Overview
This lecture reviews AP Chemistry Unit 2 topics: types of chemical bonds, bond polarity, potential energy diagrams, bond energies and lengths, periodic trends, and factors influencing Coulombic (ionic) attractions.
Types of Chemical Bonds & Electronegativity
- Electronegativity increases left to right across a period, and decreases down a group.
- Non-polar covalent bonds form when valence electrons are shared equally between atoms of similar electronegativity.
- Polar covalent bonds form when electrons are shared unequally; the more electronegative atom gains a partial negative charge.
- Greater electronegativity differences in single bonds lead to greater bond dipoles (polarity).
- The distinction between ionic and covalent bonding is a continuum, not an absolute.
- Bonds between metals and non-metals are generally ionic; between non-metals, covalent.
Assigning Partial Charges & Ionic Character
- In a polar bond, the atom with higher electronegativity carries the partial negative charge.
- Bond ionic character increases as the electronegativity difference between bonded atoms increases.
- Properties of compounds (not just element types) are best for characterizing metallic bonding.
Bond Polarity & Order
- Greater differences in electronegativity yield more polar bonds.
- To rank bonds by decreasing polarity, look for decreasing electronegativity differences.
- Bonds with higher bond orders (triple > double > single) are shorter and have higher bond energies.
Potential Energy Diagrams & Bond Energies
- Potential energy vs. internuclear distance graphs show equilibrium bond length (x-axis minimum) and bond energy (y-axis minimum).
- Bond energy equals the energy needed to break a bond or released when forming it (record as a positive value).
- Smaller atoms form shorter bonds; higher bond order means shorter, stronger bonds.
- Longer bonds (from larger atoms) are weaker and have lower bond energies.
Coulombic Attraction & Lattice Energy
- Strength of ionic interactions increases with larger ion charges and smaller ionic radii.
- Lattice energy is greater for ions with higher charges and smaller sizes.
- Bonds between small, highly charged ions (e.g., Ca²⁺ and O²⁻) have the strongest Coulombic attractions.
Key Terms & Definitions
- Electronegativity — Atom’s tendency to attract shared electrons in a bond.
- Bond Polarity — Measure of unequal electron sharing in a bond.
- Bond Energy — Energy required to break a bond; also energy released on bond formation.
- Bond Order — Number of shared electron pairs (single, double, triple).
- Lattice Energy — Energy required to separate ions in an ionic solid.
- Coulombic Attraction — Electrostatic force between oppositely charged ions.
Action Items / Next Steps
- Review and practice problems for topics 2.3 and 2.4.
- Check the provided online packet for further practice.
- Pay attention to periodic trends and practice interpreting potential energy graphs.
- Ensure understanding of how to apply electronegativity trends for partial charge assignments.