Bond Polarity & Electronegativity Notes

Bond Polarity & Electronegativity

• Electronegativity (EN)
  • A dimensionless measure of an atom’s ability to attract shared electrons.
  • The higher the EN, the stronger the pull on bonding electrons.
• Bond polarity depends on the EN difference $\Delta EN = |EN<em>A - EN</em>B|$.
  • $\Delta EN = 0$ → perfectly even sharing.
  • Larger $\Delta EN$ → greater electron displacement, producing partial charges $\delta^+$ and $\delta^-$.
• Key rule ("bigger gap, bigger pull"):
  • “The greater the difference in electronegativity, the more polar the bond.”

Electronegativity‐Difference Classification

• (Typical cutoffs; refer to your course’s exact table.)
  • $0 \le \Delta EN \le 0.4$ → Non-polar covalent (electron cloud evenly distributed).
  • 0.4 < \Delta EN \le 1.7 → Polar covalent (partial charges form, but electrons still shared).
  • \Delta EN > 1.7 → Ionic (electron transfer, not sharing).

Worked Example

1. Look up chlorine’s EN: $EN_{Cl} = 3.0$.
2. For a Cl–Cl bond (diatomic $Cl_2$): $\Delta EN = |3.0 - 3.0| = 0$.
3. Classify: $\Delta EN = 0$ → non-polar covalent.
   • Electrons are equally distributed; there is no net positive or negative end.

Visual / Metaphor Mentioned

• “Polar bear stranded on an iceberg, dissolving in water”
  • Used to dramatize polarity (anything polar tends to interact with polar water).
• “Regular bear” contrasts with “polar bear” to reinforce the idea of non-polar vs. polar species.

Molecular Polarity vs. Bond Polarity

• A molecule can contain polar bonds yet be non-polar overall if its shape is symmetrical (vector sum of dipoles cancels).
  • Determined using VSEPR (Valence Shell Electron Pair Repulsion) theory (spoken in transcript as “V-expert”).
• Example idea alluded to (not explicitly named in clip): $CO_2$ has two polar C=O bonds, but linear geometry → net dipole $=0$.

Consequences & Physical Properties

• Net polarity influences intermolecular forces:
  • Stronger dipole–dipole or hydrogen bonding → higher boiling/melting points.
  • Non-polar molecules rely on weaker London dispersion forces.
• Thus, bond-polarity analysis helps predict bp/mp trends, solubility ("like dissolves like"), and other physical behavior.

Key Takeaways

• Always start by calculating $\Delta EN$.
• Consult the cut-off table to label the bond (non-polar, polar covalent, ionic).
• Use VSEPR shapes to decide if individual bond dipoles cancel.
• Remember: Polarity is a vector property—both magnitude (from $\Delta EN$) and direction (from geometry) matter.
• Physical properties (bp, mp, solubility) correlate strongly with overall polarity.