How does the structure of a substance explain its bulk properties?
Linking structure and bonding to properties; the four main structures; allotropes of carbon including graphene and fullerenes; and predicting properties from structure.
A focused answer to AQA GCSE Chemistry 4.2.2 and 4.2.3, linking the four main structures (ionic, simple molecular, giant covalent and metallic) to their properties, and covering the allotropes of carbon including graphene and fullerenes.
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What this dot point is asking
AQA wants you to link the four main structures to their bulk properties (melting point, conductivity, hardness), predict the type of structure from given properties, and describe the carbon allotropes diamond, graphite, graphene and fullerenes. This dot point pulls together the whole bonding topic: it tests whether you can reason in both directions, from structure to property and from property back to structure.
Linking structure to properties
Each property follows from the structure: high melting points come from many strong bonds or forces throughout a giant structure, and conductivity needs charged particles (ions or delocalised electrons) that are free to move.
Identifying structure from properties
- High melting point, conducts when molten or dissolved but not as a solid: ionic.
- Low melting point, does not conduct: simple molecular.
- Very high melting point, does not conduct (or conducts, for graphite): giant covalent.
- High melting point, conducts as a solid, malleable: metallic.
Working backwards like this is a common exam task: each clue narrows the possibilities until only one structure fits.
Carbon allotropes
- Graphene: a single layer of graphite, one atom thick, with each carbon bonded to three others; it conducts electricity (delocalised electrons) and is very strong and light, useful in electronics and composites.
- Fullerenes: hollow molecules of carbon, such as buckminsterfullerene (, a hollow sphere) and carbon nanotubes (cylinders), useful for drug delivery, as catalysts and as strong, light materials.
Carbon nanotubes deserve a special mention: they have a very high length-to-diameter ratio, conduct electricity, and have an extremely high tensile strength, so they are used to reinforce materials (in sports equipment and electronics) and as nanoscale wires. The reason carbon can form such different allotropes is that each carbon atom has four outer electrons, which can be arranged into four bonds (diamond), three bonds with a delocalised electron (graphite, graphene, nanotubes) or curved sheets that close into hollow shapes (fullerenes).
Try this
Q1. A solid has a high melting point and conducts electricity only when molten. Identify its structure. [1 mark]
- Cue. Ionic.
Q2. State one use of fullerenes and one property of graphene. [2 marks]
- Cue. Fullerenes: drug delivery (or catalysts). Graphene: conducts electricity (or very strong).
Q3. A substance has a low melting point and does not conduct electricity. Identify its structure. [1 mark]
- Cue. Simple molecular.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA substance has a high melting point, does not conduct electricity when solid, but does conduct when molten or dissolved in water. Deduce the type of structure and bonding, and explain how each property follows from it.Show worked answer →
A 4-mark deduction question on identifying a structure.
Structure (1 mark): ionic (a giant ionic lattice). High melting point (1 mark): strong electrostatic forces between oppositely charged ions throughout the lattice need a lot of energy to overcome. Does not conduct when solid (1 mark): the ions are held in fixed positions, so they cannot move to carry charge. Conducts when molten or dissolved (1 mark): the ions become free to move and carry charge.
Markers reward correctly naming ionic and matching each property to the structure.
AQA 20213 marksGraphene and buckminsterfullerene () are both forms of carbon. Describe the structure of each, and give one property or use of each that depends on its structure.Show worked answer →
A 3-mark question on carbon allotropes.
Graphene (1 mark structure, part of marks): a single layer of graphite, one atom thick, with each carbon bonded to three others in a hexagonal sheet; it conducts electricity (delocalised electrons) and is extremely strong and light. Buckminsterfullerene (1 mark structure): a hollow ball-shaped molecule of 60 carbon atoms; used to deliver drugs into the body or as a lubricant or catalyst. Awarding: 1 mark each for the two structures and 1 mark for a correct property or use overall.
Markers reward the single-layer description for graphene and the hollow-molecule description for the fullerene, each with a structure-linked use.
Related dot points
- The three types of chemical bond (ionic, covalent and metallic); when each forms based on the elements involved; the link between bonding and the particles transferred or shared.
A focused answer to AQA GCSE Chemistry 4.2.1, covering the three types of chemical bond, when ionic, covalent and metallic bonding occur, and how electrons are transferred or shared in each.
- Ionic bonding; the transfer of electrons to form ions; dot and cross diagrams; the giant ionic lattice; and how the structure explains melting points and conductivity.
A focused answer to AQA GCSE Chemistry 4.2.1, covering how ions form by electron transfer, drawing dot and cross diagrams, the giant ionic lattice, and how the structure explains the high melting points and conductivity of ionic compounds.
- Covalent bonding; shared pairs of electrons; small molecules; giant covalent structures such as diamond, graphite and silicon dioxide; and how structure explains properties.
A focused answer to AQA GCSE Chemistry 4.2.1 and 4.2.2, covering how covalent bonds form by sharing electrons, small molecules, polymers and giant covalent structures such as diamond, graphite and silicon dioxide, and how structure explains their properties.
- Metallic bonding; positive ions in a sea of delocalised electrons; the properties of metals; why alloys are harder than pure metals.
A focused answer to AQA GCSE Chemistry 4.2.1 and 4.2.2, covering metallic bonding as positive ions in a sea of delocalised electrons, how this explains conductivity and malleability, and why alloys are harder than pure metals.
- The three states of matter; the particle model; changes of state; state symbols; and the limitations of the particle model.
A focused answer to AQA GCSE Chemistry 4.2.2, covering the three states of matter, the particle model, melting, boiling and the energy needed for changes of state, state symbols, and the limitations of the simple particle model.
Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)