Overview
This lecture introduces electronegativity, explains how it differs from electron affinity, and discusses how differences in electronegativity determine bond polarity and influence molecular properties.
Electron Affinity vs. Electronegativity
- Electron affinity (EA) is the energy released or required when an atom gains an electron (measured for atoms in the gas state).
- Electronegativity is an atom’s ability to attract shared electrons in a chemical bond.
Electronegativity Trends
- Electronegativity increases from left to right across a period on the periodic table.
- Electronegativity decreases down a group.
- Noble gases are not assigned electronegativity values.
- The effective nuclear charge (Z_eff) increases across a period, pulling electrons closer and increasing electronegativity.
Bond Polarity and Electronegativity Differences
- The difference in electronegativity (ΔEN) between two atoms determines bond type:
- ΔEN = 0 to 0.4: Nonpolar covalent bond (equal or nearly equal sharing).
- ΔEN = 0.5 to 1.9: Polar covalent bond (unequal sharing, partial charges).
- ΔEN ≥ 2.0: Ionic bond (electron transfer).
- Partial charges are denoted by lowercase delta (δ+ for slight positive, δ– for slight negative).
Examples and Calculations
- Calculate ΔEN by subtracting smaller electronegativity from the larger: ΔEN = |EN_large – EN_small|.
- Cl–Cl: ΔEN = 0; pure covalent, no polarity.
- H–Cl: ΔEN = 0.9; polar covalent, Cl is δ– and H is δ+.
- Na–Cl: ΔEN = 2.1; ionic, Na is δ+ and Cl is δ–.
- C–O: ΔEN = 1.0; polar covalent, O is δ– and C is δ+.
- C–H: ΔEN = 0.4; nonpolar covalent, very small dipole.
Bond Dipoles and Molecular Properties
- Bond dipole: Arrow points towards the more electronegative atom (from δ+ to δ–).
- Physical properties (solubility, melting point, etc.) are influenced by bond polarity and molecular interactions.
- Polar molecules (e.g., HCl, HF) dissolve in water; nonpolar molecules (e.g., H₂, CH₄) do not.
Biological Relevance
- C–O and N–H bonds are common in biological molecules (proteins, DNA).
- Polar covalent bonds contribute to the structural strength of proteins (hair, nails, collagen).
Key Terms & Definitions
- Electron Affinity (EA) — energy change when an atom gains an electron in the gas phase.
- Electronegativity (EN) — tendency of an atom to attract bonding electrons.
- Effective Nuclear Charge (Z_eff) — net positive charge felt by valence electrons.
- ΔEN (Delta EN) — difference in electronegativity between two bonded atoms.
- Bond Dipole — directional arrow showing partial charge separation in a bond.
- Partial Charge (δ+, δ–) — slight positive or negative charge due to unequal sharing of electrons.
Action Items / Next Steps
- Memorize electronegativity trends and key ΔEN cutoffs for bond type classification.
- Practice calculating ΔEN and classifying bond types using a periodic table.
- Review and be able to identify and draw bond dipoles on molecules.