Overview
This section explains the concepts of polar covalent bonds and electronegativity, focusing on how electron sharing and atom types influence molecular polarity.
Types of Chemical Bonds
- Bonds can be classified as ionic, covalent, or polar covalent depending on electron distribution.
- Ionic bonds involve complete electron transfer (e.g., NaCl).
- Covalent bonds involve equal sharing of electrons (e.g., C–C in ethane).
- Most bonds are polar covalent, with electrons unevenly shared due to differences in atom attraction.
Bond Polarity and Electronegativity
- Bond polarity arises when one atom attracts bonding electrons more strongly than the other.
- Electronegativity is an atom's ability to attract shared electrons in a bond.
- Fluorine is the most electronegative element (EN = 4.0); cesium is the least (EN = 0.7).
- Electronegativity increases left to right on the periodic table and decreases top to bottom.
- Carbon's electronegativity is intermediate (EN = 2.5).
Determining Bond Type by Electronegativity Difference
- EN difference < 0.5: nonpolar covalent bond.
- EN difference 0.5–2.0: polar covalent bond.
- EN difference > 2.0: ionic bond.
Examples of Bond Polarization
- C–H bonds are relatively nonpolar due to similar EN values.
- C–O and C–N bonds are polar, with electrons drawn toward O or N (higher EN).
- C–Li or C–Mg bonds are polar, with electrons drawn toward C (higher EN than metals).
- Partial charges are indicated as δ+ (electron-poor) and δ– (electron-rich).
Visualizing Polarity
- Dipole arrows point from positive (electron-poor) to negative (electron-rich) regions.
- Electrostatic potential maps use color (red for electron-rich, blue for electron-poor) to show charge distribution.
Inductive Effects
- The inductive effect is the shift of electrons in a bond due to electronegativity differences.
- Metals inductively donate electrons; nonmetals inductively withdraw electrons.
- Inductive effects help explain chemical reactivity.
Key Terms & Definitions
- Polar covalent bond — a bond with unequal sharing of electrons between atoms.
- Electronegativity (EN) — an atom's tendency to attract shared electrons in a bond.
- Partial charge (δ+, δ–) — slight charge assigned to atoms in a polar bond.
- Inductive effect — electron shifting through σ bonds due to electronegativity differences.
- Electrostatic potential map — a visual showing electron distribution in a molecule.
Action Items / Next Steps
- Complete Problems 2-1 to 2-4 (identify more electronegative elements, assign bond polarity, rank bonds by polarity, interpret electrostatic maps).
- Review Figures 2.2–2.4 for visual examples of bond types and charge distribution.