Solids: Structure and Physical Properties

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.

Build on atomic structure, then use your course’s Chemical Bonding topic navigation 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).

Key Ideas (What Earns Marks)

• Start every answer by naming the structure type (ionic / metallic / simple molecular / hydrogen-bonded / giant covalent).
• Then state the particles and forces present (ions + ionic bonding; molecules + IMFs; delocalised electrons; covalent network).
• Conductivity must name the charge carriers:
  • ions (molten/aqueous ionic substances)
  • delocalised electrons (metals; graphite)

Detailed Explanations

A. Quick structure–property links

Melting Points by Structure Type (Approx.)

Data table

Substance	Melting point
I2	114
NaCl	801
Cu	1085
SiO2	1710

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)

Heat, test and strike each kind of lattice. The chart keeps the melting and boiling points you find, so you can compare them on one scale.

Reset

Sodium chloride, a giant ionic structure, at 25 °C: a solid. The circuit is off.

State

solid

Melting point

—

Boiling point

—

Bulb

switched off

Melting overcomes

—

Sodium chlorideIodineCopperGraphite

Substance Sodium chloride, giant ionicMagnesium oxide, giant ionicDiamond, giant covalentGraphite, giant covalentCopper, giant metallicIodine, simple molecularWater, simple molecularMethane, simple molecular

Temperature

°C

Try this

0 of 4 done

1. Melt a giant structure and a simple molecular substance. (not done yet)

2. Test sodium chloride as a solid, as a melt and in water. (not done yet)

3. Strike sodium chloride, then copper. (not done yet)

4. Test diamond and graphite for conductivity. (not done yet)

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

Structures, electron availability and conduction

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

Neutral H₂O moleculesMobile H⁺ and O²⁻ ions

Check my attempt

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

Both structures, forces and melting comparison

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).

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Credit each structure and its force-to-melting link.

Mark points

1. 1 mark: Identifies simple molecular iodine.
2. 1 mark: Links weak London forces to lower melting point.
3. 1 mark: Identifies giant covalent diamond.
4. 1 mark: Links many strong covalent bonds to very high melting point.

Calculate my mark

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

Charge carrier and layer-sliding explanation

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.

Next step

Try the unknown ionic-solid Mind Stretcher independently.

Common Mistakes

• Saying ionic solids conduct (ions are fixed in the lattice).
• Saying all giant covalent structures conduct (diamond does not).
• Calling iodine or ice “giant covalent”: both contain discrete molecules, with intermolecular attractions between them.
• Deducing structure from one property when the evidence is not unique.

Use the topic check in Practise and check below to practise and check your understanding.

Exam Tips

• Start structure–property answers with the structure type (ionic / metallic / giant covalent / simple molecular).
• Then state the particles and forces involved (ions + ionic bonds; molecules + IMFs; electrons + metallic bonding).
• Conductivity: specify the charge carriers (mobile ions in melt/aq; delocalised electrons in metals/graphite).

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.