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Resonance Structures

Resonance Structures . Two completely equivalent Lewis structures can be drawn for ozone, O 3 . Experiment, however, shows that both bonds in ozone are equivalent.  The reality is a blend of the two Lewis structures. Resonance Structures .

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Resonance Structures

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  1. Resonance Structures Two completely equivalent Lewis structures can be drawn for ozone, O3. Experiment, however, shows that both bonds in ozone are equivalent.  The reality is a blend of the two Lewis structures.

  2. Resonance Structures The actual ozone molecule with its equal bond distances is a blend of the two Lewis structures. Each of these structures is called aresonance structure.

  3. Resonance Structures Three resonance structures can be drawn for the nitrate ion, NO3-. Again, reality is a blend of the three. There are no double bonds in the nitrate ion, but each bond is more stable than just a single bond.

  4. Resonance in Benzene and Other Aromatic Molecules Benzeneis a cyclic compound, C6H6, andis very stable. Often, this resonance is represented by: where the circle shows the blending of the resonance structures. Other compounds containing this ring structure are called aromatic compounds.

  5. Name HCN(g) HCN(aq) HC2H3O2 HBr(g) HI(aq) HClO3 FeN Zn3N2 CuNO3 AgNO3 PI3 (NH4)2CO3

  6. Write the formula bismuth(V) nitrate strontium chloride silver acetate sulfurous acid hydrosulfuric acid copper(II) sulfate pentahydrate nitrogen triiodide hydrofluoric acid rubidium oxide barium hydroxide calcium chlorite

  7. Lewis Structures that are Exceptions to the Octet Rule Exceptions to the Octet rule are: 1. Molecules in which an atom has more than an octet (this can occur with period 3 and higher elements…NEVER with period 2 elements). 2. Molecules in which an atom has less than an octet: H, Be, B. 3. Molecules with an odd number of electrons. An example is NO, but we are not going to dwell on this category.

  8. Lewis Structures that are Exceptions to the Octet Rule Exceptions to the Octet rule are: Molecules in which an atom has more than an octet (this can occur with period 3 and higher elements…NEVER with period 2 elements).

  9. Lewis Structures that are Exceptions to the Octet Rule Exceptions to the Octet rule are: 2. Molecules in which an atom has less than an octet (generally occurs with B or Be and always with H).

  10. Electron Domains • Being able to draw the correct Lewis structure is the first step in determining the geometry of the covalent compound. • This allows the identification of the electron domainsof the molecule. Domains are the regions in the molecules where it is most likely to find electrons.

  11. Electron Domains • Domains are the regions in the molecules where it is most likely to find electrons. • For a bond (single, double, or triple), the electron domain is between the two atoms in the bond and consists of all the electrons involved in the bond. • For nonbonding pairs of electrons, the domain is the nonbonding pair and is centered on a single atom.

  12. Identifying Electron Domains It is important to differentiate between bonding domainsand nonbonding domains. The nitrate ion has three electron domains, all bonding. Xenon difluoride has five electron domains, two bonding, three nonbonding.

  13. VSEPR Theory • ValenceShell Electron Pair Repulsion • Electrons in each domain are subject to electrostatic repulsion from the electrons in the other domains. • The domains will orient themselves so as to minimize this repulsion. • The orientation of these domains is a function of the number of domains around the central atom and is one of several simple geometric figures. Memorize these geometries and angles

  14. Trigonal Bipyramid Geometry angles BAC = 90° angles CAC = 120° angle BAB = 180° B C 90° 120° A C C B

  15. Electron Domain Geometries Trigonal planar Bond angles all 120° TrigonalbipyramidBondangle is 180°

  16. Different Ways to Represent Molecular Geometry a tetrahedron Space-filling model Ball and stick model

  17. Molecular Geometries for Molecules with No Nonbonding Electron Domains CCl4 BCl3 CO2 PCl5 SF6 Memorize both the geometries and the bond angles.

  18. Molecular Geometries for Molecules with Nonbonding Electron Pairs An important thing to remember is that the shape of ANY molecule is based on the location of its atoms (NOT the location of its electrons).

  19. Molecular Geometries for Molecules with Nonbonding Electron Pairs • The shape of ANY molecule is based on the location of its atoms(NOT the location of its electrons). • However, the nonbonding electron pairs take up more space than bonding pairsand have the effect of squeezing (decreasing) the bond angles among the atoms.

  20. Molecular Geometries for Molecules with Nonbonding Electron Pairs The shape of ANY molecule is based on the location of its atoms(NOT the location of its electrons). The e- pair here takes up extra space. The two e- pairs take up extra space. Example: CH4 Bond angles 109.5° Example: NH3 Bond angles <109.5° Example: H20 Bond angles <109.5°

  21. Going from the Molecular Formula to the Molecular geometry .. NH3 molecular formula H—N—H | H Lewis structure molecular geometry: trigonal pyramid electron domain geometry: tetrahedral

  22. Trigonal Bipyramid Geometry The nonbonding electron pairsgo to the equatorial positions to minimize repulsions.

  23. Octahedral Geometry

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