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.
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 (London 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.
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 London forces (instantaneous dipole-induced dipole forces). Iodine has 106 electrons per molecule, so its London 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 London 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 London forces between I2 molecules have to be overcome; the strong covalent bonds within the molecules are not broken.
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 London 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.
Comparing the Molecular Crystals
Simple Molecular Lattices: Iodine, Ice and Buckminsterfullerene
Drag to rotate, scroll or pinch to zoom. Each crystal is built from whole molecules held in place by weak forces between them. Press Heat it to see which bonds actually break when a simple molecular solid melts.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
| Crystal | Molecule | Intermolecular force | Melting or sublimation | Why |
|---|---|---|---|---|
| Iodine | I2 | London forces | 114 °C | Large, polarisable molecules give fairly strong London 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.
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
AQA 3.1.3.4 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 (3.1.3.7) 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.
Check Your Understanding
Use these short activities to check simple molecular lattices on substances that do not appear on this page.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
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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