Overview
This lecture covers the differences between ionic and molecular (covalent) compounds, covalent bond formation and dissociation energy, and the concepts of bond polarity and electronegativity.
Ionic vs. Molecular Compounds
- Ionic compounds consist of ions in a crystal lattice; formulas represent ratios, not discrete units.
- Molecular compounds are made of discrete molecules held by covalent bonds; formulas reflect actual molecular structure (e.g., Hâ‚‚O).
- Ionic compounds have higher melting and boiling points than molecular compounds.
- At room temperature, ionic compounds are usually solids; molecular compounds can be solids, liquids, or gases.
- Ionic bonds involve electron transfer; covalent bonds involve electron sharing.
Covalent Bond Formation & Dissociation Energy
- Covalent bonds form when atoms share electrons to complete their valence shells (octets).
- Two atoms form a covalent bond when attractive forces between nuclei and electrons overcome repulsive forces.
- Bond forms at the distance where energy released (energy minimum) is maximized.
- Energy is released when a bond forms; energy (bond dissociation energy) must be input to break a bond.
- Bond dissociation energy equals the energy released during bond formation but with opposite sign (energy released is negative; energy required is positive).
Bond Polarity and Electronegativity
- Electronegativity is an atom’s ability to attract shared electrons in a bond.
- Electronegativity increases left-to-right and bottom-to-top on the periodic table (F is most electronegative, Cs is least).
- Ionic bonds: electronegativity difference ≥ 1.9.
- Covalent bonds: electronegativity difference < 1.9; subdivided into:
- Polar covalent: 0.5 ≤ difference < 1.9 (unequal sharing; partial charges develop).
- Nonpolar covalent: difference < 0.5 (equal sharing; no partial charges).
- In polar bonds, partial negative charge is on the more electronegative atom; partial positive on the less.
- Direction of bond polarity is shown with an arrow pointing toward the partial negative end (tail with plus sign at partial positive).
Quick Rules for Bond Type
- Metal + non-metal: ionic bond.
- C bonded to H or an element bonded to itself: nonpolar covalent.
- All other combinations: polar covalent.
Key Terms & Definitions
- Molecular Compound — Compound made of molecules with atoms held by covalent bonds.
- Covalent Bond — Bond formed by sharing electrons between atoms.
- Bond Dissociation Energy — Energy required to break a bond; equal in magnitude to energy released when bond forms.
- Electronegativity — Atom’s ability to attract electrons in a bond.
- Polar Covalent Bond — Bond with unequal electron sharing, resulting in partial charges.
- Nonpolar Covalent Bond — Bond with equal electron sharing, no partial charges.
- Partial Charge (δ+/δ−) — Indicates a small positive or negative charge due to unequal sharing in a polar bond.
Action Items / Next Steps
- Print and use the provided handout summarizing bonding and trends for homework.
- Review electronegativity trends and bond type classification for upcoming assignments and exams.