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Polarity

Polarity. Molecules with many bonds. Polar or non-polar? That is the question. A molecule is polar if the centers of positive and negative charge do not coincide. How do we determine this? Rigorous approach needs consideration of symmetry and mathematical calculations

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Polarity

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  1. Polarity Molecules with many bonds

  2. Polar or non-polar? That is the question. • A molecule is polar if the centers of positive and negative charge do not coincide. • How do we determine this? • Rigorous approach needs consideration of symmetry and mathematical calculations • Approximate approach considers arrangements of bonds

  3. How many polar bonds? • Bond is polar if electronegativity difference greater than 0.4 • Zero bonds: always nonpolar • Except when lone pairs lower symmetry • One bond: always polar • Two or more bonds: may or may not… • Consider the molecular shape • Do individual bond polarities cancel? • If yes, nonpolar. If no, polar • Example

  4. Two bonds • Equal bonds oppose (linear) • Nonpolar (CO2) • Unequal bonds oppose (linear) • Polar (HCN) • Equal bonds do not oppose (bent) • Polar (H2O)

  5. Three bonds • Equal bonds oppose in trigonal planar arrangement • Nonpolar • Unequal bonds in trigonal planar arrangement • Polar

  6. Gets more complicated • Planar or pyramidal? • Depends on number of groups of charge • BCl3 is trigonal planar – nonpolar • NCl3 is trigonal pyramidal – polar • Four bonds works better with models

  7. Symmetry: Another approach to thinking about polarity • Consider the molecule as a whole. • If it looks symmetrical it is nonpolar • If it looks unsymmetrical it is polar

  8. Roadmap to polarity • Establish skeleton of molecule • Determine Lewis dot structure using S = N – A • Determine electronic geometry using VSEPR • Identify molecular geometry from molecular • Count number of polar bonds • Perform polarity analysis using rules described above

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