Structures & properties
Structures & properties
Four structure types: particles and attractions
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Start a structure question by identifying the particles: ions, atoms or molecules. Then name the attraction between them. This stops you using the right explanation for the wrong structure.
- Choose the structure type from the evidence: giant ionic, metallic, giant covalent or simple molecular.
- For melting or boiling, name the attractions that must be overcome and connect their strength to the energy needed.
- For conductivity, identify the charged particles and say whether they can move.
- Use all the evidence together. A solid conductor could be a metal or graphite, so one property alone may not identify it.
| Structure | Particles | Bond or attraction |
|---|---|---|
| Giant ionic | Positive and negative ions | Electrostatic attractions between oppositely charged ions |
| Metallic | Positive metal ions and delocalised electrons | Electrostatic attraction between positive ions and delocalised electrons |
| Giant covalent | Atoms | Many strong covalent bonds throughout a network |
| Simple molecular | Molecules | Forces between molecules |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
For each structure below, state the particles present and the bond or attraction that holds the structure together. Give one correct entry in each empty cell. [8]
| Structure | Particles present | Bond or attraction |
|---|---|---|
| Giant ionic | … | … |
| Metallic | … | … |
| Giant covalent | … | … |
| Simple molecular | … | … |
Total: 8 marks
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| Structure | Particles present | Bond or attraction |
|---|---|---|
| Giant ionic | Positive and negative ions | Electrostatic attractions between oppositely charged ions |
| Metallic | Positive metal ions and delocalised electrons | Electrostatic attraction between positive ions and delocalised electrons |
| Giant covalent | Atoms | Many strong covalent bonds throughout a network |
| Simple molecular | Molecules | Forces between molecules |
Award one mark for each completed cell: 8 marks.
Repair each statement using precise particle language.
- “Electron transfer is the ionic bond.” [3]
- “Molten iodine contains iodine atoms because its covalent bonds have broken.” [2]
- “Solid magnesium conducts because Mg2+ ions move through the lattice.” [3]
Total: 8 marks
Show answer & marking points
- Transfer forms positive and negative ions [1]. Ionic bonding is electrostatic attraction [1] between oppositely charged ions in a lattice [1].
- Molten iodine still contains I2 molecules [1]; only forces between molecules are overcome, not I–I bonds [1].
- Mg2+ ions are fixed in the solid [1]; delocalised electrons are mobile [1] and carry charge [1].
Identifying structures from physical properties
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Treat the properties as clues that must fit the same structure. A high melting point suggests strong attractions, but it does not by itself tell you which particles are present.
Conductivity is especially useful. A solid metal conducts through mobile electrons. An ionic compound normally conducts only when its ions can move, such as when molten or dissolved.
Check the full set of observations against the table. Graphite is an important exception to the idea that giant covalent substances never conduct.
| Structure | Melting point | Conductivity |
|---|---|---|
| Giant ionic | High | Solid: no; molten/aqueous: yes, via ions |
| Metallic | Often high | Solid and liquid: yes, via delocalised electrons |
| Giant covalent | Very high | Usually no; graphite is an exception |
| Simple molecular | Usually low relative to giant structures | No mobile charged particles in the pure substance |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
Four solids give the following data.
| Solid | Melting point / °C | Conducts as solid? | Conducts when molten? |
|---|---|---|---|
| W | 801 | No | Yes |
| X | 650 | Yes | Yes |
| Y | 3550 | No | No |
| Z | 114 | No | No |
- Identify the most likely structure type of W, X, Y and Z. [4]
- For each solid, quote the decisive evidence supporting your choice. [4]
Total: 8 marks
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- W: giant ionic; X: metallic; Y: giant covalent; Z: simple molecular. One mark each.
- W conducts only when molten, consistent with mobile ions [1]. X conducts in both states, consistent with mobile delocalised electrons [1]. Y has a very high melting point and no mobile charge carriers [1]. Z has a low melting point and does not conduct [1].
Three samples are sodium chloride, graphite and iodine in an unknown order.
| Sample | Thermal observation | Conducts as solid? | Conducts when molten? |
|---|---|---|---|
| P | Melts at 114 °C | No | No |
| Q | Melts at 801 °C | No | Yes |
| R | Remains solid above 3000 °C | Yes | Not tested |
- Identify P, Q and R. [3]
- State the structure type of each. [3]
- For each, explain one decisive observation using particles, attractions or mobile charge carriers. [3]
Total: 9 marks
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- P: iodine; Q: sodium chloride; R: graphite. One mark each.
- P: simple molecular; Q: giant ionic; R: giant covalent. One mark each.
- P melts at relatively low temperature because forces between I2 molecules are overcome [1]. Q’s molten ions carry charge [1]. R has mobile delocalised electrons [1].
A contains atoms joined in a continuous three-dimensional covalent network. B contains discrete X8 molecules. Neither conducts electricity.
- Classify their structures. [2]
- Explain why A has a much higher melting point than B. [5]
- Explain why neither conducts. [2]
Total: 9 marks
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- A: giant covalent [1]; B: simple molecular [1].
- A has many strong covalent bonds throughout a network [1]; many must be broken [1], requiring much energy [1]. B has discrete X8 molecules [1]; melting overcomes forces between them, not X–X bonds [1].
- A has no mobile charged particles [1]; neither does B [1].
Samples A–D are diamond, graphite, magnesium and iodine in an unknown order.
| Sample | Physical evidence |
|---|---|
| A | Conducts as solid and liquid; melts at 650 °C |
| B | Conducts as solid; very high melting point; soft, layered |
| C | Does not conduct; very high melting point; extremely hard |
| D | Does not conduct; melts at 114 °C |
- Identify A–D. [4]
- Compare B and C: explain their high melting points and why only B conducts. [5]
- Explain why D melts far below C, without referring to weak covalent bonds. [3]
Total: 12 marks
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- A: magnesium; B: graphite; C: diamond; D: iodine. One mark each.
- Both giant covalent with many strong bonds [1], requiring much energy to break [1]. Graphite has delocalised electrons [1] that are mobile and carry charge [1]. Diamond’s outer electrons are used in bonds, so it has no mobile charge carriers [1].
- D has discrete I2 molecules [1]. Melting overcomes forces between molecules [1], requiring much less energy than breaking diamond’s covalent bonds [1].
Diamond: a rigid covalent network
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Diamond is one giant covalent network, rather than a collection of small carbon molecules. Each carbon atom forms four strong covalent bonds in a rigid three-dimensional arrangement.
Melting requires many strong covalent bonds to be broken, so a lot of energy is needed. The rigid network also makes diamond very hard.
All four outer electrons of each carbon are used in bonds. There are no mobile charge carriers, so diamond does not conduct electricity.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
- State the number of covalent bonds formed by each carbon in diamond. [1]
- Explain diamond’s high melting point and lack of electrical conductivity. [4]
Total: 5 marks
Show answer & marking points
- Four [1].
- A giant covalent network [1] has many strong covalent bonds requiring much energy to break [1]. All four outer electrons are used in bonds [1], leaving no mobile charge carriers [1].
Graphite: layers and delocalised electrons
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Graphite contains flat layers of carbon atoms. Each carbon makes three strong covalent bonds, leaving one electron per carbon delocalised.
The strong bonds within the layers explain the high melting point. The delocalised electrons can move along the layers and carry charge, so graphite conducts electricity.
Weak van der Waals attractions between the layers allow them to slide over one another. This explains softness; it is a different explanation from the one for melting point.
Graphite is a giant covalent structure. The diagram's forces between layers do not mean that each layer is a collection of small molecules.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
- State the structure type shared by diamond and graphite. [1]
- State how many covalent bonds each carbon atom forms in diamond and graphite. [2]
- Explain why both have very high melting points. [3]
- Explain why graphite conducts electricity but diamond does not. [3]
- Explain why graphite’s layers can slide over one another. [1]
Total: 10 marks
Show answer & marking points
- Giant covalent [1].
- Diamond: four [1]; graphite: three [1].
- Many strong covalent bonds [1] extend throughout giant structures [1], so much energy is required to break them [1].
- Graphite has one mobile delocalised electron per carbon [1] that carries charge [1]. Diamond’s four outer electrons are used in bonds, so it has no mobile charge carriers [1].
- Weak forces between the layers allow them to slide [1].
C consists of flat covalent layers. Each atom forms three covalent bonds and contributes one delocalised electron. Weak forces act between the layers.
- Classify C’s structure. [1]
- Predict and explain its melting point. [3]
- Predict and explain its electrical conductivity. [2]
- Predict and explain whether its layers can slide. [2]
- Explain why C should not be described as simple molecular. [2]
Total: 10 marks
Show answer & marking points
- Giant covalent [1].
- Very high [1]; many strong covalent bonds within continuous layers [1] require much energy to break [1].
- Conducts [1] because mobile delocalised electrons carry charge [1].
- Layers can slide [1] because forces between them are weak [1].
- Atoms form continuous covalent layers [1], not separate molecules [1].
Molecular crystals: iodine and ice
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Iodine and ice are made of separate molecules arranged in a solid. There are covalent bonds inside each molecule and attractions between neighbouring molecules.
Melting partly overcomes the attractions between molecules. Boiling separates the molecules further. The molecules themselves remain intact.
For iodine, name van der Waals forces between I₂ molecules. For ice, name hydrogen bonds between water molecules. Do not say that melting breaks the I-I or O-H covalent bonds.
| Crystal | Particles | Within molecules | Between molecules |
|---|---|---|---|
| Iodine | I2 molecules | Strong I–I covalent bonds | van der Waals forces |
| Ice | H2O molecules | Strong O–H covalent bonds | Hydrogen bonds |
| Change | Energy |
|---|---|
| Melting or boiling | Energy absorbed to overcome attractions |
| Freezing or condensing | Energy released as attractions form |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
A student writes: “Iodine melts at a low temperature because the weak covalent bonds in I2 molecules break.”
- Identify two errors. [2]
- Explain correctly what happens when solid iodine melts. [3]
- Explain why diamond melts at a much higher temperature than iodine. [2]
Total: 7 marks
Show answer & marking points
- The I–I covalent bonds are not broken on melting [1]; iodine’s melting point is not low because those bonds are weak [1].
- Iodine is a molecular crystal of I2 molecules [1]. Melting overcomes forces between molecules [1]; the molecules remain intact [1].
- Diamond is a giant covalent network. Melting iodine overcomes only weak intermolecular (van der Waals) forces between I2 molecules [1]; breaking the strong C–C covalent bonds in diamond requires much more energy [1].
- State the particles present in ice. [1]
- State which attractions are overcome when ice melts and which bonds are not broken. [2]
- State whether melting absorbs or releases energy. [1]
- Describe the attraction and energy changes when water freezes. [2]
- Explain why each water molecule’s chemical identity is unchanged during melting. [1]
Total: 7 marks
Show answer & marking points
- H2O molecules [1].
- Forces between water molecules are partly overcome [1]; O–H covalent bonds remain intact [1].
- Energy is absorbed [1].
- An ordered network of attractions forms between water molecules [1]; energy is released [1].
- The O–H bonds remain intact, so the molecules remain H2O [1].
Drawing exact numbers of particles
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Before drawing, underline what you have been asked to count: atoms, ions or molecules. The number of circles you need depends on that choice.
- For I₂, draw separate touching pairs. One pair is one molecule, so four molecules need eight atoms.
- For NaCl, show an alternating arrangement of touching positive and negative ions, with the charges labelled.
- Count the requested entities again at the end. Check that the overall composition and charge match the substance.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
- Draw exactly five I2 molecules. [2]
- Draw a touching, alternating lattice containing exactly four Na+ ions and four Cl- ions. Show every charge. [3]
- State the particle represented in each diagram. [1]
Total: 6 marks
Show answer & marking points
- Five distinct I–I pairs [1], with ten atoms arranged as five molecules [1].
- Exactly four Na+ and four Cl- ions [1]; opposite charges alternate in a touching lattice [1]; all charges correct [1].
- Molecules for iodine; ions for sodium chloride [1].