2.4.2Structure 2 — Models of bonding and structure
Bonding and Properties — IB Chemistry HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 6 of 6 Unit quiz
Part 1 of 6Properties by Bonding Type
The structure and bonding of a substance determine how it behaves.
Ionic
- Brittle solids with high melting points that conduct when molten or aqueous.
Simple molecular
- Low melting and boiling points and no conductivity in any state.
Giant covalent
- Very high melting points, hard, and poor conductors apart from graphite.
Metallic
- Variable but generally high melting points, malleable and good conductors in the solid state.
Part 2 of 6Comparison Table
Comparison Table
Melting point
- High for ionic, giant covalent and most metals; low for simple molecular.
Conductivity
- Only metals conduct when solid; ionic compounds conduct when molten or dissolved.
Solubility
- Ionic and polar covalent substances dissolve in water; non-polar ones dissolve in non-polar solvents.
Hardness
- Giant covalent substances are hard, ionic substances are hard but brittle, and metals are malleable.
| Type | Melting point | Conducts solid? | Conducts liquid? |
|---|---|---|---|
| Ionic | High | No | Yes |
| Simple molecular | Low | No | No |
| Giant covalent | Very high | No (graphite yes) | No |
| Metallic | Usually high | Yes | Yes |
Part 3 of 6Identifying an Unknown
1
Look first at the conductivity of the solid, because only metals and graphite conduct.
2
If the solid does not conduct, test the molten or dissolved substance to detect mobile ions.
3
A low melting point suggests a simple molecular substance.
4
Very high melting point with no conduction in any state suggests a giant covalent network.
5
Combine several observations to justify the conclusion, since one property is rarely enough.
Part 4 of 6Linking to Intermolecular Forces
Polar molecules
- Dipole–dipole forces and hydrogen bonds raise boiling points above non-polar molecules.
Non-polar molecules
- Only London forces act, so boiling points rise with molar mass.
Hydrogen bonding
- Substances with N–H, O–H or F–H bonds, like water and ethanol, have unusually high boiling points for their size.
Part 5 of 6Worked Example
Question
- Substance X melts at 801 °C, is brittle, does not conduct as a solid but conducts when molten.
- Identify its bonding.
- High melting point rules out simple molecular.
- Not conducting as a solid rules out metallic and graphite-like structures.
- Conducting when molten shows mobile ions are present, and brittleness agrees.
Answer X is an ionic compound with a giant ionic lattice, for example sodium chloride.
Part 6 of 6Exam Checklist & Tips
2.4.2 – Key Points for the Exam
- Ionic compound A lattice of oppositely charged ions.
- Simple molecular Discrete molecules attracted by intermolecular forces.
- Giant covalent A network of atoms joined by covalent bonds.
- Metallic solid A lattice of cations in a sea of delocalised electrons.
Common Mistakes
- Identifying a substance as ionic only because it is a solid at room temperature.
- Saying that ionic solids conduct because ions are present.
- Predicting that all covalent substances have low melting points.
- Giving one property as complete proof of bonding type.
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