Overview
This lecture covers scientific models in chemistry, emphasizing their purpose, limitations, and evolution, with a focus on chemical bonds and Lewis structures.
Scientific Models: Purpose and Types
- Models help us understand complex, large, small, or dangerous things by representing them in a more manageable form.
- Scientific models can be physical or conceptual (e.g., ball-and-stick models, musical notation).
- All models are imperfect representations of reality.
Models in Chemistry
- Chemists use models to visualize atoms and chemical bonds.
- The ball-and-stick model illustrates atoms as spheres and bonds as sticks, helpful but simplistic.
- More advanced models portray bonds as electrons shared between atoms.
Evolution of the Bonding Model
- Early models (Newton) proposed atoms stuck together physically (like hooks or Velcro).
- 19th-century models focused on positive and negative charges (like magnets).
- Discovery of electrons led to new understanding of bonds as electron interactions.
- Gilbert Lewis (1916) described covalent bonds as shared pairs of electrons (Lewis structures).
Lewis Structures and the Octet Rule
- Lewis structures use lines for bonds and dots for lone electrons.
- Bonds consist of shared pairs of valence electrons.
- Atoms are most stable with filled outer shells, usually 8 electrons (octet rule), with exceptions for hydrogen and certain elements.
- Ionic bonds (e.g., NaCl) involve electron transfer; covalent bonds (e.g., Hâ‚‚O, COâ‚‚, Nâ‚‚) involve electron sharing.
Drawing Lewis Structures: Steps and Examples
- Count total valence electrons for all atoms involved.
- Arrange atoms (central atom often less electronegative, or unique).
- Use pairs of electrons to form bonds; allocate remaining electrons as lone pairs.
- Form double or triple bonds if needed to satisfy octet with available electrons.
- Example: Hâ‚‚O (2 single bonds, 2 lone pairs), COâ‚‚ (2 double bonds), Nâ‚‚ (triple bond).
Modern Model of Chemical Bonding
- Linus Pauling applied quantum mechanics to chemical bonding, seeing bonds as overlaps of electron clouds.
- Pauling's model replaced simple electron sharing with a more accurate quantum view.
Key Terms & Definitions
- Model — A representation of something else to aid understanding.
- Covalent Bond — A chemical bond formed by sharing electron pairs between atoms.
- Ionic Bond — A chemical bond formed by the transfer of electrons from one atom to another.
- Valence Electrons — Electrons in the outermost shell involved in bonding.
- Lewis Structure — A diagram showing covalent bonds as lines and lone electrons as dots.
- Octet Rule — Atoms are stable with 8 electrons in their outer shell.
- Lone Pair — A pair of valence electrons not shared with another atom in a bond.
- Double/Triple Bond — Bonds where two or three pairs of electrons are shared between two atoms.
Action Items / Next Steps
- Practice drawing Lewis structures for various molecules.
- Prepare to learn more about atomic orbitals and quantum mechanics in bonding.