Simple Molecular Structures
A concise revision guide to simple molecular lattices: iodine and ice, 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.
- 3.1.3.4i-d
- 3.1.3.4ii-c
- 3.1.3.4ii-d
What these spec points say
- 3.1.3.4i-d know molecular crystal structures
- 3.1.3.4ii-c know the structure of ice as an example of crystal structure
- 3.1.3.4ii-d know the structure of iodine as an example of crystal structure
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 (van der Waals forces, permanent dipoles, 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.
Heating iodine separates whole I₂ molecules: only the weak 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 van der Waals forces (induced dipole-dipole forces).
Iodine has 106 electrons per molecule, so its van der Waals forces are strong enough for it to be a solid at room temperature, but 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 van der Waals 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 van der Waals 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; far higher than for molecules of similar size such as H2S (−86 °C) or CH4 (−182 °C), because hydrogen bonds are stronger than van der Waals 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.
Quick Check: Water Against Hydrogen Sulfide
Two molecules of similar shape, melting almost 82 degrees apart. Choose the reason.
Comparing the Molecular Crystals
| Crystal | Molecule | Intermolecular force | Melting or sublimation | Why |
|---|---|---|---|---|
| Iodine | I2 | van der Waals forces | 114 °C | Large, polarisable molecules give fairly strong van der Waals forces |
| Ice | H2O | Hydrogen bonds | 0 °C | Hydrogen bonds are the strongest intermolecular force, but there are only four per molecule |
Key idea: The melting point of a molecular crystal is set by the intermolecular force, never by the covalent bonds inside the molecules.
Iodine, ice and C₆₀ all keep their molecules intact on heating; the force between the molecules sets each property.
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.
- 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 AQA Specification
The specification names ice and iodine as its molecular crystal examples and asks you to relate their melting points and conductivity to structure and bonding, and to explain the energy changes on melting. The intermolecular forces themselves are on the Intermolecular Forces pages.
AQA focus: Be ready to draw a fragment of the iodine lattice showing separate I2 molecules, and to explain why ice floats.
FAQs
These questions cover the simple molecular ideas students most often get wrong.
Does iodine break its I-I bonds when it sublimes?
No. Only the van der Waals forces between I2 molecules are overcome. The purple vapour is made of intact I2 molecules.
Why does ice float on water?
Each molecule in ice forms four hydrogen bonds, holding the molecules in an open hexagonal lattice. When ice melts some hydrogen bonds break and the molecules move closer, so liquid water is denser than ice.
Why does iodine have a higher melting point than ice?
An I2 molecule has 106 electrons against 10 in H2O, so its van der Waals forces outweigh the hydrogen bonds in ice. Hydrogen bonds are stronger only when molecules of similar size are compared.
Does molten iodine conduct electricity?
No. Molten iodine is still neutral I2 molecules. There are no ions and no delocalised electrons to carry charge.
Why must I name the intermolecular force in an answer?
Writing “weak forces” alone rarely earns the mark. Name the force, van der Waals forces for iodine and hydrogen bonds for ice, and say that it acts between the molecules.
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.
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