Mini Chemistry
Part of Mini EducationSwitch subject

Molecular structures and properties

Distinguish bonds within molecules from attractions between molecules, and use structure to explain melting, boiling and electrical behaviour.

  • SEC G3 Pure Chemistry 2027

Reading settings

On this page

Methane boils at a low temperature even though its carbon–hydrogen covalent bonds are strong. The key is what separates during boiling: whole molecules move apart, while the atoms within each molecule stay bonded.

Use Covalent bonding and electron diagrams first if you need to identify shared electron pairs.

Bonds within molecules, attractions between molecules

A simple molecular substance consists of separate small molecules, such as methane, CH₄, or iodine, I₂. Strong covalent bonds join atoms within each molecule. Attractions between molecules are called intermolecular forces.

Covalent bonds within methane and attractions between methane moleculesThe left panel shows one carbon atom joined to four hydrogen atoms by solid lines, representing strong covalent bonds. The right panel shows two intact methane molecules inside separate dashed boundaries; a dotted line between the boundaries represents weaker intermolecular attraction. Boiling separates whole molecules while retaining each molecule’s four carbon–hydrogen bonds.Within one methane moleculeBetween methane moleculesHHHHCHHHHCHHHHCStrong C–H covalent bondsstay intact when methane boils.Weaker intermolecular attractionsare overcome as molecules separate.
Two levels of attraction in methane: strong covalent bonds join C and H inside each molecule; weaker intermolecular attractions act between whole molecules. Melting and boiling separate intact molecules, rather than breaking C–H bonds. The dotted line is a schematic attraction between molecules, not another covalent bond. These sketches do not show methane’s three-dimensional tetrahedral shape.

In methane and iodine, intermolecular forces are much weaker than the covalent bonds within their molecules. “Weak” is a comparison with those bonds; it does not mean there is no attraction or that every molecular substance has the same melting temperature.

What changes when a molecular substance melts or boils?

Melting allows molecules to move past one another. Boiling separates molecules into a gas. Both overcome attractions between molecules, while the molecules themselves remain intact, provided the substance does not react or decompose.

CH₄(l) → CH₄(g)

The formula stays CH₄ because boiling does not break the C–H bonds. Breaking those bonds would change the molecule and would be a chemical change.

Simple molecular substances typically melt and boil at relatively low temperatures compared with giant ionic or covalent structures: less energy is needed to overcome their intermolecular attractions. Their actual temperatures vary with the strength of those attractions; water’s intermolecular attractions, for example, are stronger than methane’s.

A covalent bond does not always mean a small molecule

Diamond, graphite and silicon dioxide contain covalent bonds, but do not consist of separate small molecules. Their bonds extend through giant structures. Disrupting those networks requires much more energy than separating methane molecules.

StructureExampleInteraction overcome in an ordinary physical changeElectrical behaviour
Simple molecularMethane, iodineIntermolecular attractions during melting or boiling; internal covalent bonds stay intact.No mobile charged particles in the pure material, so it does not conduct.
Giant covalentDiamond, silicon dioxideMany strong covalent bonds must be disrupted to break down the network.No mobile charge carriers in these examples, so they do not conduct.
Layered giant covalentGraphiteLayers can slide without breaking their internal bonds; destroying the network requires much energy.Delocalised electrons move along the layers and carry charge.

See Giant covalent structures for the complete structure–property explanations. Carbon does not simply melt at ordinary pressure: graphite sublimes at very high temperature. This still requires disrupting strong covalent bonding, rather than just sliding layers.

Neutral molecules and electrical conduction

Pure methane and iodine contain neutral molecules. Motion alone is not enough for electrical conduction: the moving particles must carry charge. These substances have neither mobile ions nor delocalised electrons available to carry current.

Check what happens in water

Do not extend this explanation to every solution of a molecular substance. Hydrogen chloride contains molecules as a gas, but forms ions when it dissolves in water. Hydrochloric acid therefore conducts through mobile ions.

Correct a melting or boiling explanation

Common misconception 1

Error Analysis (Fix the Student)

Find and correct the mistake

Learner response

A student writes: “Methane has a low boiling point because covalent bonds are weak.” Locate the first error and correct the explanation.

Identify what boiling overcomes

Force overcome during boiling
Covalent bonds in methane

View solution step by step

  1. Locate the first error

    Method

    Reject the claim that C-H covalent bonds are weak.

    Reason

    Boiling does not decompose methane molecules, so the strong bonds within each molecule remain intact.

    Working

    CH₄(l) → CH₄(g) keeps each methane molecule intact.

  2. Name the relevant attraction

    Method

    Focus on forces between separate methane molecules.

    Reason

    Only weak intermolecular forces need to be overcome to separate the molecules.

    Working

    Weak intermolecular forces → little energy needed for boiling.

  3. Write the correction

    Working

    Methane has a low boiling point because the intermolecular forces between its molecules are weak, not because its covalent bonds are weak.

Examiner practice 2

Property Comparison (Ionic vs Covalent)

4 marks

Examination question

Explain why sodium chloride, NaCl, has a much higher melting point than methane, CH₄. [4 marks]

Compare structure, force and energy

View solution step by step

  1. Explain sodium chloride

    2 marks

    Method

    Identify its giant ionic lattice and strong attractions.

    Reason

    Much energy is required to overcome electrostatic attraction between oppositely charged ions.

    Working

    Giant lattice + strong ionic attractions → high melting point.

  2. Explain methane

    2 marks

    Method

    Identify its simple molecular structure.

    Reason

    Only weak intermolecular forces between methane molecules are overcome during melting; the C-H bonds remain intact.

    Working

    Less energy is needed, so methane has the lower melting point.

Try these independently

Mind stretcher 1: Data Interpretation (Which Is Giant Covalent?)Extension

Question: Two pure covalent substances are compared. Substance A melts at -183°C. Substance B melts at 1610°C. Which is more likely to be a giant covalent structure? Explain.

Show Answer

Compared with A, B’s much higher melting temperature supports a giant network with strong covalent bonds, rather than separate small molecules with weaker attractions between them. Melting data alone do not prove the complete structure; conductivity and other supplied evidence can help.

Conclusion: B is the stronger candidate for a giant covalent structure under the stated comparison.

Mind stretcher 2: Conductivity Trap (Graphite Exception)Extension

Question: A student says: “All covalent substances do not conduct electricity.” Explain why this statement is unsafe.

Show Answer

Simple molecular substances such as methane do not conduct because their neutral molecules provide neither mobile ions nor delocalised electrons.

Graphite is an exception: it has delocalised electrons that can move through the layers and carry charge.

Practise and check

Topic check

See what you know across this topic, then go back to anything you got wrong.

Take the topic check

Syllabus and review details

Last reviewed: