In short: A substance's structure decides its properties. Covalent molecular substances melt easily and never conduct, covalent networks like diamond are very hard with very high melting points, ionic lattices conduct only when molten or dissolved, and metals conduct as solids.
Four types of structure
Almost every solid fits one of four structures.
| Structure | Particles | Bonding | Examples |
|---|---|---|---|
| Ionic lattice | Positive and negative ions | Ionic | Sodium chloride, magnesium oxide |
| Covalent molecular | Small separate molecules | Covalent inside molecules, weak forces between them | Water, carbon dioxide, sugar, iodine |
| Covalent network (giant covalent) | Atoms | Covalent bonds everywhere, in a huge 3D or layered network | Diamond, graphite, silicon dioxide (sand) |
| Metallic lattice | Positive ions in a sea of electrons | Metallic | Copper, iron, aluminium |
The property table
This table is one of the most useful in all of chemistry. Learn it well.
| Property | Ionic | Covalent molecular | Covalent network | Metallic |
|---|---|---|---|---|
| Melting point | High | Low | Very high | Usually high |
| Conducts as a solid | No | No | No (graphite yes) | Yes |
| Conducts when molten | Yes | No | No | Yes |
| Conducts dissolved in water | Yes | Usually no | Does not dissolve | Does not dissolve |
| Hard or soft | Hard, brittle | Soft | Very hard (graphite soft) | Malleable, ductile |
| State at room temperature | Solid | Often gas or liquid | Solid | Solid (except mercury) |
Diamond and graphite
Diamond and graphite are allotropes of carbon: different forms of the same element.
| Diamond | Graphite | |
|---|---|---|
| Bonds per carbon | 4, in a 3D tetrahedral network | 3, in flat sheets of hexagons |
| Hardness | Hardest natural substance. Used in drill tips and cutting tools | Soft and slippery: layers slide. Used in pencils and as a lubricant |
| Conducts electricity | No: every electron is in a bond | Yes: the fourth electron of each carbon is delocalised and moves along the layers |
| Melting point | Very high (about 3500 °C) | Very high (over 3600 °C) |
Silicon dioxide (quartz, sand) has a similar 3D network to diamond, so it is also hard with a very high melting point.
Identifying a mystery substance
Work through the properties like a detective.
- Low melting point? It is covalent molecular.
- Conducts as a solid? It is metallic (or graphite).
- Conducts only when melted or dissolved? It is ionic.
- Very high melting point, never conducts, does not dissolve? It is a covalent network.
- High melting point
- Conducts molten or dissolved
- Brittle
- Low melting point
- Never conducts
- Very high melting point
- Diamond, graphite, SiO₂
- Graphite conducts
- Conducts as solid
- Malleable
- Melting point first
- Then conductivity
Quick summary
- Four structures: ionic lattice, covalent molecular, covalent network, metallic lattice.
- Covalent molecular: low melting point, never conducts.
- Covalent network: very high melting point, hard, does not conduct (except graphite).
- Ionic: high melting point, brittle, conducts only when molten or dissolved.
- Metallic: conducts as a solid, malleable.
- Diamond (4 bonds, hard, no conduction) and graphite (3 bonds, layers, soft, conducts) are allotropes of carbon.
Quiz
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Try 3 questions now
Which structure has the lowest melting points?
- A Ionic lattice
- B Covalent molecular
- C Covalent network
- D Metallic lattice
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B: Covalent molecular
Only weak forces between molecules need to be overcome.
A substance conducts electricity as a solid and is malleable. It is most likely:
- A Metallic
- B Ionic
- C Covalent molecular
- D Covalent network
Show answer
A: Metallic
Conducting as a solid and bending are signs of metallic bonding.
A substance melts at 801 °C and conducts only when molten or dissolved. It is:
- A Metallic
- B Ionic
- C Covalent molecular
- D Covalent network
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B: Ionic
Ions are fixed in the solid but free to move when molten or dissolved.