2.4.2Structure 2 — Models of bonding and structure
Bonding and Properties — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 6 of 6 Unit quiz
Part 1 of 6Four Types of Structure
Physical properties are evidence for the type of bonding present.
Ionic lattice
- Strong electrostatic forces between ions give high melting points, brittleness, and conduction only when molten or dissolved.
Simple molecular
- Weak intermolecular forces give low melting and boiling points, softness and no electrical conduction.
Giant covalent
- A network of covalent bonds gives very high melting points, hardness and no conduction in most cases.
Metallic
- Cations in a sea of electrons give good conductivity, malleability and a range of melting points.
Part 2 of 6Comparison Table
Comparison Table
Melting point
- Ionic and giant covalent are high, simple molecular is low and metals vary from low to very high.
Electrical conductivity
- Metals conduct as solids, ionic compounds only when molten or dissolved, and molecular substances do not.
Solubility in water
- Many ionic compounds and polar molecules dissolve, whereas metals, giant covalent and non-polar substances do not.
Mechanical
- Ionic solids are brittle, metals malleable and giant covalent solids hard.
| Property | Ionic | Simple molecular | Giant covalent | Metallic |
|---|---|---|---|---|
| Melting point | high | low | very high | variable, often high |
| Conducts when solid | no | no | no (graphite yes) | yes |
| Conducts when molten | yes | no | no | yes |
| Hardness | hard, brittle | soft | very hard | malleable |
Part 3 of 6Deducing the Type
1
First check conductivity of the solid: if it conducts, the substance is a metal (or graphite).
2
If the solid does not conduct, test the molten or dissolved material: if that conducts, it is ionic.
3
If it never conducts, a very high melting point suggests a giant covalent structure.
4
If it never conducts and has a low melting point, it is probably a simple molecular substance.
5
Use solubility in water or hexane to decide whether a molecular substance is polar or non-polar.
Part 4 of 6Pitfalls
Explanations not descriptions
- When asked to explain, link each property to the particles and the forces between them.
Correct bonds
- Name the forces that are overcomecovalent bonds for giant covalent, intermolecular forces for molecular.
Exceptions
- Graphite conducts and diamond is hard, so check the structure rather than relying on the element's type.
Mixed evidence
- Use at least two properties together, because a single test can mislead.
Part 5 of 6Worked Example
Question
- A substance melts at 1410 °C, is insoluble in water and does not conduct as a solid or liquid.
- Deduce its structure.
- Not conducting as a solid rules out a metal.
- Not conducting when molten rules out an ionic compound, since ions would carry charge.
- The very high melting point and insolubility indicate a giant covalent network.
Answer Giant covalent structure.
Part 6 of 6Exam Checklist & Tips
2.4.2 – Key Points for the Exam
- Structure The arrangement of particles in a substance.
- Brittle Breaking easily when struck.
- Conductivity The ability to allow electric charge to flow.
- Volatile Having a low boiling point.
Common Mistakes
- Saying molten ionic compounds do not conduct.
- Stating that metals conduct because ions flow.
- Using the same explanation for the melting of molecular and giant covalent substances.
- Forgetting that melting a molecular compound does not break covalent bonds.
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