Lecture Notes: Distinguishing Polar and Nonpolar Molecules

Introduction

• Ways to distinguish polar from nonpolar molecules.
• Importance of recognizing characteristics of nonpolar molecules first.

Nonpolar Molecules

1. Single Element Composition

   • Monoatomic or diatomic molecules made of one element are nonpolar.
   • Examples:
     • Diatomic: N2, O2, Cl2, F2, H2.
     • Monatomic: Noble gases like helium, neon, argon, xenon.

2. Molecules with Only Carbon and Hydrogen

   • Known as hydrocarbons.
   • Examples: Methane, Alkanes (e.g., Ethane - C2H6), Ethene (CH2=CH2).
   • Carbon-hydrogen bonds are nonpolar.

3. Symmetrical Molecules

   • Symmetry in molecules often leads to nonpolarity.
   • Examples:
     • Carbon tetrafluoride, sulfur hexabromide, carbon dioxide, phosphorus trichloride, BH3.

4. Electronegativity Difference

   • Nonpolar if EN difference < 0.5.
   • Examples: Iodine monobromide (EN difference ~0.3), bromine monochloride.

Polar Molecules

1. Hydrogen Bonding

   • Molecules with hydrogen directly attached to nitrogen, oxygen, or fluorine.
   • Examples: Water, NH3, HF, CH3OH, CH3NH2, CH3COOH.

2. Lack of Symmetry and Polar Bonds

   • Presence of polar bonds without symmetry often results in polarity.
   • Examples:
     • Molecules with carbon-oxygen bonds where carbon has 2.5 electronegativity and oxygen 3.5.
     • Asymmetrical molecules (e.g., sulfur dioxide).

3. Molecular Shape and Dipole Moments

   • Shape can influence polarity.
   • Linear shapes (e.g., CO2) allow dipole moments to cancel, leading to nonpolarity.
   • Bent shapes (e.g., SO2) prevent cancellation, leading to polarity.

4. NH3 Example

   • Polarity due to hydrogen bonding and molecular geometry.
   • Dipole moments do not cancel, enhancing polarity.

Summary

• Nonpolar molecules typically:
  • Are diatomic with the same element, monatomic noble gases, pure hydrocarbons, or symmetrical.
• Polar molecules:
  • Lack symmetry, have hydrogen bonding, and exhibit significant electronegativity differences.
• Use electronegativity values and molecular symmetry as key indicators.

Conclusion

• Understanding the properties that define polar and nonpolar molecules can aid in chemistry courses.
• Importance of practicing these identification methods.