Distinguishing Polar and Non-Polar Molecules

Non-Polar Molecules

• Single Element Molecules: Molecules composed of one element are non-polar.

  • Examples: N₂, O₂, Cl₂, F₂, H₂ (diatomic molecules).
  • Monoatomic noble gases like helium, neon, argon, xenon.

• Hydrocarbons: Molecules containing only carbon and hydrogen are non-polar.

  • Examples: Methane, alkanes (e.g., ethane, C₂H₆, CH₂=CH₂).
  • The C-H bond is non-polar.

• Symmetrical Molecules: Molecules with symmetrical geometry.

  • Examples: Carbon tetrafluoride (CF₄), sulfur hexabromide, carbon dioxide (CO₂), phosphorus trichloride (PCl₃), boron trihydride (BH₃).
  • Symmetrical molecules have identical outer elements and are non-polar.

Polar Molecules

• Unsymmetrical Molecules with Polar Bonds:

  • Molecules lacking symmetry with a polar bond are polar.
  • Example: Carbon monoxide (CO), sulfur dioxide (SO₂).

• Electronegativity Difference:

  • If the electronegativity difference is greater than 0.5, the bond is polar.
  • Example: Iodine monobromide (I-Br), bromine monochloride (Br-Cl) are non-polar due to a small electronegativity difference.

• Hydrogen Bonding:

  • If hydrogen is bonded directly to nitrogen, oxygen, or fluorine, the molecule is polar.
  • Examples: Water (H₂O), ammonia (NH₃), hydrogen fluoride (HF), methanol (CH₃OH).

• Molecules Lacking Symmetry:

  • If a molecule lacks symmetry and has polar bonds, it tends to be polar.
  • Example: Methanol (CH₃OH), due to the presence of oxygen.

• Molecular Shape Influence:

  • Shape can affect polarity; for example, CO₂ is linear and non-polar due to canceling dipole moments, whereas SO₂ is bent and polar.

Key Points

• Non-Polar Criteria: Look for diatomic molecules, monoatomic noble gases, hydrocarbons, and symmetrical molecules.
• Polar Criteria: Check for hydrogen bonds, lack of symmetry, and significant electronegativity differences (>0.5).
• Polarity Determination: Draw dipole moment arrows and check if they cancel.

Conclusion

• This guide should aid in quickly distinguishing between polar and non-polar molecules, focusing on molecular composition, symmetry, and electronegativity.