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9476 / 2027
Chemical Bonding

Lesson 6 of 6

Identify a structure from its properties

Compare all five required crystalline structures and reason from evidence.

A-Level 9476 (2026-2027)

  • I can describe the required crystal structures.Syllabus

    Syllabus 9476, 2(l). Describe the required crystal structures.

  • I can predict properties from structure and bonding.Syllabus

    Syllabus 9476, 2(m). Predict properties from structure and bonding.

  • I can infer structure and bonding from supplied evidence.Syllabus

    Syllabus 9476, 2(n). Infer structure and bonding from supplied evidence.

Different solids contain different moving charges and attractions

Name the particles, arrangement and attraction before predicting a property.

The required crystalline structures
Example and structureParticles and attractionProperties to explain
NaCl and MgO: ionicAn extended array of oppositely charged ions; electrostatic lattice attraction.High melting points. Fixed ions prevent solid conduction; mobile ions conduct when molten or suitably dissolved. MgO has stronger attractions than NaCl.
I2: simple molecularDiscrete I2 molecules; strong covalent bonds within each, weaker attractions between them.Relatively low melting point; no mobile charged particles in the pure solid or liquid.
Diamond: giant covalentEach C covalently bonds to four C atoms in a three-dimensional network.Very hard; high thermal stability; no mobile electrons for electrical conduction.
Graphite: giant covalent layersEach C bonds to three in a sheet; delocalised electrons within sheets and weaker attractions between sheets.Conducts along sheets; layers slide, so it is soft despite strong in-plane bonding.
Ice: hydrogen-bonded molecularDiscrete H2O molecules in an open hydrogen-bond network.Lower density than liquid water; melting changes intermolecular organisation.
Copper: metallicPositive metal ions and delocalised electrons throughout the metal.Electrical/thermal conduction; layers can shift while metallic attraction persists.

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 and graphite are required H2 examples. The silica comparison reinforces the distinction between an extended network and discrete molecules.

Crystalline means a regular, repeating arrangement; it does not mean that every crystalline material is ionic. The unit-cell construction is not required. Electrical conductivity needs mobile charged particles, so identify whether the carrier is an electron or ion and whether it can move in the stated physical state.

Solubility depends on the balance of attractions broken and formed, as well as disorder, not a universal "ionic means soluble" rule. Some ionic compounds dissolve very little. Molecular substances capable of strong interactions with water can dissolve; non-polar substances often dissolve better in non-polar solvents.

Check your understanding

Graphite conducts electricity but diamond does not. Why?

  1. Graphite contains ions between its layers, which move when a voltage is applied.
  2. Diamond's covalent bonds are too strong to break, while graphite's bonds break to free electrons.
  3. Each C in graphite bonds to three others, leaving one delocalised electron per atom.
Show the answer

Each C in graphite bonds to three others, leaving one delocalised electron per atom.

In graphite the fourth outer electron of each carbon is delocalised within the layers and can move. In diamond all four electrons are in localised covalent bonds.

Use several observations to identify a structure

A property is evidence; it is rarely a unique label by itself.

Worked exampleAn unknown solid

A solid melts at a high temperature, does not conduct when solid, but conducts when molten. What is the most plausible structure?

Try it first. Then show the worked stepsHide the worked steps
  1. High melting temperature suggests strong attractions, but could fit several giant structures.
  2. Failure to conduct as a solid argues against an ordinary metal and conducting graphite.
  3. Conduction after melting is consistent with charged ions becoming mobile.
Answer

A giant ionic lattice is the best explanation from these observations. Say which result distinguishes it; high melting point alone is insufficient.

Worked exampleA soft conductor

Another solid conducts electricity, is soft and has very high thermal stability. Why is graphite plausible?

Try it first. Then show the worked stepsHide the worked steps
  1. Delocalised electrons account for conduction.
  2. Weak interlayer attractions permit sliding and account for softness.
  3. Strong covalent bonds within layers explain the high thermal stability.
Answer

Different parts of the structure explain different properties. Calling the whole structure "weakly bonded" contradicts its thermal behaviour.

A practical evidence sequence
  1. Observe before identifying

    Record appearance, melting behaviour, solubility and conductivity under stated conditions.

  2. Control the conductivity comparison

    Use comparable temperature, electrode arrangement and solution concentration; avoid confusing a wet sample with the dry solid.

  3. Match carriers to state

    Electron conduction can occur in a solid metal; ionic conduction requires mobile ions.

  4. Keep conclusions proportional

    Suggest a structure consistent with the evidence and identify any extra test needed to distinguish alternatives.

Check your understanding

A solid has a very high melting point and does not conduct as a solid or when molten. Which structure fits best?

  1. Simple molecular, such as iodine or sulfur
  2. Giant covalent, such as diamond or silicon dioxide
  3. Giant ionic, such as sodium chloride or MgO
Show the answer

Giant covalent, such as diamond or silicon dioxide

A very high melting point needs strong bonds throughout. No conduction even when molten rules out ions and delocalised electrons.