chem lecture 13

chem lecture 13 - Key ideas of Valence Bond theory: The...

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Chemistry 6A 1 Key ideas of Valence Bond theory: • The overlap of atomic orbitals, each of which contains one electron of opposite spin, results in a covalent bond, wherein the localized bonding electrons are associated with two particular atoms. The greater the amount of orbital overlap, the stronger the bond. We can account for observed molecular geometries by assuming that an atom in a molecule can adopt a set of atomic orbitals, called hybrid orbitals , that are different from those in the free state. The various combination of s, p , and d atomic orbitals results in linear ( sp ), trigonal planar ( sp 2 ), tetrahedral ( sp 3 ) , trigonal bipyramidal ( sp 3 d) and octahedral ( sp 3 d 2 ) geometries about the atom. In a typical multiple bond, the σ bond results from overlap of hybrid atomic orbitals and the π bond(s) result from overlap of pure atomic orbitals.
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Chemistry 6A 2 Key ideas of Molecular Orbital theory: Molecular orbitals are to molecules what atomic orbitals are to atoms. That is, a molecular orbital (a wave function) describes a region of space in a molecule where electrons are most likely to be found, and it has a specific size, shape, and energy level. • The combination of n atomic orbitals gives n molecular orbitals. Relative to the starting AOs of the free atoms, MOs that are lower in energy are bonding, and MOs that are higher in energy are anti -bonding for a given molecule. • The rules of MO filling are similar to those for atomic orbitals. If the number of bonding electrons is greater than the number of antibonding electrons, the molecule is predicted to be stable.
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Chemistry 6A 3 Select the statement regarding Valence Bond theory that is incorrect: (a) A pi bond has no electron density along the internuclear axis. (b) Molecular geometry depends on the σ -bonded framework. (c) A sigma bond can be formed from end-to-end overlap of p orbitals (d) The angles generated by sp 2 orbitals are approximately 109.5° (e) The octahedral hybrid configuration is composed of a sp 3 d 2 orbital combination.
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Chemistry 6A How many sp 2 -hybridized carbon centers are in caffeine? (a) 1 (b) 2 (c) 3 (d) 4 (e) 5 4
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Chemistry 6A How many sp 2 -hybridized carbon centers are in caffeine? (a) 1 (b) 2 (c) 3 (d) 4 (e) 5 5
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Chemistry 6A Which of the following statements are true? (a) The number of MOs in a molecule equals twice the number of constituent atomic orbitals. (b) As bonding MOs become more stable, antibonding MOs become equally less stable. (c) In MOs, the number of bonding electrons equals the number of antibonding electrons. (d) Each bonding MO can accommodate only one electron. (e) None of the above is true. 6
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Chemistry 6A 7 How does Molecular Orbital theory yield deeper insights into the nature of the (covalent) chemical bond? Next to consider: Homonuclear diatomic molecules of Period 2 elements How do the 1 s, 2s, and 2 p orbitals mix? Heteronuclear diatomic molecules
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chem lecture 13 - Key ideas of Valence Bond theory: The...

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