The four crystal structures
Crystalline solids contain particles arranged in a regular, repeating pattern. The four types considered here are ionic, metallic, molecular and macromolecular (giant covalent).
| Crystal type | Particles or units | What holds the structure together? | Examples |
|---|---|---|---|
| Giant ionic | Positive and negative ions | Electrostatic attractions between oppositely charged ions | Sodium chloride |
| Giant metallic | Positive metal ions and delocalised electrons | Electrostatic attractions between metal ions and delocalised electrons | Magnesium |
| Molecular | Separate molecules | Intermolecular forces between molecules | Iodine, ice |
| Macromolecular / giant covalent | Atoms connected throughout a giant network | Strong covalent bonds | Diamond, graphite |
Giant ionic and metallic crystals
Sodium chloride
Sodium chloride forms a giant ionic lattice in which oppositely charged ions alternate throughout the three-dimensional structure.
Magnesium
Magnesium forms a giant metallic lattice of closely packed positive ions surrounded by mobile delocalised electrons.
Molecular crystals
Iodine
Solid iodine contains separate molecules arranged regularly, with intermolecular attractions between neighbouring molecules.
Ice
In ice, water molecules are held in an open molecular lattice by hydrogen bonds. The molecules are further apart than in liquid water.
Exam Tip: Use molecules and intermolecular forces only when describing molecular structures. Giant ionic, metallic and giant covalent crystals do not consist of separate molecules.