Bonding and Structure
Specification Reference Physical Chemistry: States of Matter 4.2
Quick Notes
- Solids can be crystalline with four main types of structure:
- Giant ionic (e.g. NaCl, MgO)
- Simple molecular (e.g. I2, ice, C60)
- Giant molecular (e.g. SiO2, diamond, graphite)
- Giant metallic (e.g. copper)
- Physical properties (melting point, conductivity, solubility) depend on structure and bonding:
- Giant ionic → high mp, soluble in water, conducts when molten
- Simple molecular → low mp, doesn’t conduct, often insoluble in water
- Giant covalent → very high mp, doesn’t conduct (except graphite)
- Metallic → high mp, good electrical/thermal conductor
- You can deduce bonding and structure based on melting point, solubility, and conductivity data.
Full Notes
Solids can be classified by their crystal lattice structure and type of bonding. There are four main types:
Giant Ionic Structures
ExamplesSodium chloride (NaCl), Magnesium oxide (MgO)
- Made from a regular 3D lattice of alternating positive and negative ions.
- Held together by strong electrostatic attraction (ionic bonds).
- Key properties:
- High melting and boiling points (due to strong ionic bonds)
- Conduct electricity when molten or dissolved in water (ions are free to move)
- Soluble in water but often insoluble in organic solvents
Simple Molecular Structures
Examples Iodine (I2), Ice (H2O), Buckminsterfullerene (C60)
- Consist of small molecules held together by intermolecular forces (van der Waals, hydrogen bonds).
- The bonds within each molecule are covalent, but molecules are not bonded to each other.
- Key properties:
- Low melting and boiling points (weak intermolecular forces)
- Do not conduct electricity (no free-moving charged particles)
- Usually insoluble in water, but soluble in non-polar solvents
- Special cases:
- Ice: Hydrogen bonding gives it a relatively high melting point and low density (ice floats).
- C60 (fullerene): Weak London forces between molecules, soft solid, poor conductor.
Giant Molecular (Covalent) Structures
Examples Silicon dioxide (SiO2), Diamond, Graphite
- Atoms are bonded in a continuous network via strong covalent bonds.
- No discrete molecules — instead, one large molecule or crystal.
- Key properties:
- Very high melting points (many strong covalent bonds to break)
- Insoluble in all solvents
- Do not conduct electricity — except graphite
Special case: Graphite: Layers of carbon atoms with delocalised electrons between them make graphite a good electrical conductor and lubricant.
Giant Metallic Structures
ExampleCopper (Cu)
- Positive metal ions arranged in a lattice, surrounded by a sea of delocalised electrons.
- Bonded by metallic bonding (electrostatic attraction between ions and electrons).
- Key properties:
- High melting and boiling points
- Good conductors of heat and electricity
- Malleable and ductile (layers of ions can slide)
- Insoluble in water and most solvents
How Structure Affects Properties
| Type | Bonding Explanation |
|---|---|
| Ionic | Melts at high temp due to strong ionic bonds; conducts when ions can move |
| Molecular | Weak intermolecular forces → low mp; no free charges to conduct |
| Covalent network | All atoms strongly bonded → very high mp; no delocalised electrons |
| Metallic | Free electrons carry charge and energy; structure is flexible |
We can often identify the bonding type by looking at a few simple properties:
- High melting point + conducts when molten → Ionic
- Low melting point + no conductivity → Simple molecular
- Very high melting point + no conductivity → Giant covalent (diamond/SiO2)
- Very high melting point + good conductivity → Metallic (or graphite)
- Soft solid with poor conductivity → Likely molecular (e.g. I2 or C60)
Summary
- Four main solid structures: giant ionic, simple molecular, giant molecular, metallic.
- Properties depend on type of bonding and structure.
- Ionic: high mp, soluble in water, conducts when molten.
- Molecular: low mp, no conduction, usually insoluble in water.
- Giant covalent: very high mp, insoluble, generally non-conductive (except graphite).
- Metallic: high mp, good conductor, malleable and ductile.
- Bonding type can be deduced from melting point, solubility and conductivity data.