The 4 types of crystal lattice structure

There are 4 main types of crystal lattice structure:

1. Ionic - e.g. NaCl.
2. Metallic - e.g. Mg.
3. Simple molecular - e.g. I₂ and H₂O.
4. Giant covalent - e.g. diamond, graphite, and graphene

The table below illustrates the key structural features of the four main types of crystal lattice:

Physical properties of the four crystal types

Explaining the properties of ionic compounds

High melting and boiling points:

• Strong electrostatic forces between oppositely charged ions require significant energy to overcome.

Electrical conductivity:

• When molten or dissolved, charged ions can move and conduct electricity.

Solubility in water:

• Charged ions interact strongly with polar water molecules, allowing ionic compounds to dissolve.

Explaining the properties of metals

High melting points:

• Strong electrostatic forces between positive ions and delocalised electrons require significant energy to overcome.

Thermal conductivity:

• Delocalised electrons readily conduct thermal energy, making metals good heat conductors.

Electrical conductivity:

• Delocalised electrons can move and carry charge through metals.

Insolubility:

• Metallic bonds are very strong, preventing dissolution.

Explaining the properties of simple molecules

Low melting and boiling points:

• Weak intermolecular forces between molecules require little energy to overcome.

Peculiar properties of ice and water due to hydrogen bonding:

• Ice is less dense than water.
• Ice has a higher than expected melting point.
• Water has a higher than expected boiling point.

Poor electrical conductivity:

• Absence of charged particles or delocalised electrons prevents electrical conduction.

Solubility depends on molecular polarity:

• Polar molecules (e.g., alcohols) can form hydrogen bonds with water and are soluble.
• Non-polar molecules (e.g., oils) cannot form strong interactions with water and are insoluble.

Explaining the properties of giant covalent substances

Very high melting and boiling points:

• Many strong covalent bonds throughout the structure require significant energy to break.

Electrical conductivity:

• Graphite: Delocalised electrons between carbon layers allow electrical conduction.
• Diamond: Localised electrons in four covalent bonds per atom prevent electrical conduction.
• Graphene: Delocalised electrons in a single layer of carbon atoms allow electrical conduction.

Insolubility:

• Strong covalent bonds prevent interaction with water molecules.

Predicting structure from properties

You can use the properties of a material to predict its structure.

Example:

1. Substance X has a melting point of 801°C. When solid, it is an insulator, but when molten it conducts electricity well.
2. The high melting point and electrical conductivity when molten indicate substance X is ionic. It is not giant covalent because it conducts electricity when molten.

So substance X likely has an ionic crystal lattice structure.