structures of a molecule? How is electronegativity and molecular structure is used to predict polarity of a molecule? What is the difference between Valence-Bond Theory and Molecular Orbital Theory? How is molecular orbital theory applied to homonuclear and heteronuclear diatomic molecules? How is intermolecular forces used to explain physical properties of chemical compounds? Learning Objectives Valence Shell Electron Pair Repulsion Theory, Valence Bond Theory, Molecular Orbital Theory, Molecular Orbitals for Period 1and 2 Diatomic Molecules, Molecular Orbitals for Heteronuclear Diatomic Molecules, and Intermolecular Forces:Dispersion (London) Forces, Dipole-Dipole Forces and Hydrogen Bonding. Apply these concepts to chemical problems. Resources Inorganic Chemistry By Peter Atkins, Tina Overton, Jon Rourke, Mark Weller, Fraser Armstrong, 5th Edition 2010. Prerequisites Atomic structure was discovered, the Schrödinger wave equation shapes of the atomic orbitals, valence electron configurations of molecules and ions its relationship to properties of elements, modern periodic table and its organization based on electronic configuration, the effective nuclear charge and its relation to the atomic size, ionization energy, electron affinity and electronegativity, New Concepts Valence Shell Electron Pair Repulsion Theory This theory assumes that the molecular structure is determined by the lone pair and bond pair electron repulsion around the central atom. Possible Molecular Geometry No unpaired electrons With unpaired electrons 1. Linear (180) Boding nonbonding 2. Trigonal Planar (120) 6. Angular 2 1 3. Tetrahedral (109) 7. Pyramidal, 3 1 Angular 2 2 4. Trigonal bipyramid (90, 120, 8., Sea-saw 4 1 180) 9. T-shape 3 2 Linear 2 3 5. Octahedral (90, 180) 10. Square pyramidal, 5 1 11. Square planar 4 2 Electronegativity in predicting polarity of molecules Electronegativity: The ability of an atom that is bonded to another atom or atoms to attract electrons to itself. Bond polarity is a useful concept for describing the sharing of electrons between atoms a) Nonpolar covalent bond is one in which the electrons are shared equally between two atoms
b) Polar covalent bond is one in which one atom has a greater attraction for the electrons than the other atom. c) Ionic bond: If this relative attraction is great enough, then the bond is an ionic bond Electronegativity Difference 0-0.5 0.6-1.6 1.6, or greater Nonpolar covalent Polar covalent bond Ionic C-H O-H Na-Cl Hybridization of atomic orbitals: Linear combination of atomic orbitals (LACO) Mixing of valence atomic orbitals on the central atom. Possible hybridizations: sp, sp 2, sp 3, sp 3 d, sp 3 d 2 Valence-Bond Theory Describes covalent bonding in molecule using valence atomic orbitals. Hybrid or unhybrid atomic orbital of one atom occupy the same region with a orbital from another atom. The total number of electrons in both orbital is equal to two. Possible overlaps Head to head: ó bonds single bond resulting from head to head overlap of two valence atomic orbital Lateral or side way overlap: π bond double and triple bond resulting from lateral or side way overlap of two unhybrid p atomic orbitals δ bond Quadruple bond resulting from lateral or side way overlap of two unhybrid d atomic orbitals Introduction to Molecular Orbital Theory LACO of the molecular orbitals (M.O.'s) to produce a set of combinations by combining the original valence level atomic orbitals of the all atoms that were involved in the covalent bonding process. 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) Bond order: is equal to the number of bonding electrons, Nb.e, subtract number of nonbonding electrons, Nnb.e,. and divided by 2. Bond order = 1/2 ( N b.e. - Nnb.e.)
Molecular Orbitals for Period 1 Diatomic Molecules Molecular Orbitals for Period 2 Diatomic Molecules Electron configuration of a molecule: Concentrating only on the valence molecular orbitals, one can write the electron configuration of O2: O 2: (2 s ) 2 (2 s *) 2 (2p) 2 (2 p) 4 (2 p*) 2 Molecular Orbitals for Heteronuclear Diatomic Molecules Intermolecular Forces Dispersion (London) Forces Dipole-Dipole Forces Hydrogen Bonding Exercises (Applying concepts)
GHW#3 CHEM 281 Your Name: Valence Shell Electron Pair Repulsion Theory 1. Determine the electron pair arrangement and the molecular shape (geometry and angles) according to VSEPR theory. a) CCl4 b) BH3 c) PCl3 d) PF5 e) Xe O 3 Polarity of molecules 2. Predict the polarity of following molecules: a. CCl4 b. BH3 c. PC l 3 d. PF5 e. XeO 3 Hybridization 3. Predict the hybridization in the central atom: a. CCl4 b. BH3 c. PC l 3 d. PF5 e. XeO3 f. [Fe(CN) 6] -3 Valence-Bond Theory 4. What is valence bond(vb) theory and use following as examples a. PF5 b. XeO3 c. [Fe(CN)6]-3
5. Define the following terms: a) Hybridization (LCAO) b) Molecular orbitals (LCAO) c) What are the main features of the molecular orbital theory? 6. Outline the main differences between VB theory and MO theory 7. Define the following terms: a. σ molecular orbital: b. π molecular orbital: 8. molecular orbital and quadruple bonding: 9. Use a molecular orbital diagram to determine the bond order of the H2 10. Would you expect Be2 to exist? Use a molecular orbital energy diagram to explain your reasoning. Molecular Orbitals for Period 2 Homonuclear Diatomic Molecules 11. Are there any differences in molecular orbital diagrams of B2, C2, N2, O2 and F2? 12. Draw a molecular orbital diagram for O2 determine the bond order and magnetic properties.
13. Use a molecular orbital diagram to determine the bond order in the O2 + ion. Write an electron configuration [KK(σ2 s) 2,... ] for this ion. Draw a molecular orbital diagram for CO and determine the bond order and magnetic properties. 14. Assuming that it has similar molecular orbital energies to those of carbon monoxide, deduce the bond order of the NO + ion. 15. Assuming that it has similar molecular orbital energies to those of carbon monoxide, deduce the bond order of the NO - ion. 16. Predict which of the following gas-phase reactions are the more favored and give your reasoning. a. NO + CN ---> NO + + CN b. NO + CN ---> NO - + CN + Intermolecular Forces 17. What the main differences in following intermolecular forces? a. Hydrogen bonding b. Dipole-Dipole forces c. Dispersion (London) Forces 18. Predict the intermolecular forces in following a) N2 b) CO2 c) CCl4 d) H2O e) HCl