Polar covalent bonds form when atoms share electrons unequally. This uneven sharing creates partial charges, leading to molecular polarity. Understanding how these charges distribute across molecules is key to predicting their behavior and properties.

Molecular polarity affects everything from boiling points to solubility. By learning to calculate dipole moments and predict molecular dipoles, you'll gain insight into how molecules interact with each other and their environment.

Polar Covalent Bonds and Molecular Polarity

Origins of molecular polarity

• Molecular polarity determined by sum of individual bond dipole moments and lone pair contributions
  • Bond dipole moments arise from electronegativity differences between bonded atoms
    • More electronegative atom attracts bonding electrons more strongly, creating partial negative charge ($\delta-$) (oxygen in H$_2$O)
    • Less electronegative atom has partial positive charge ($\delta+$) (hydrogen in H$_2$O)
  • Lone pairs contribute to molecular polarity
    • Nonbonding electrons held closer to atom, creating partial negative charge ($\delta-$) (oxygen in H$_2$O, nitrogen in NH$_3$)
• Vector sum of all bond dipole moments and lone pair contributions determines overall molecular dipole moment
  • Bond dipole moments and lone pair contributions cancel each other out, molecule is nonpolar (CO$_2$, CCl$_4$)
  • Bond dipole moments and lone pair contributions do not cancel each other out, molecule is polar (H$_2$O, NH$_3$)
• Electron density distribution affects molecular polarity

Calculation of dipole moments

• Dipole moment ($\mu$) measures polarity of molecule
  • Calculated using formula: $\mu = Q \times r$
    • $Q$: magnitude of partial charges (Coulombs)
    • $r$: distance between partial charges (meters)
  • Unit for dipole moment is Debye (D)
    • 1 D = 3.336 × 10$^{-30}$ C·m
• Calculating dipole moment:
  1. Determine partial charges ($Q$) on atoms based on electronegativity differences
  2. Measure distance ($r$) between partial charges
  3. Multiply partial charge ($Q$) by distance ($r$) to obtain dipole moment ($\mu$)

Prediction of molecular dipoles

• Molecular geometry and symmetry crucial in determining presence and direction of dipole moments
  • Molecules with symmetric charge distribution are nonpolar
    • CO$_2$: linear geometry, opposing bond dipoles cancel out
    • CCl$_4$: tetrahedral geometry, bond dipoles cancel out
    • BF$_3$: trigonal planar geometry, bond dipoles cancel out
  • Molecules with asymmetric charge distribution are polar
    • H$_2$O: bent geometry, bond dipoles and lone pairs create net dipole
    • NH$_3$: trigonal pyramidal geometry, bond dipoles and lone pair create net dipole
    • CH$_3$Cl: tetrahedral geometry, C-Cl bond dipole creates net dipole
• Predicting presence and direction of dipole moments:
  1. Identify molecular geometry using VSEPR theory
  2. Determine individual bond dipole moments based on electronegativity differences
  3. Consider contribution of lone pairs to overall molecular polarity
  4. Visualize vector sum of all bond dipole moments and lone pair contributions
     • Vector sum is zero, molecule is nonpolar
     • Vector sum is non-zero, molecule is polar, direction of dipole moment is from positive to negative end of molecule
• Bond angle affects the overall dipole moment of a molecule

Molecular Structure and Polarity

• Lewis structures help visualize electron distribution and predict molecular polarity
• Intermolecular forces are influenced by molecular polarity
• Polarity scale helps compare relative polarity of different molecules