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Solids: Structure and Physical Properties

Learn and apply Solids: Structure and Physical Properties in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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Solids: Structure and Physical Properties: Orientation

Structure–property questions are “chain” questions: structure type → particles/forces → what can move → property. This lesson gives you the templates to write those chains quickly and safely.

Treat this as an extension of Atomic Structure (A Level), then use the Chemical Bonding hub to compare models across the topic.

Definitions (Must Know)

A. Ionic lattice (giant ionic structure)

An ionic lattice is a giant structure of alternating ions held by ionic bonding in all directions.

B. Metallic lattice

A metallic lattice is a giant structure of positive ions in a “sea” of delocalised electrons.

C. Simple molecular substance

A simple molecular substance contains molecules held together by intermolecular forces.

D. Giant covalent structure (network)

A giant covalent structure is a network of atoms linked by covalent bonds throughout the structure.

E. Hydrogen-bonded structure

A hydrogen-bonded structure is a lattice of molecules held together by hydrogen bonds (e.g. ice).

Detailed Explanations

StructureWhy melting point is high/lowConductivityTypical example
Ionic latticestrong ionic bonding throughout lattice → highsolid: no (ions fixed); molten/aq: yes (mobile ions)NaCl
Metallic latticestrong attraction between ions and delocalised electrons → highyes (delocalised electrons)Cu
Simple molecularweak intermolecular forces between molecules → lowno (no mobile charged particles)iodine, I₂
Hydrogen-bonded (molecular)hydrogen bonds between molecules → higher than similar Mᵣ moleculesno (no mobile ions/electrons)ice, H₂O
Giant covalentmany covalent bonds throughout network → very highgraphite: yes; diamond: noC (graphite/diamond)
Five syllabus solid structures: alternating ions in sodium chloride or magnesium oxide, positive ions and delocalised electrons in copper, iodine molecules, covalent networks in diamond and layered graphite, and bent water molecules in ice linked by hydrogen bonds into an open hexagonal network, with covalent O–H bonds drawn solid and hydrogen bonds dashed
The specified lattices differ in particles, attractions and mobile charge carriers; these microscopic differences determine their physical properties.
Melting Points by Structure Type (Approx.)Simple molecular solids melt at low temperatures (weak intermolecular forces), while ionic/metallic/giant covalent structures melt at much higher temperatures (strong bonding throughout).Melting Points by Structure Type (Approx.)SubstanceMelting point (°C)
Simple molecular solids melt at low temperatures (weak intermolecular forces), while ionic/metallic/giant covalent structures melt at much higher temperatures (strong bonding throughout).
Data table
SubstanceMelting point
I2114
NaCl801
Cu1085
SiO21710

B. Identifying structure from properties (workflow)

  1. Look at conductivity:
    • conducts as a solid → metallic or graphite (delocalised electrons)
    • does not conduct as a solid → could be ionic, simple molecular, or diamond-like giant covalent
  2. Look at melting point:
    • very high → ionic/metallic/giant covalent
    • low → simple molecular
  3. If it is ionic, check conductivity when molten/aqueous:
    • conducts when molten/aqueous → mobile ions

Mini example: “high melting point + conducts as a solid” strongly points to a metallic lattice (mobile delocalised electrons).

Do not identify a structure from a single property if several models fit. For example, high melting point alone could indicate ionic, metallic or giant molecular structure. Combine all available evidence.

C. What each specified lattice looks like

  • Ionic — NaCl and MgO: a repeating three-dimensional arrangement of oppositely charged ions. Ionic attractions act in all directions; these are not discrete molecules.
  • Simple molecular — iodine: discrete I₂ molecules occupy lattice positions, with London forces between molecules and covalent I–I bonds within them.
  • Giant molecular — diamond: each C is covalently bonded to four others in a three-dimensional tetrahedral network; there are no mobile electrons.
  • Giant molecular — graphite: each C is bonded to three others in planar layers. One electron per C is delocalised; weak attractions between layers allow them to slide.
  • Hydrogen-bonded — ice: water molecules form an open lattice held by hydrogen bonds; each molecule remains a discrete H₂O molecule.
  • Metallic — copper: positive metal ions form a lattice surrounded by mobile delocalised electrons.

D. Melting/boiling point

  • ionic/giant covalent/metallic: generally high (strong bonding in lattice)
  • simple molecular: generally low (weak intermolecular forces)

E. Electrical conductivity

  • ionic: conducts when molten/aqueous (mobile ions), not when solid
  • metallic: conducts as solid (delocalised electrons)
  • simple molecular: usually does not conduct (no mobile charges)
  • giant covalent:
    • graphite conducts (delocalised electrons)
    • diamond does not (no free electrons)

F. Solubility

  • “like dissolves like” is a starting idea:
    • ionic compounds often dissolve in water (ion–dipole attraction)
    • non-polar molecules dissolve in non-polar solvents (London forces)

Worked Examples

Modelled example 1

Explain Sodium Chloride Conduction

Core

Problem

Explain why solid NaCl does not conduct electricity but molten NaCl does.
Study the worked solution

  1. Analyse the solid

    Method

    State that the ions occupy fixed lattice positions.

    Reason

    Charged particles must be mobile to carry current through the sample.

    Working

    Solid NaCl: ions present but fixed; no conduction.

  2. Analyse the melt

    Method

    State that melting frees the ions to move.

    Reason

    Mobile Na⁺ and Cl⁻ ions transport charge to the electrodes.

    Working

    Molten NaCl: mobile ions; conducts.

Guided practice 2

Compare Graphite and Diamond Conduction

About 7 min

Problem

Explain why graphite conducts electricity but diamond does not.

Try this before viewing the solution

Hints

Hint 1: compare carbon bonding
Each graphite carbon bonds to three others; each diamond carbon bonds to four.
Hint 2: find a mobile carrier
Ask which structure retains one delocalised electron per carbon.
View solution step by step

  1. Analyse graphite

    Method

    State that each carbon makes three covalent bonds in a layer.

    Reason

    One electron per carbon remains delocalised and mobile through the layer.

    Working

    Graphite has mobile delocalised electrons and conducts.

  2. Analyse diamond

    Method

    State that each carbon makes four covalent bonds.

    Reason

    All four valence electrons are localised in bonds, leaving no mobile charge carrier.

    Working

    Diamond has no delocalised electrons and does not conduct.

Common misconception 3

Correct an Ice-Conduction Claim

Find and correct the mistake

Learner claim

Asked why ice does not conduct electricity, a learner says, “Hydrogen bonds contain hydrogen ions, so ice should have mobile charged particles.” Identify the error and give the correct explanation.

Classify the particles

Ice consists primarily of

View solution step by step

  1. Classify the solid

    Method

    Identify ice as a hydrogen-bonded molecular solid.

    Reason

    Hydrogen bonds are intermolecular attractions between discrete H₂O molecules, not reservoirs of free ions.

    Working

    Ice lattice particles: neutral H₂O molecules.

  2. Apply the carrier test

    Method

    State that there are no mobile ions or delocalised electrons.

    Reason

    Without mobile charged particles, electrical current cannot flow.

    Working

    Ice does not conduct electricity.

Examiner practice 4

Compare Iodine and Diamond Melting Points

4 marks

Problem

Explain why solid iodine has a much lower melting point than diamond. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Describe iodine

    1 mark

    Method

    Identify discrete I₂ molecules in a simple molecular lattice.

    Reason

    The covalent I–I bonds remain within each molecule during melting.

    Working

    Iodine melts by overcoming London forces between molecules.

  2. Explain iodine's melting point

    1 mark

    Method

    State that intermolecular London forces are relatively weak.

    Reason

    Only modest energy is needed to separate the molecules.

    Working

    Solid iodine has a comparatively low melting point.

  3. Describe diamond

    1 mark

    Method

    Identify a giant covalent three-dimensional network.

    Reason

    Carbon atoms are linked by strong covalent bonds throughout the structure.

    Working

    Diamond has no discrete molecules to separate.

  4. Explain diamond's melting point

    1 mark

    Method

    State that many strong covalent bonds must be overcome.

    Reason

    This requires far more energy than overcoming intermolecular forces.

    Working

    mp(diamond)≫ mp(I₂).

Challenge 5

Explain Two Graphite Properties

Minimal support

Problem

Graphite conducts electricity and is soft enough for its layers to slide over one another. Explain both properties from its structure and bonding.

Try this before viewing the solution

Hints

Hint 1: within each layer
One electron per carbon is delocalised through the bonded layer.
Hint 2: between layers
Strong covalent bonds act within layers, but only weak attractions act between layers.
View solution step by step

  1. Explain conduction

    Method

    Identify mobile delocalised electrons within the layers.

    Reason

    Each carbon bonds to three neighbours, leaving one electron able to move and carry charge.

    Working

    Delocalised electrons → conduction along layers.

  2. Explain softness

    Method

    Contrast strong covalent bonding within layers with weak attraction between layers.

    Reason

    The weak interlayer attractions allow whole layers to slide without breaking the covalent network inside a layer.

    Working

    Weak interlayer forces → layers slide.

Mind Stretchers

Mind stretcher 1Extension

A solid has a high melting point, conducts electricity as a solid and is malleable. Deduce the structure type and justify using bonding and particles.

Show Answer

Mark scheme:

  • Metallic lattice.
  • High melting point: strong attraction between metal ions and delocalised electrons.
  • Conducts as a solid: delocalised electrons are mobile.
  • Malleable: layers of positive ions can slide while remaining attracted to the non-directional sea of delocalised electrons.

Mind stretcher 2Extension

Solid X has a high melting point, does not conduct electricity as a solid, but conducts when molten. Deduce its structure and explain each observation.

Show Answer

Mark scheme:

  • X has a giant ionic lattice.
  • Strong electrostatic attractions between oppositely charged ions require much energy to overcome, giving a high melting point.
  • In the solid, ions are fixed and cannot carry charge.
  • When molten, ions are mobile and carry charge.