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.

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Presentation transcript:

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.A molecule is polar if the centers of positive and negative charge do not coincide. How do we determine this?How do we determine this? –Rigorous approach needs consideration of symmetry and mathematical calculations –Approximate approach considers arrangements of bonds

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

Two bonds Equal bonds oppose (linear)Equal bonds oppose (linear) –Nonpolar (CO 2 ) Unequal bonds oppose (linear)Unequal bonds oppose (linear) –Polar (HCN) Equal bonds do not oppose (bent)Equal bonds do not oppose (bent) –Polar (H 2 O)

Three bonds Equal bonds oppose in trigonal planar arrangementEqual bonds oppose in trigonal planar arrangement –Nonpolar Unequal bonds in trigonal planar arrangementUnequal bonds in trigonal planar arrangement –Polar

Gets more complicated Planar or pyramidal?Planar or pyramidal? Depends on number of groups of chargeDepends on number of groups of charge BCl 3 is trigonal planar – nonpolarBCl 3 is trigonal planar – nonpolar NCl 3 is trigonal pyramidal – polarNCl 3 is trigonal pyramidal – polar Four bonds works better with modelsFour bonds works better with models

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

Roadmap to polarity Establish skeleton of moleculeEstablish skeleton of molecule Determine Lewis dot structure using S = N – ADetermine Lewis dot structure using S = N – A Determine electronic geometry using VSEPRDetermine electronic geometry using VSEPR Identify molecular geometry from molecularIdentify molecular geometry from molecular Count number of polar bondsCount number of polar bonds Perform polarity analysis using rules described abovePerform polarity analysis using rules described above