Chemistry · Bonding, Structure and Properties of Matter
Structure and Properties of Small Molecules
Spec 5.2.2.4📙 Higher
Theory · read, then commit
Simple molecular substances are made of small, separate molecules — each holds a fixed number of atoms joined by strong covalent bonds.
Checkpoint · quick check
Describe what a simple molecular substance is.
🔗Bonds WITHIN a molecule
Type of forceCovalent bonds
StrengthStrong
Broken when it melts?No — they stay intact
🌫️Forces BETWEEN molecules
Type of forceWeak intermolecular forces
StrengthWeak
Broken when it melts?Yes — easily overcome
Melting overcomes the WEAK forces between molecules — not the strong bonds inside them. That is why the melting point is low.
Checkpoint · quick check
State what is overcome when a simple molecular substance boils.
Properties of small molecules
🚫Do not conduct electricity
Molecules have no overall charge and no free electrons or ions to carry current.
🌡️Bigger molecule → higher melting point
More electrons → stronger intermolecular forces → more energy to separate.
💧Solubility varies
“Like dissolves like” — some dissolve in water, others do not.
Checkpoint · quick check
Explain why simple molecular substances do not conduct electricity.
🧪Example — HCl gas dissolving in water
HCl(g) + water → H⁺(aq) + Cl⁻(aq).
Dissolving splits the molecule into ions.
The solution (hydrochloric acid) now conducts — because ions are present.
Retrieval · sort the cards
Test yourself · 9 questions, exam order
⭐Examiner Tip
Never write "the bonds break" when a simple molecular substance melts. Say the weak intermolecular forces between molecules are overcome — the strong covalent bonds inside each molecule stay intact.
0 of 9 answered
Warm-up · 2 questions
Water (H₂O) boils at 100°C but methane (CH₄) boils at −161°C, although both are simple molecular. Explain why the difference is so large.
Explain why simple molecular substances have low melting and boiling points.
Exam standard · 3 questions
Explain why larger molecules generally have higher melting and boiling points than smaller molecules.
Explain why melting a simple molecular substance does not break its covalent bonds.
Many simple molecular substances are gases at room temperature, but iodine is a solid. Suggest why iodine is a solid.
Stretch · 4 questions
A substance has a low melting point and does not conduct electricity in any state. Deduce its structure and bonding.
Explain why the melting point of a simple molecular substance tells you nothing about the strength of its covalent bonds.
Chlorine (Cl₂) is a gas at room temperature but iodine (I₂) is a solid, even though both are simple molecular. Explain this difference.
Explain one limitation of representing a simple molecular substance by drawing just a single molecule.
Key note · cover it, say it, check it
Revision card · 5 stepsStructure and Properties of Small Molecules