Bonding as a Continuum
Specification Reference S2.4.1
Quick Notes
- Bonding is a spectrum, not just three separate types.
- Most bonds fall somewhere between pure ionic, covalent, or metallic bonding.
- The bonding triangle shows this continuum:
- Ionic corner: electrons transferred
- Covalent corner: electrons shared
- Metallic corner: electrons delocalized
- A material’s structure and bonding type determine its:
- Melting/boiling point
- Electrical conductivity
- Malleability or brittleness
- Solubility
Full Notes:
The Three Bonding Models
Traditionally, bonds are classified into three types:
| Bond Type | Description | Example |
|---|---|---|
| Ionic | Electrons transferred between metal and non-metal | NaCl |
| Covalent | Electrons shared between non-metals | H2O, CO2 |
| Metallic | Delocalized electrons between metal cations | Cu, Fe |
But in reality, bonding is not strictly one or the other – it exists on a continuum.
Electronegativity values of each bonded atom can be used to predict where on the continuum the bond exists.
The Bonding Triangle
The bonding triangle is a conceptual tool to show this continuum:
Corners represent pure forms of bonding:
- Top = Ionic
- Bottom left = Metallic
- Bottom right = Covalent
Most substances lie between these points, showing mixed bonding character.
For Example Electronegativity and bond type
- NaCl: large difference → ionic
- HCl: moderate difference → polar covalent
- Cl2: no difference → non-polar covalent
Explaining Material Properties Using Bonding Models
Bonding models help explain physical properties:
| Property | Ionic | Covalent | Metallic |
|---|---|---|---|
| Melting point | High | Variable (low to very high) | Generally high |
| Electrical conductivity | Conductive when molten or aqueous | Usually non-conductive | Conductive (solid and liquid) |
| Solubility | Often soluble in water | Soluble in non-polar solvents | Insoluble in most solvents |
| Malleability | Brittle | Brittle (if giant covalent) | Malleable and ductile |
Linked Course Questions
Structure 3.1 – Linked Course Question
How do the trends in properties of period 3 oxides reflect the trend in their bonding?
| Oxide | Structure | Melting Point (°C) |
|---|---|---|
| Na2O | Giant Ionic | 1275 |
| MgO | Giant Ionic | 2800 |
| Al2O3 | Giant Ionic (some covalent character) | 2072 |
| SiO2 | Giant Covalent | 1713 |
| P4O10 | Simple Molecular | 580 |
| SO2 | Simple Molecular | -72 |
| SO3 | Simple Molecular | 16 |
Explanation of the trend:
- Ionic oxides (Na2O, MgO, Al2O3) have high melting points due to strong electrostatic forces (ionic bonding) holding the solid lattice together.
- SiO2 has a very high melting point due to its giant covalent structure — lots of strong covalent bonds need to be broken to melt the structure, meaning high amounts of energy required.
- Molecular oxides (P4O10, SO2, SO3) have low melting points due to weak intermolecular forces between molecules.
Application: Using the Bonding Model
When analysing a material:
- Identify the atoms involved (metal vs non-metal)
- Determine type of bonding based on electronegativity difference and position on the triangle
- Predict physical properties using the bonding model
Examples:
- NaCl (ionic): High melting point, conducts when molten
- Diamond (giant covalent): Very hard, high melting point, non-conductive
- Copper (metallic): Conductive, malleable, high melting point
Summary
- Bonding is best viewed as a spectrum or triangle, not in fixed categories.
- Most real substances have bonding characteristics that overlap.
- The bonding model provides a useful framework to explain observable properties of materials.