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Electronegativity, Dipole Moment

electronegativity - ability of atom in a molecule to attract electrons

  • used to see if bond will be nonpolar covalent, polar covalent, or ionic
  • bond polarity - describes sharing of electrons between atoms
    • nonpolar covalent bond - electrons shared equally (electronegativities equal)
    • polar covalent bond - 1 atom attracts bonding electrons more than the other
    • ionic bond forms when difference in electronegativity > 3
  • based on ionization energy, electron affinity
  • fluorine - most electronegative
  • cesium - least electronegative
  • in molecules, electron density (and negative charge) shift towards more electronegative atom

dipole moment - measure of dipole’s magnitude

  • dipole - formed when electrical charges of opposite sign, equal magnitude separated by distance
  • m = Qr
    • m = dipole moment
    • Q = electronic charge (unit e = 1.60 x 10-19 C)
    • r = distance separating 2 atoms
  • debyes (D) - unit equal to 3.34 x 10-30 coulomb-meters (C-m)

Calculate the dipole moment of HCl if the H and Cl are separated by 1.27Å 

  • Q = 1e = 1.60 x 10-19 C
  • r = 1.27Å
  • m = (1.60 x 10-19 C)(1.27Å)(10-10m/1.00Å)(1D/3.34 x 10-30 C-m)
  • m = 6.08 D

In units of e, what is the charge on the atoms of a compound if they're separated by 2.74Å and have a measured dipole moment of 1.97 D? 

  • Q = m/r
  • m = 1.97D = (1.97D)(3.34 x 10-30 C-m / D) = 6.5798 x 10-30 C-m
  • r = 2.74Å = (2.74Å)(10-10m/1.00Å) = 2.74 x 10-10m
  • Q = m/r = (6.5798 x 10-30 C-m) / (2.74 x 10-10m) = 2.40 x 10-20 C
  • (2.40 x 10-20 C)(1 e / 1.60 x 10-19 C) = 0.150 e
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