Comparing covalent structures
Compare small molecules, poly(ethene) chains and giant covalent networks. Use bonds, forces and mobile charge carriers to explain their properties.
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Methane, poly(ethene) and diamond all contain covalent bonds, but they behave very differently when heated. The useful question is where the covalent bonds are, and what must change for the material to flow.
Build on Molecular structures and properties and Giant covalent structures. We now compare those structures with long polymer chains.
Small molecules: bonds within, forces between
Methane, CH₄, consists of separate small molecules. Each molecule has strong C–H covalent bonds. Relatively weak intermolecular forces act between molecules.
When methane melts or boils, the molecules move relative to one another or separate. Their C–H bonds remain intact. Much less energy is needed to overcome the intermolecular attractions than to break covalent bonds throughout a giant network, so methane has low melting and boiling points.
Iodine, I₂, is also simple molecular. Each molecule contains two covalently bonded iodine atoms. It is a solid at room temperature because its intermolecular attractions are stronger than methane’s. Simple molecular does not mean “always a gas”, and different molecular substances need not have the same melting point.
Neither pure methane nor pure iodine has mobile ions or delocalised electrons to carry current. They do not conduct electricity.
Poly(ethene): long molecules, separate chains
A polymer contains large molecules built from many repeating units. In poly(ethene), strong covalent bonds join carbon atoms along each long chain; hydrogen atoms are bonded to the carbons. Each chain is a molecule, and intermolecular forces act between chains.
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Poly(ethene) can soften and melt when heated because its chains become able to move past one another as intermolecular attractions are overcome. This does not require breaking the strong C–C bonds along the chain. Breaking those bonds would change the molecules chemically.
Long chains have many points of contact, so their attractions can be greater overall than those between small molecules such as methane. The temperature at which poly(ethene) softens or melts depends on its chain structure and packing. Avoid a fixed rule that every polymer has a high melting point.
Poly(ethene) does not conduct electricity: it has neither mobile ions nor delocalised electrons. This makes it useful as electrical insulation, within its suitable temperature range.
Many covalent bonds along one chain do not create covalent bonds between all chains. The explanation above concerns poly(ethene). Other polymers can have different structures, including covalent links between chains, so do not apply its softening behaviour to every polymer.
You will learn how repeat units form in Polymers. Here, the focus is how the existing structure explains the material’s properties.
Giant covalent networks: bonds throughout
Diamond and silicon dioxide have covalent bonds extending throughout rigid three-dimensional networks. Graphite has covalent bonds throughout each extended layer, with weaker forces between layers.
Disrupting these covalent networks requires breaking many strong covalent bonds and therefore a large energy input. Silicon dioxide has a high melting point. At ordinary pressure, graphite instead sublimes at a very high temperature.
Diamond and silicon dioxide do not conduct electricity because they have no mobile charge carriers. Graphite is the conducting exception in this set: its delocalised electrons move along the layers. The presence of covalent bonds alone does not decide conductivity.
Put graphite and then diamond in the circuit, strike each one, and compare them with iodine, a simple molecular solid.
Graphite, a giant covalent structure, at 25 °C: a solid. The circuit is off.
- State
- solid
- Melting point
- —
- Boiling point
- —
- Bulb
- switched off
- Melting overcomes
- —
Try this
0 of 4 doneMelt a giant structure and a simple molecular substance. (not done yet)
Melting a giant structure breaks strong bonds all through it; melting a simple molecular substance only overcomes weak forces between molecules, which takes far less energy.
Test sodium chloride as a solid, as a melt and in water. (not done yet)
Ions carry the current, but only when they can move: in the melt or the solution, not when held in the lattice.
Strike sodium chloride, then copper. (not done yet)
A blow pushes ions of the same charge next to each other in NaCl, so they repel and the crystal shatters. In copper the layers of ions slide in the sea of electrons, so the metal bends instead.
Test diamond and graphite for conductivity. (not done yet)
Each carbon in graphite bonds to three others, leaving one electron per atom delocalised along its layer. Diamond uses all four outer electrons in bonds, so nothing can carry a current.
Compare the change, not just the bond name
| Structure | Examples | Strong covalent bonds occur… | What permits melting, boiling or flow? | Electrical conductivity of these examples |
|---|---|---|---|---|
| Simple molecular | Methane, iodine | Within separate small molecules | Overcoming intermolecular attractions lets molecules move or separate; covalent bonds remain intact | Do not conduct: no mobile charge carriers |
| Long-chain molecular polymer | Poly(ethene) | Along each long molecule | Overcoming attractions between chains lets chains move; the chain bonds remain intact | Does not conduct: no mobile charge carriers |
| Giant covalent | Diamond, graphite, silicon dioxide | Throughout a network or extended layers | Breaking up the network requires many strong covalent bonds to break; graphite sublimes at ordinary pressure | Graphite conducts through delocalised electrons; diamond and silicon dioxide do not |
For any structure-property question:
- Identify the particles and their arrangement from the information supplied.
- Name the interaction relevant to the change, or the mobile charge carrier relevant to conduction.
- Link that feature to the observation. Check whether another structure could also explain the same single property.
A high melting point alone does not prove a giant covalent structure: many ionic and metallic substances also have high melting points. Use the stated candidates, diagrams and other observations together.
Worked examples
Modelled example 1
Why poly(ethene) can be reshaped
Problem
Poly(ethene) consists of long chains with no covalent links between separate chains. Explain why warming it enough allows it to be reshaped without changing its molecules.
Study the worked solution
Identify the two kinds of connection
Method
Separate covalent bonds within chains from intermolecular attractions between chains.
Reason
The material consists of separate long molecules, rather than one giant covalent network.
Working
Strong bonds along each chain; intermolecular forces between chains.
Explain chain movement
Method
Overcome enough of the attractions between chains for them to move past one another.
Reason
The C–C bonds along each chain remain intact, so softening does not require the molecules to break apart.
Working
Chain movement allows the warmed material to flow and be reshaped.
Challenge 2
Identify a substance from several properties
Property-evidence transfer
An unlabelled sample is known to be diamond, graphite or silicon dioxide. It conducts electricity, has a slippery surface and remains solid at a supplied high test temperature. Identify the sample and explain how its structure accounts for these observations.
Use every observation
Hints
Hint 1: use the conducting exception
Of the three listed giant covalent substances, only one has delocalised electrons.
Hint 2: account for softness separately
Slipperiness is explained by layers that can slide. Many strong covalent bonds within layers require much energy to break.
View solution step by step
Use conductivity
Method
Identify graphite from mobile delocalised electrons.
Reason
Diamond and silicon(IV) oxide lack mobile charge carriers.
Working
Conducts → graphite.Use slipperiness
Method
Identify graphite’s layered structure.Reason
Weak forces between layers allow them to slide.Working
Sliding layers → slippery feel.Account for remaining solid
Method
Identify strong covalent bonds within the giant layers.
Reason
Many such bonds require much energy to break.Working
Strong bonds within graphite layers are consistent with the observed heat resistance. The conductivity and slipperiness identify graphite among the stated candidates.
Try it yourself
Mind stretcher 1: Two materials that do not conductExtension
Both poly(ethene) and silicon dioxide do not conduct electricity. Explain why that observation does not make their structures the same. Then compare the interactions involved in allowing each material to flow on heating.
Show answer
Both lack mobile charge carriers, which explains their non-conduction.
Poly(ethene) consists of separate long-chain molecules. Its chains can move as intermolecular attractions between them are overcome, while the covalent bonds along each chain remain intact.
Silicon dioxide has a continuous giant covalent network. Melting requires disrupting that network and breaking many strong covalent bonds, so much more energy is needed than for overcoming the attractions between poly(ethene) chains.
Mind stretcher 2: What does a high melting point tell you?Extension
A solid has a high melting point. A learner concludes, “It must be giant covalent.” Explain why this conclusion is not secure and suggest additional evidence that could help distinguish an ionic solid from silicon dioxide.
Show answer
A high melting point is consistent with strong interactions in several structures, including ionic lattices and giant covalent networks. It is not enough to identify the structure.
Electrical conductivity when molten can help: an ionic liquid contains mobile ions and conducts, whereas silicon dioxide has no mobile ions or delocalised electrons and does not conduct. A structural diagram or information about the elements and bonding can also help. Do not assume that every solid dissolves in water.
Practise and check
See what you know across this topic, then go back to anything you got wrong.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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