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wise otter.Chemistry
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K324 / 2027
Bonding and Structure

Lesson 4 of 6

Large molecules and giant structures

Compare polymer chains, diamond, graphite and silicon dioxide through their structures.

O-Level 2026 SEC G3 2027. Extensions are labelled Pure only.

Pure only
  • I can compare molecular and giant structures.Syllabus

    Syllabus K324, 3.4(e). Compare molecular and giant structures

  • I can compare diamond and graphite.Syllabus

    Syllabus K324, 3.4(f). Compare diamond and graphite

  • I can reason from properties to structure and back.Syllabus

    Syllabus K324, 3.4(g). Reason from properties to structure and back

A long molecule is different from a giant network

Compare poly(ethene), diamond, graphite and silicon dioxide.

Poly(ethene) consists of very long covalent molecules called macromolecules. Each chain contains many repeating units. Strong covalent bonds run along a chain, while intermolecular forces act between chains. Longer chains have more opportunities for intermolecular attraction than small molecules such as methane.

Long molecules, separate chains

Two uncrosslinked polyethene chainsTwo separate rows of six CH2 groups. Solid lines join groups along each chain and continue beyond the drawing. Dashed amber lines between the chains show weaker attractions, not covalent cross-links. Each labelled CH2 group contains one carbon and two hydrogens.CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2
Strong covalent bonds run along each chain. Weaker attractions act between the separate chains.

Example: uncrosslinked poly(ethene). Each CH2 label is a group, not one atom. The straight rows simplify the chain shapes. Cross-linked polymers have different structures.

Separate poly(ethene) chains: covalent bonds along each chain, intermolecular forces between chains. On softening, chains can move relative to one another.

Poly(ethene) needs more energy to separate its chains than methane needs to separate its small molecules. Heating can soften it without breaking its carbon backbone. It is an electrical insulator because it lacks mobile charged particles. A giant covalent structure, by contrast, has a continuous network of strong covalent bonds rather than separate molecules.

Diamond: four bonds per carbon

Diamond network fragment with four bonds at the central carbonThe central carbon bonds to four surrounding carbon atoms. Two plain bonds, a wedge towards the viewer and a dashed bond away from the viewer indicate a three-dimensional arrangement. Each outer carbon has three additional bond stubs continuing into the network.CCCCC
Strong covalent bonds extend through a three-dimensional network. There are no small diamond molecules.

Graphite: three bonds per carbon

One hexagon within a graphite sheetSix carbon atoms form a hexagon. Each has two bonds around the hexagon and a third bond continuing outwards into the sheet. These are covalent bonds. Each carbon also contributes one electron to delocalisation within the sheet; those electrons are not individually drawn here.CCCCCCGraphite sheets stacked with weaker attractions between themThree broad parallelograms represent whole sheets, not individual bonds. Dashed amber lines between sheets represent weaker attractions. The sheets can slide relative to each other.
Covalent bonds within sheets; weaker attractions between sheets. Each carbon supplies one delocalised electron within its sheet.

Silicon dioxide: a repeating network

Silicon dioxide coordination fragmentThe central silicon bonds to four oxygen atoms. Each of these oxygens bridges to a second silicon. Additional bonds from the four outer silicons continue beyond this fragment. The flat arrangement shows connectivity only; the real network is three-dimensional and the bonds are not at these drawn angles.SiSiSiSiSiOOOO
Each Si bonds to four O atoms; each O joins two Si atoms. Shared oxygen atoms give the ratio Si:O = 1:2.

Original network fragments, not to scale. Bond stubs continue beyond each drawing. Diamond and silica extend in three dimensions; graphite forms extended sheets. The drawings show connections, not measured bond angles.

Diamond: four bonds per carbon. Graphite: three bonds per carbon within layers. Silicon dioxide: a network of silicon and oxygen atoms. Learn to interpret the structures. Drawing the diamond and graphite structures is not required.
Use the arrangement to explain the property
StructureBonding and arrangementProperties and uses
DiamondEach C bonds to four others in a rigid three-dimensional network.Very hard: cutting tools. Very high melting point. No mobile electrons: does not conduct.
GraphiteEach C bonds to three others in a layer. One electron per C is delocalised. Weak forces act between layers.Conducts through mobile electrons. Layers slide: soft lubricant. Strong bonds within layers give a very high melting point.
Silicon dioxide, SiO2Each Si bonds to four O; each O bridges two Si. The formula is a ratio, not a separate molecule.Hard, high melting point; no mobile charged particles, so does not conduct. A principal constituent of sand.

Check your understanding

How can graphite be soft and still have a very high melting point?

  1. Its atoms are joined by weak bonds, but there are so many atoms that it takes a long time to melt.
  2. Its delocalised electrons make it soft, and the same electrons hold the layers together very tightly.
  3. Its layers slide over each other easily, but melting means breaking the strong covalent bonds within the layers.
Show the answer

Its layers slide over each other easily, but melting means breaking the strong covalent bonds within the layers.

Weak forces hold the layers together, so they slide and graphite feels soft. Melting breaks up the giant structure, which needs a lot of energy to break the covalent bonds in each layer.