Lesson 15 Chapter 8 Lesson 15 ? Chapter 8 Molecular Shape Goals Determine the shape of a molecule with no lone pairs of electrons on the central atom Understand how lone pairs of electrons will affect the shape of a molecule Determine the shape of any molecule Why? Molecules are not two dimensional or flat. Because the physical properties of molecules have as much to do with the shape of the molecule as chemical properties of molecules have to do with the composition of the molecules, it is important to be able to determine the shape of a molecule given the Lewis dot structure. Additional Resources Introduction to Chemistry, Chapter 8, p. 310-316 Prerequisites Understand the number of valence electrons for an element Be able to read a formula Understand the octet rule Be able to make Lewis dot structures of simple molecules Terms Bent, bond angle, coordination, central atom, linear, molecular shape, tetrahedral, trigonal planar, trigonal pyramidal Homework Problems: Online homework portions of Chapter 8 Textbook Chapter 8: 83-106, 131-132 Lewis Dot Structures ? Part I For CO2 (carbon dioxide), assign which representation is the chemical formula, the structural formula and the Lewis dot structure. CO2 ________________ ____________________ ____________________ Which representation tells us the most about the molecule, CO2? Why? Is this molecule actually flat (like the drawings infer)? Why? Lewis Dot Structures ? Part II Give the chemical formula, the structural formula and the Lewis dot structure for nitrogen tricloride, NCl3. Chemical Formula Structural Formula Lewis Dot Structure Which representation is best for NCl3? What vital piece of information is lost for the molecule between the structural formula and the Lewis dot structure with regards to the nitrogen atom? Is this molecule flat? Do you think the molecule has 90( bond angles (between Cl?N?Cl)? Molecular Shape ? Part I Complete the table. Electron Pairs Shape Bond Angle EMBED ChemDraw.Document.5.0 EMBED ChemDraw.Document.5.0 EMBED ChemDraw.Document.5.0 EMBED ChemDraw.Document.5.0 EMBED ChemDraw.Document.5.0 Molecular Shape ? Part II Give the coordination (number of bonded atoms + non bonding electron pairs) for Freon, CF2Cl2 ____________ Formaldehyde, COH2 ____________ PCl5 ____________ SF6 ____________ COS ____________ Give the bond angle and the name of the shape for Freon, CF2Cl2 ____________ ________________________ Formaldehyde, COH2 ____________ ________________________ PCl5 ____________ ________________________ SF6 ____________ ________________________ COS ____________ ________________________ How would you assign the electron pairs to nitrogen trifluoride, NF3? Does the lone pair on the central atom make a difference? Valence Shell Electron Pair Repulsion or VSEPR ? Part I Complete the table. Electron pairs # of lone pairs Shape Bond Angle Example 2 0 CO2 3 0 COF2 3 1 O3 4 0 CH4 4 1 NH3 4 2 H2O Complete the table. Molecule Electron Pairs Number of lone pairs (on central atom) Shape Bond Angle NF3 OCl2 SO2 N2O Valence Shell Electron Pair Repulsion or VSEPR ? Part II In the table is actual bond angle data for the three molecules, methane (CH4), ammonia (NH3), and water (H2O). Molecule Electron Pairs Number of lone pairs Shape Bond Angle CH4 4 0 Tetrahedral 109.5( NH3 4 1 Trigonal pyramidal 107( H2O 4 2 Bent 105( Considering the central atom, how does methane differ from ammonia and water? How does this affect the bond angle in a molecule? (Hint: What wants the most space on the central atom?) 16. Complete the table. Ion Lewis dot Structure Electron Pairs Number of lone pairs on the central atom Shape Bond Angle Nitrite Sulfite Nitrate Sulfate Carbonate PAGE 84 PAGE 83
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