Simple Molecular Structures
A concise revision guide to simple molecular lattices: iodine, ice and buckminsterfullerene, the strong covalent bonds inside the molecules and the weak forces between them, and why that gives low melting points, no conductivity and solubility that follows polarity.
- 4.2.1(b)-i
- 4.2.1(b)-ii
- 4.2.1(b)-iii
What these spec points say
- 4.2.1(b)-i describe, in simple terms, the lattice structure of a crystalline solid which is simple molecular, including iodine
- 4.2.1(b)-ii describe, in simple terms, the lattice structure of a crystalline solid which is simple molecular, including buckminsterfullerene C60
- 4.2.1(b)-iii describe, in simple terms, the lattice structure of a crystalline solid which is simple molecular, including ice
Simple Molecular Lattices: Iodine, Ice and C₆₀
Choose a step to watch a crystal built from whole molecules, then heat each one to see which forces melting has to overcome.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Molecules, Bonds and Conducting
Before you start, check the GCSE ideas this page builds on.
What a Simple Molecular Lattice Is
Some solids are made of molecules: small groups of atoms held together by strong covalent bonds.
In the solid the molecules pack into a regular lattice, but the forces between the molecules are weak intermolecular forces, not covalent bonds. This gives every simple molecular solid the same family of properties.
| Feature | Inside each molecule | Between molecules |
|---|---|---|
| Type of force | Covalent bonds | Intermolecular forces (id-id forces, pd-pd forces, hydrogen bonds) |
| Strength | Strong (hundreds of kJ mol−1) | Weak (a few to about 40 kJ mol−1) |
| Broken on melting or boiling? | No | Yes |
| Consequence | The molecules survive changes of state | Low melting and boiling points, soft solids |
Key idea: When iodine sublimes, purple I2 molecules leave the crystal intact. Nothing covalent breaks.
In a simple molecular lattice the covalent bonds inside each molecule stay intact on melting and boiling, and only the weak intermolecular forces between molecules are overcome.
Quick Check: Phosphorus and Chlorine
Drag each term into place to compare two molecular solids the page does not use.
Iodine
Solid iodine is a lattice of I₂ molecules. Each molecule has one strong I-I covalent bond; the molecules are held in the lattice only by instantaneous dipole-induced dipole (id-id) forces.
Iodine has 106 electrons per molecule, so its id-id forces are strong enough for it to be a solid at room temperature.
They are still far weaker than covalent bonds: iodine sublimes on gentle warming to give a purple vapour.
| Property | Observation | Explanation |
|---|---|---|
| Melting point | 114 °C (sublimes readily) | Only weak id-id forces between I2 molecules are overcome |
| Electrical conductivity | None, solid or molten | No ions and no delocalised electrons; the molecules are neutral |
| Solubility | Almost insoluble in water; dissolves in hexane | A non-polar molecule interacts best with a non-polar solvent |
| Hardness | Soft, flaky crystals | The layers of molecules are easily separated |
Exam sentence: Iodine has a low melting point because only the weak id-id forces between I2 molecules have to be overcome; the strong covalent bonds within the molecules are not broken.
Quick Check: Which Solid Is Molecular?
Use the melting point and the conductivity of each solid to decide.
Quick Check: Explain the Solubility of Naphthalene
Write a short explanation, then compare it with the mark points and the model answer.
Ice
Ice is a lattice of H₂O molecules held together by hydrogen bonds.
Each water molecule can form up to four hydrogen bonds, two through its hydrogen atoms and two through the lone pairs on oxygen, so in ice every oxygen is surrounded tetrahedrally by four others.
This makes an open, hexagonal lattice with a lot of empty space.
When ice melts some of the hydrogen bonds break and the molecules move closer together, so liquid water is denser than ice and ice floats.
Hydrogen bonds are the strongest intermolecular force, which is why water has a far higher melting and boiling point than a molecule of its size would otherwise have.
But they are still weak next to the O-H covalent bonds, which survive melting and boiling.
| Property | Observation | Explanation |
|---|---|---|
| Melting point | 0 °C | Hydrogen bonds between molecules are overcome; higher than for I2-sized non-polar molecules because hydrogen bonds are stronger than id-id forces |
| Density | Ice is less dense than water | The hydrogen-bonded open lattice holds the molecules further apart than in the liquid |
| Conductivity | None as a pure solid or liquid | Neutral molecules; no mobile charge carriers |
| Solubility | Miscible with other hydrogen-bonding liquids | Water hydrogen bonds to alcohols and dissolves ionic solids by hydrating the ions |
Exam sentence: In ice each water molecule is hydrogen bonded to four others in an open lattice, so the molecules are further apart than in liquid water and ice is less dense.
Each water molecule in ice is hydrogen bonded to four others in an open lattice, so ice is less dense than liquid water and floats.
Quick Check: Water Against Hydrogen Sulfide
Two molecules of similar shape, melting almost 82 degrees apart. Choose the reason.
Buckminsterfullerene, C60
Buckminsterfullerene is a form of carbon made of separate C60 molecules: sixty carbon atoms bonded into a hollow ball of twelve pentagons and twenty hexagons, like a football.
Within each ball every carbon is covalently bonded to three others (with delocalised electrons over the surface); between the balls there are only id-id forces.
Because there are molecules, C60 is a simple molecular solid, unlike graphite and diamond, which are giant molecular.
| Property | C60 | Graphite (for comparison) |
|---|---|---|
| Structure | Simple molecular: separate C60 molecules | Giant molecular: layers extend through the whole crystal |
| Melting behaviour | Sublimes above about 600 °C | Sublimes above 3600 °C |
| Conductivity | Very poor: electrons are delocalised over each ball but cannot move between balls | Good along the layers |
| Solubility | Dissolves in non-polar solvents such as benzene | Insoluble |
Exam sentence: C60 has a much lower sublimation temperature than graphite because only weak id-id forces between C60 molecules need to be overcome, whereas in graphite strong covalent bonds must be broken.
Quick Check: Two Forms of Carbon
Both are pure carbon, but only one is made of molecules. Choose the explanation.
Comparing the Molecular Crystals
| Crystal | Molecule | Intermolecular force | Melting or sublimation | Why |
|---|---|---|---|---|
| Iodine | I2 | id-id forces | 114 °C | Large, polarisable molecules give fairly strong id-id forces |
| Ice | H2O | Hydrogen bonds | 0 °C | Hydrogen bonds are the strongest intermolecular force, but there are only four per molecule |
| Buckminsterfullerene | C60 | id-id forces | above 600 °C | A very large molecule has many electrons and strong id-id forces |
Key idea: The melting point of a molecular crystal is set by the intermolecular force, never by the covalent bonds inside the molecules.
Quick Check: Rank Five Molecular Solids
Drag the five solids into order of melting temperature, lowest at the top.
Quick Check: The Whole Page in Five Rounds
Pick the one accurate statement in each round to pull the molecular crystals together.
Common Exam Mistakes
- Writing that iodine or ice “has weak covalent bonds”. The covalent bonds are strong; the intermolecular forces are weak.
- Saying ice floats because “ice is lighter”. Give the structural reason: the open hydrogen-bonded lattice.
- Claiming a molecular solid conducts when molten. There are no ions; a melt of I2 is still neutral molecules.
- Calling C60 a giant structure because it is carbon. It is a molecule of exactly sixty atoms.
- Forgetting to name the intermolecular force. “Weak forces” alone rarely earns the mark.
Exam sentence: Strong inside, weak between: molecular crystals melt at low temperatures because only the weak intermolecular forces are overcome.
Where This Sits in the Cambridge Syllabus
Cambridge 4.2.1(b) names three simple molecular lattices: iodine, buckminsterfullerene C60 and ice.
You must describe each in simple terms and, under 4.2.2, interpret and predict melting point, boiling point, conductivity and solubility from the structure and bonding.
C60 is the example that separates a simple molecular solid from a giant molecular one such as graphite, even though both are pure carbon.
Cambridge focus: C60 is a molecule of 60 atoms; graphite has no molecules. That single difference explains why C60 melts (sublimes) far below graphite.
FAQs
These questions target the points students most often get wrong about molecular crystals.
Do covalent bonds break when iodine sublimes?
No. Whole I2 molecules leave the crystal. Only the weak id-id forces between the molecules are overcome; the strong I-I covalent bond is not broken.
Why does ice float on water?
In ice each molecule is hydrogen bonded to four others in an open lattice with a lot of empty space. On melting some hydrogen bonds break and the molecules move closer, so liquid water is denser than ice.
Why does molten iodine not conduct electricity?
Molten iodine still contains only neutral I2 molecules. There are no ions and no delocalised electrons, so there are no charged particles free to move.
Why does iodine dissolve in hexane but not in water?
Iodine and hexane are both non-polar, so the id-id forces between them are similar to those they replace. Water molecules hydrogen bond strongly to each other, and iodine cannot replace those hydrogen bonds.
C60 is pure carbon, so is it a giant covalent structure?
No. C60 is a molecule of exactly sixty carbon atoms. Strong covalent bonds hold each ball together, but only id-id forces act between the balls, so it is simple molecular and sublimes far below the melting point of graphite.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
Keep this note — free
Save your progress across every Cambridge topic. A free account remembers which topics you have covered, saves your question scores, and syncs across your phone and laptop.
- Track every topic you have finished
- Keep your practice-question scores
- No payment, no card, free forever
Already registered? Log in