Bonding and Physical Properties
Specification Reference Physical Chemistry, Bonding 3.1.3.4
Quick Notes
- There are four main types of crystal structures:
- Ionic:
High melting point, conducts when molten, soluble in water (e.g., NaCl) - Metallic:
High melting point, conducts electricity, malleable (e.g., Mg) - Macromolecular (Giant Covalent): Very high melting point, variable conductivity (e.g., Diamond, Graphite)
- Molecular (Simple Molecular):
Low melting point, does not conduct (e.g., Ice, I2) - Melting point and conductivity depend on bonding and structure.
- Energy changes during state changes arise from overcoming bonds or intermolecular forces.
Full Notes
The Four Types of Crystal Structures
There are four main types of crystal structure for substances in a solid state:
| Structure Type | Example | Bonding Type between particles | Key Properties |
|---|---|---|---|
| Ionic | Sodium Chloride (NaCl) | Ionic | High melting point, conducts when molten, soluble in water |
| Metallic | Magnesium (Mg) | Metallic | High melting point, conducts electricity, malleable |
| Macromolecular (Giant Covalent) | Diamond, Graphite | Covalent | Very high melting point, variable conductivity |
| Molecular (Simple Molecular) | Ice, Iodine (I2) | Intermolecular (van der Waals, hydrogen bonding) | Low melting point, does not conduct |
Matt’s exam tip
Remember structure and bonding refer to different things - be careful when asked about either (or both) in a question. For example, both macromolecular (giant covalent) and molecular substances contain covalent bonding however have very different structures and properties.
Structure and Properties of AQA-Specified Examples
1. Sodium Chloride (NaCl) – Ionic Structure
Strong electrostatic forces between Na+ and Cl− ions form a giant ionic lattice.
- High melting point (~800°C) → large amounts of energy required to break ionic bonds.
- Soluble in water → water molecules separate ions during solvation.
- Conducts electricity when molten or dissolved → free ions carry charge.
2. Magnesium (Mg) – Metallic Structure
Strong electrostatic forces between Mg2+ ions and a sea of delocalised electrons form a giant metallic lattice.
- High melting point (~650°C) → strong metallic bonds require much energy to break.
- Good electrical conductor → delocalised electrons carry charge.
- Malleable and ductile → layers of ions can slide without breaking metallic bonds.
3. Diamond (C) – Giant Covalent Structure
Each carbon atom bonds to four others in a 3D tetrahedral lattice of unlimited size.
- Very high melting point (~3550°C) → covalent bonds need very high energy to break.
- Does not conduct electricity → no free electrons or ions.
- Very hard → strong bonding in all directions.
4. Graphite (C) – Giant Covalent Structure
Each carbon bonds to three others, forming hexagonal layers with delocalised electrons and weak forces between layers.
- High melting point (~3700°C) → strong covalent bonds in layers.
- Conducts electricity → delocalised electrons move freely between layers.
- Soft and slippery → layers slide easily due to weak forces between them.
5. Ice (H2O) – Molecular Structure
Water molecules form a regular lattice held together by hydrogen bonds.
- Low melting point (~0°C) → hydrogen bonds weaker than atomic bonds.
- Less dense than water → open structure forms when frozen.
- Does not conduct electricity → no free electrons or ions.
6. Iodine (I2) – Molecular Structure
Weak van der Waals forces hold I2 molecules in a simple molecular lattice.
- Low melting point (~114°C) → weak intermolecular forces require little energy to break.
- Sublimes → solid iodine changes directly to gas.
- Does not conduct electricity → no delocalised electrons or ions.
Relating Structure to Melting Point and Conductivity
| Example | Structure Type | Melting Point | Conducts Electricity? | Energy Needed for State Change |
|---|---|---|---|---|
| Sodium Chloride (NaCl) | Ionic | High | Yes (molten/solution) | High |
| Magnesium (Mg) | Metallic | High | Yes | High |
| Diamond | Giant Covalent | Very High | No | Very High |
| Graphite | Giant Covalent | Very High | Yes | Very High |
| Ice (H2O) | Molecular | Low | No | Low |
| Iodine (I2) | Molecular | Low | No | Low |
Summary
- Ionic structures have high melting points, conduct when molten/dissolved.
- Metallic structures have high melting points, good conductors, malleable.
- Giant covalent structures have very high melting points, variable conductivity (diamond = insulator, graphite = conductor).
- Molecular structures have low melting points, do not conduct, weak intermolecular forces.
- Physical properties (melting point, conductivity, hardness, solubility) depend on bonding type and structure.