Overview
This lecture covers the concepts of bonding, electronegativity, and how the difference in electronegativity determines whether a compound is ionic or molecular (covalent), including key trends and examples.
Types of Compounds
- Compounds are made from more than one type of atom, combined as molecules or ionic compounds.
- Ionic compounds have formula units representing the simplest whole number ratio of ions (e.g., NaCl: 1 Na⁺ per 1 Cl⁻).
- Molecules have distinct, countable groups of atoms (e.g., H₂O: 2 H and 1 O per molecule).
Electronegativity and Bonding
- Electronegativity is the ability of an atom to attract electrons in a bond.
- The difference in electronegativity (ΔEN) between two bonded atoms determines bond type.
- Calculate ΔEN by subtracting the lower value from the higher value for the bonded atoms.
- ΔEN ranges from 0 (equal sharing) up to about 3 (complete transfer).
Bond Types Based on Electronegativity Difference
- ΔEN 0: Non-polar covalent bond (pure, equal sharing).
- ΔEN 0 < x < 1.7: Polar covalent bond (unequal sharing; partial charges result).
- ΔEN ≥ 1.7: Ionic bond (electrons mostly transferred, ions formed).
Classification and Exceptions
- The 1.7 value is a guideline; real bonds fall on a spectrum.
- Some compounds have properties between ionic and molecular, depending on their ΔEN and actual observed properties.
Lewis Structures and Examples
- Draw Lewis structures to represent shared (covalent) or transferred (ionic) electrons.
- Assign partial charges (δ⁺, δ⁻) for polar covalent bonds.
- Example: In CF₄, each C-F bond is polar covalent with F more electronegative (δ⁻).
Electronegativity Trends in the Periodic Table
- Electronegativity increases up and to the right across the periodic table; fluorine is the most electronegative.
- Noble gases typically have no electronegativity value as they rarely bond.
- The trend is due to increased nuclear charge and decreased shielding.
Key Terms & Definitions
- Electronegativity — The ability of an atom to attract electrons in a chemical bond.
- Ionic bond — Bond formed by the transfer of electrons (ΔEN ≥ 1.7).
- Covalent bond — Bond formed by sharing electrons (ΔEN < 1.7).
- Non-polar covalent — Bond with equal sharing of electrons (ΔEN = 0).
- Polar covalent — Bond with unequal sharing of electrons, creating partial charges (0 < ΔEN < 1.7).
- Partial charge (δ⁺, δ⁻) — Small charge difference due to uneven electron sharing.
Action Items / Next Steps
- Practice calculating ΔEN for given compounds using a periodic table.
- Draw Lewis structures and assign bond types and charges based on ΔEN.
- Review periodic table electronegativity trends and be able to explain the reasons for them.