Overview
This lecture covers the different types of bonding in materials, properties arising from atomic structure, and how models help us represent substances, all within the context of the periodic table and chemical formulas.
The Periodic Table: Structure and Development
- Mendeleev arranged elements by increasing atomic mass, grouping similar properties and predicting undiscovered elements.
- Modern table is ordered by atomic number, showing elements in groups (columns) with similar properties and periods (rows) with the same electron shells.
- Metals (left side) tend to lose electrons; non-metals (right side) tend to gain electrons; noble gases are inert.
Ions and Ionic Bonding
- Ions are atoms or groups of atoms with a charge, formed by losing (cations, metals) or gaining (anions, non-metals) electrons.
- Ionic compounds have a lattice structure with strong electrostatic attraction between oppositely charged ions.
- Ionic compounds have high melting points and conduct electricity when molten or dissolved, but not when solid.
- Common ions include oxide (O²⁻), hydroxide (OH⁻), halides (F⁻, Cl⁻, Br⁻, I⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), and sulfate (SO₄²⁻).
Covalent Bonding and Molecular Compounds
- Covalent bonds involve sharing pairs of electrons between non-metal atoms, forming molecules.
- Simple molecules are small, have low melting/boiling points due to weak intermolecular forces, and do not conduct electricity.
- Dot-and-cross diagrams for covalent bonds show only outer shell electrons and shared pairs.
Giant Covalent Structures and Allotropes of Carbon
- Diamond: each carbon atom bonded to four others in a 3D lattice, very hard, high melting point, does not conduct electricity.
- Graphite: layers of carbons bonded in hexagonal rings, conducts electricity via delocalised electrons, layers slide easily.
- Graphene is a single layer of graphite; fullerenes ("buckyballs") and nanotubes are other carbon allotropes with unique electrical and mechanical properties.
Polymers
- Polymers are long chains of repeating monomer units, bonded by strong covalent bonds, usually with a carbon backbone.
- The repeating unit is used to represent long polymer chains in diagrams.
Metallic Bonding and Properties
- Metals consist of a lattice of positive ions surrounded by a sea of delocalised electrons.
- This structure explains metals’ high melting points, conductivity, malleability, and ductility.
- Alloys are mixtures of metals with irregular layers, making them harder and less malleable.
- Non-metals are dull, have low melting/boiling points, and are poor conductors.
Bonding Models and Their Limitations
- Dot and cross diagrams, chemical formulas, space-filling, ball-and-stick, and straight-line models each represent different bonding aspects but have limitations (e.g. not showing charges, relative sizes, or formation of bonds).
Key Terms & Definitions
- Ion — Atom or group with a positive or negative charge from losing or gaining electrons.
- Cation — Positively charged ion (lost electrons).
- Anion — Negatively charged ion (gained electrons).
- Lattice — Regular arrangement of particles in a solid.
- Covalent bond — Bond made by sharing electron pairs.
- Intermolecular forces — Weak forces between molecules.
- Allotrope — Different structural forms of the same element.
- Polymer — Large molecule made of repeating monomer units.
- Alloy — Mixture of a metal with other elements for improved properties.
Action Items / Next Steps
- Practice drawing dot-and-cross diagrams for given ionic and covalent substances.
- Review and memorise common ion names and formulas.
- Read about properties of metals, non-metals, and allotropes of carbon in textbook.
- Complete assigned questions on bonding and properties.