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.
Structure and properties
Ionic lattice
- High melting point
- Conducts molten or dissolved
- Brittle
Covalent molecular
- Low melting point
- Never conducts
Covalent network
- Very high melting point
- Diamond, graphite, SiO₂
- Graphite conducts
Metallic
- Conducts as solid
- Malleable
Detective steps
- 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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