Identify the particles in a solid, explain the forces connecting them and use several physical observations together to deduce an unfamiliar structure.
Bond type and structure answer different questions
Bonding describes attractions between particles. Structure describes how those particles are arranged and how far the connected arrangement extends. Covalent bonding can occur inside a small molecule or throughout a giant network; the word ‘covalent’ alone therefore cannot predict a melting temperature.
Ionic solids and metals have giant lattices. Diamond, graphite and silicon(IV) oxide have giant covalent structures. Solid iodine and ice have molecular structures: their molecules form a lattice, but the covalent bonds are confined within separate I₂ or H₂O units. The lattice in ice is connected by hydrogen bonds, not an uninterrupted covalent network.
| Structure | Particles | Main binding interaction | Examples |
|---|---|---|---|
| Giant ionic | Cations and anions | Oppositely charged ions attract | NaCl, MgO |
| Giant metallic | Metal cations and delocalised electrons | Cation–electron attraction | Mg, Al |
| Giant covalent | Atoms linked throughout the structure | Covalent bonds | Diamond, graphite, SiO₂ |
| Simple molecular | Separate molecules | Intermolecular attractions between molecules | I₂, ice, solid CO₂ |
Diamond and silicon dioxide: strong continuous networks
In diamond, each carbon forms four covalent bonds to four other carbons in a tetrahedral network. The rigid three-dimensional network gives hardness; extensive strong bonding gives a very high temperature for disruption. All four outer electrons per carbon participate in localised bonding, so pure diamond does not conduct electricity.
In silicon(IV) oxide, each silicon is bonded to four oxygens and each oxygen links two silicons, giving the overall SiO₂ ratio. It is a giant covalent network, not separate SiO₂ molecules. Its strong network explains high melting temperature, poor electrical conductivity and insolubility in water under ordinary conditions.
Water cannot simply pull individual molecules out of either network, because there are no separate molecules to remove. Dissolution would require extensive disruption of strong covalent bonding without sufficiently favourable replacement interactions.
Graphite and graphene separate strength from slipperiness
In graphite, each carbon forms three covalent bonds in a planar hexagonal layer. One outer electron per carbon contributes to a delocalised system extending across the layer. Mobile delocalised electrons allow electrical conduction, especially along the planes; do not say electrons are merely trapped in gaps between layers.
Strong covalent bonds within the sheets give high resistance to thermal disruption. Weaker attractions between the sheets allow them to slide, explaining softness and lubrication. ‘Weak bonds’ alone is misleading: it confuses the weak interlayer attractions with the strong intralayer covalent network.
Graphene is one carbon sheet with the same three-coordinate hexagonal bonding and delocalised electrons. It conducts along the sheet and has high tensile strength because of the strong covalent network. Its atomically thin structure differs from the many stacked layers in graphite. Properties of larger devices also depend on defects and contacts.
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Carbon structures: diamond, graphite and graphene
Labels to include:
- Diamond: four tetrahedral covalent bonds per carbon
- Graphite: planar hexagonal sheets, three covalent bonds per carbon
- Graphite: weaker attractions between sheets
- Delocalised electrons associated with each sheet; conduction along sheets
- Graphene: one sheet, three bonds per carbon
The distinction between strong bonds within a sheet and weaker attractions between sheets explains how graphite can be soft yet resist heating. A finished lattice drawing remains outstanding; the connectivity and property explanations are complete.
Specify which charged particles are mobile
A molecular substance such as hydrogen chloride can form ions by reacting with water, so its aqueous solution conducts even though pure molecular HCl is not an ionic lattice. Similarly, a reactive metal disappearing in acid is undergoing a chemical reaction, not merely dissolving as neutral metal particles. Always state physical state and solvent when interpreting conductivity or solubility.
| Structure | Melting/boiling | Conductivity | Water behaviour |
|---|---|---|---|
| Giant ionic | Often high: strong ionic attractions | Solid: no; molten/dissolved: mobile ions | Variable; depends on lattice and hydration |
| Metallic | Often high, but variable | Solid and molten: mobile electrons | Usually insoluble; some react with water |
| Giant covalent | Usually very high | Usually poor; graphite/graphene have mobile electrons | Usually insoluble under ordinary conditions |
| Molecular | Often lower; depends on intermolecular attractions | Usually poor: no mobile charged particles | Depends on solute–solvent attractions; some react/ionise |
Use all observations before making a deduction
Constructed unknown A melts at a high temperature, does not conduct as a solid, conducts when molten and dissolves to give a conducting aqueous solution. A giant ionic structure is the consistent model: strong attractions resist melting, fixed solid ions cannot carry current, and molten or hydrated ions can move.
Constructed B is soft, conducts as a solid, is insoluble in water and remains solid at very high temperatures. A layered giant covalent structure such as graphite fits: delocalised electrons explain current, weak interlayer attractions explain sliding, and strong intralayer bonds explain thermal stability. ‘It conducts, so it must be metallic’ would discard the other evidence.
Constructed C melts at low temperature and does not conduct as a solid or melt. A simple molecular structure is plausible, because weak intermolecular attractions can be overcome with little energy and there are no mobile ions or delocalised electrons. Its water solubility still requires information about polarity, hydrogen bonding or reaction with water.
For an ‘explain’ response, connect structure → relevant particles/interactions → energy or mobility → observed property. A label plus an unconnected list of facts is weaker than a causal explanation. In the 2023 AS Q5(d) context, the scheme linked named compounds directly to properties from the supplied table.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.
Q1. Why can both diamond and iodine be called covalent, yet have very different melting behaviour?Show answer
Diamond is a giant covalent network: melting/disruption requires breaking many strong covalent bonds. Iodine consists of I₂ molecules; melting overcomes weaker London attractions between molecules while the I–I covalent bonds remain intact.
Q2. Explain graphite’s conductivity without saying it has free ions.Show answer
Each carbon forms three covalent bonds and contributes an electron to a delocalised system across a layer. These electrons can move and carry charge, especially along the planes. The carbon atoms do not become mobile ions.
Q3. Why is SiO₂ described as a formula ratio rather than a molecule?Show answer
Its covalent connectivity extends throughout a giant network. Each Si has four O neighbours and each O bridges two Si, giving Si:O = 1:2; there are no separate SiO₂ molecules in the ordinary solid.
Q4. An unknown conducts as a solid. Give two possible structural types and one useful extra observation.Show answer
It could be metallic or a conducting giant covalent material such as graphite. Malleability versus easy layer sliding, thermal data or other structural evidence could help distinguish them. Conductivity alone is insufficient.
Q5. Why is ‘all ionic compounds dissolve in water’ scientifically unsafe?Show answer
Dissolving requires lattice disruption and formation of hydration interactions, with the overall balance also involving mixing. Some lattices, including barium sulfate, remain only sparingly soluble. The presence of ions alone does not determine solubility.
Sources
Sources and examiner guidance (reviewed 9 October 2026)
- Pearson Edexcel 9CH0 specification, Issue 3 — Topic 2, printed pp. 9–11, checked against 8CH0 printed pp. 7–9. Reviewed 9 October 2026.
- Chemrevise — Edexcel Bonding — All 14 pages reviewed as a secondary coverage check. Explanations, worked applications and exercises are original.
- Pearson 8CH0/01 June 2023 mark scheme — Q3(a)(iii), Q3(b)(i), Q4(a), Q5(c–d), Q7(a–b); PDF pp. 11–12, 14, 18–19, 22–23. Question-specific evidence.
- Pearson 8CH0/01 June 2023 examiner report — Q3–5 and Q7, PDF pp. 4–6. Reviewed with the mark scheme and question context on 9 October 2026.
- Pearson 8CH0/01 June 2023 question paper — Context for the cited bonding questions; no official question reproduced here.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.