How does electronegativity help predict polarity of molecules?

Short Answer

Electronegativity is the ability of an atom to attract shared electrons in a covalent bond.

  • When two atoms in a bond have different electronegativities, the electrons are unequally shared, creating a dipole moment → the bond becomes polar.
  • If the electronegativity difference is small or zero, electrons are shared equally → non-polar bond.
  • Molecular polarity depends on both bond polarity and molecular geometry.
Detailed Explanation :

Electronegativity

  • Definition: Electronegativity is a measure of an atom’s tendency to attract bonding electrons.
  • Scale: Usually measured on the Pauling scale.
    • Fluorine: 3.98 (most electronegative)
    • Cesium: 0.79 (least electronegative)

Bond Polarity

  1. Electronegativity Difference (ΔEN):
    • ΔEN = |EN₁ − EN₂|
    • ΔEN > 0.5–1.7 → polar covalent bond
    • ΔEN ≈ 0 → non-polar covalent bond
    • ΔEN > 2 → ionic bond
  2. Dipole Moment:
    • Polar bonds have partial positive (δ⁺) and negative (δ⁻) charges.
    • Dipole points towards more electronegative atom.

Examples:

  • HCl: ΔEN = 3.16 − 2.20 ≈ 0.96 → polar bond → H δ⁺, Cl δ⁻
  • Cl₂: ΔEN = 3.16 − 3.16 = 0 → non-polar bond

Molecular Polarity

  • A molecule is polar if it has polar bonds and an asymmetric shape.
  • Vector sum of dipoles:
    • If dipoles cancel out → non-polar molecule
    • If dipoles do not cancel → polar molecule

Examples:

  1. Water (H₂O):
    • Bonds O–H are polar (ΔEN ≈ 1.4)
    • Bent shape → dipoles do not cancel → polar molecule
  2. Carbon Dioxide (CO₂):
    • Bonds C=O are polar (ΔEN ≈ 0.9)
    • Linear shape → dipoles cancel out → non-polar molecule

Predicting Polarity Using Electronegativity

  1. Step 1: Determine ΔEN for each bond.
  2. Step 2: Assign partial charges (δ⁺ and δ⁻) based on electronegativity.
  3. Step 3: Consider molecular geometry to see if dipoles cancel or add.
  4. Step 4: Conclude molecule polarity.

Key Points:

  • Greater ΔEN → stronger bond polarity
  • Asymmetric molecules → more likely polar
  • Symmetric molecules with polar bonds → may be non-polar

Applications

  • Predicting solubility: Polar molecules dissolve in polar solvents (e.g., water), non-polar molecules in non-polar solvents (e.g., hexane)
  • Understanding intermolecular forces: hydrogen bonding, dipole-dipole, London dispersion
  • Explaining chemical reactivity and physical properties
Conclusion

Electronegativity is a critical tool to predict bond and molecular polarity.

  • Differences in electronegativity create polar bonds, while molecular shape determines if a molecule is overall polar or non-polar.
  • By analyzing ΔEN and geometry, chemists can predict solubility, reactivity, and intermolecular interactions, making electronegativity essential in understanding chemical behavior.