Lesson 2.12 · AQA Combined Science

Compare structures and use models

Use evidence to identify a structure

Structure	Melting point	Electrical conduction
Ionic	High	Molten or dissolved: mobile ions
Small molecular	Usually low	Usually none: no mobile charges
Giant covalent	Very high	Usually none; graphite/graphene are exceptions
Metallic	Usually high	Solid or liquid: delocalised electrons

Use more than one property as evidence. A high melting point alone does not identify the bonding type.

Build a clear explanation

Try this sentence pattern: “It has… so… because…”
Example: “Sodium chloride has a high melting point because a lot of energy is needed to overcome strong electrostatic attractions between oppositely charged ions.”
For electricity, always name what moves and carries charge.

Models show different things

Model	Useful for	Limitation
Dot-and-cross	Outer electrons and sharing/transfer	Does not show real sizes or usually the 3D shape
Ball-and-stick	Which atoms are joined and their arrangement	Atoms are not solid balls joined by rigid sticks; sizes and gaps are not to scale
2D lattice drawing	Repeating pattern and charge ratio	Real lattice extends in 3D
Displayed formula	Atoms and bond numbers	Does not show all electrons or usually the 3D shape

Higher Tier: limits of the simple particle model

Higher Tier only. Simple state diagrams often draw particles as solid, inelastic spheres and leave out the forces between them. Real particles may be atoms, ions or molecules. Atoms are not solid balls. The model is useful for arrangement and movement but does not fully explain bonding or internal structure.

Avoid these easy mistakes

Do not call all particles molecules: an ionic lattice contains ions. Do not say weak covalent bonds explain a low boiling point of a molecular substance: use weak intermolecular forces. Do not give an individual atom a bulk property such as “this atom is liquid”. State describes a collection of particles.

Remember

Explain properties using structure, forces and mobile charge carriers. Every model has useful features and limitations.