1.3.4Physical Chemistry
Bonding and Physical Properties
5 min read 5 parts Lesson 17 of 63
Part 1 of 5Four Structure Types
Giant ionic
- Positive and negative ions in a lattice, as in NaCl and MgO.
Simple molecular
- Small discrete molecules held together by weak intermolecular forces, as in I₂, H₂O and CO₂.
Giant covalent
- A network of atoms joined by covalent bonds, as in diamond, graphite and silicon dioxide.
Giant metallic
- A lattice of positive ions in a sea of delocalised electrons, as in Cu and Mg.
Part 2 of 5Comparing Properties
Melting point
- Giant structures have high melting points, but simple molecular substances have low ones as only weak forces are overcome.
Electrical conductivity
- Only substances with mobile charge carriers conduct,
- which means metals, graphite,
- and ionic compounds when molten or dissolved.
Solubility
- Ionic compounds often dissolve in water, and non-polar molecular substances tend to dissolve in non-polar solvents.
Hardness
- Diamond and silicon dioxide are very hard, while graphite is soft and slippery.
| Structure | Melting point | Conducts? |
|---|---|---|
| Giant ionic | High | Only molten or aqueous |
| Simple molecular | Low | No |
| Giant covalent | Very high | No (except graphite) |
| Metallic | Usually high | Yes, solid and liquid |
Part 3 of 5Allotropes of Carbon
Allotropes of Carbon
Diamond
- Each carbon bonds to four others in a tetrahedral network, so it is very hard with a very high melting point.
Graphite
- Each carbon bonds to three others in layers, with weak forces between layers allowing them to slide.
Graphite conducts
- The fourth outer electron on each carbon is delocalised along the layers and can carry charge.
Graphene
- A single layer of graphite that is strong and a good conductor.
Part 4 of 5Explaining Conductivity
Always name the mobile particle that carries charge when you explain conduction.
1
Ask whether the substance contains any charged particles that are free to move.
2
In metals and graphite the charge carriers are delocalised electrons, so they conduct in the solid state.
3
In ionic compounds the ions are fixed in the lattice when solid but free to move when molten or dissolved.
4
Simple molecular substances and diamond have no mobile charged particles, so they do not conduct.
5
Remember that pure water and covalent liquids conduct only very slightly unless ions are formed.
Part 5 of 5Exam Checklist & Tips
1.3.4 – Key Points for the Exam
- Giant covalent structure A network of atoms joined by covalent bonds throughout the crystal.
- Simple molecular structure Discrete molecules held together by weak intermolecular forces.
- Allotropes Different structural forms of the same element in the same physical state.
- Intermolecular forces Weak forces of attraction between neighbouring molecules.
Common Mistakes
- Saying that covalent bonds break when a simple molecular substance melts;
- only intermolecular forces are overcome.
- Claiming all covalent substances have low melting points, which is false for giant covalent structures.
- Saying graphite conducts because of ions rather than delocalised electrons.
- Mixing up hardness with brittleness in descriptions of ionic and covalent solids.