Structures and Physical Properties
Quick Notes
- Substances form different types of structure depending on their bonding:
- Giant ionic lattice: repeating pattern of ions (e.g. NaCl)
- Giant covalent lattice: 3D network of atoms bonded covalently (e.g. diamond, SiO2)
- Giant metallic lattice: metal cations in a sea of delocalised electrons (e.g. Mg, Cu)
- Simple molecular structure: small molecules held together by weak IMFs (e.g. I2, ice)
- Carbon forms different structures:
- Diamond: tetrahedral, strong covalent bonds, no conductivity
- Graphite: layers of hexagons, delocalised electrons, conducts
- Graphene: single layer of graphite, excellent conductor
- Physical properties depend on the particles present (atoms, ions, molecules), the structure and bonding type, and intermolecular forces (for molecular substances).
Full Notes
Giant Ionic Lattices
Ionic compounds (e.g. NaCl, MgO) form giant ionic lattices, where each positive ion is surrounded by negative ions.
Key properties of giant ionic lattices include:
- High melting/boiling points: strong electrostatic attractions between oppositely charged ions
- Conduct electricity when molten or in solution (ions are free to move)
- Soluble in water: water hydrates ions
- Brittle: shifting layers brings like charges together, causing repulsion and fracture
Example Sodium chloride
Giant Covalent Lattices
Some non-metal elements and compounds form giant covalent structures.
These have no maximum size and all atoms are covalently bonded in a repeating pattern. Common examples include diamond, graphite and silicon dioxide.
Diamond
In diamond, carbon atoms are bonded in a tetrahedral structure, with each carbon atom forming 4 bonds.
Key properties:
- Very hard, very high melting point
- Does not conduct electricity (no delocalised electrons)
Graphite
In graphite, carbon atoms form 3 bonds to other carbon atoms, forming hexagonal layers with delocalised electrons between the layers. Weak forces of attraction exist between layers.
Key properties:
- Conducts electricity (delocalised electrons between layers)
- Soft and slippery (layers held by weak forces, slide easily)
Silicon(IV) oxide (SiO2)
In Silicon(IV) oxide (SiO2), silicon and oxygen atoms are bonded together in a tetrahedral arrangement, with strong Si–O covalent bonds.
Key properties:
- Hard, high melting point
- Does not conduct electricity
Giant Metallic Lattices
Metals form giant metallic structures, with metal cations in a sea of delocalised electrons.
Key properties include:
- High melting points (strong metallic bonding)
- Conduct electricity in solid and liquid states
- Malleable and ductile (layers can slide)
Example Magnesium (Mg)
Simple Molecular Structures
Some covalently bonded substances exist as molecules held together by intermolecular forces, such as iodine (I2) and water (H2O).
Iodine (I2)
Iodine (I2) is made up of non-polar I2 molecules, held together by London forces.
Key properties:
- Low melting and boiling point
- Does not conduct electricity
- Sublimes from solid to gas
Ice (H2O)
Ice (H2O) is made up of water molecules (H2O) which are held together by hydrogen bonds, forming a regular open structure.
Key properties:
- Lower density than liquid water (ice floats)
- Low melting point relative to ionic or covalent lattices
Predicting Structure and Physical Properties
You should be able to:
- Predict bonding and structure from chemical formula or description
- Use data (e.g. boiling point, solubility, conductivity) to deduce the bonding/structure of a substance
- Explain physical properties in terms of bonding and forces
Summary
| Structure Type | Examples | Key Properties |
|---|---|---|
| Giant ionic lattice | NaCl, MgO | High mp/bp, conduct when molten/aqueous, soluble in water, brittle |
| Giant covalent lattice | Diamond, graphite, SiO2 | Very high mp, hardness, conductivity depends on delocalised electrons |
| Giant metallic lattice | Mg, Cu | High mp, conduct, malleable, ductile |
| Simple molecular | I2, H2O | Low mp/bp, non-conductors, ice less dense than liquid water |