How Molecule Size Affects Properties and Electrical Conductivity Revision Notes | GCSE AQA Higher Chemistry

1. Why Size Matters in Chemistry

Picture two magnets - one tiny, one huge. The bigger magnet has a stronger pull, right? Molecules work similarly. Larger molecules have stronger attractions between them, which completely changes how they behave.

This isn't just theory - it explains why petrol evaporates quickly but cooking oil doesn't, why some plastics melt easily while others need extreme heat. The size of molecules determines whether something is a gas, liquid, or solid at room temperature.

2. The Molecular Attraction Game

Think of molecules like people at a party. Small molecules are like children - they move around easily and don't stick together much. Large molecules are like adults holding hands - they're harder to separate and move more slowly.

The key insight: as molecules get bigger, the intermolecular forces between them get stronger. This happens because larger molecules usually have more electrons and a larger surface area, so the London forces between molecules are stronger. More contact points mean stronger overall attraction.

3. From Molecules to Melting Points

Here's where it gets practical. When you heat a molecular substance, you're trying to break the intermolecular forces holding the molecules together. Stronger forces need more energy to break.

Small molecules like methane ($CH_4$) have weak intermolecular forces. They break apart easily, so methane boils at -164°C. Large molecules like octane ($C_8H_{18}$) have much stronger intermolecular forces, so octane boils at 126°C.

The pattern is clear: larger molecules → stronger intermolecular forces → higher melting and boiling points. This explains why cooking oil (large molecules) stays liquid when water (small molecules) boils away.

4. The Electricity Problem

Now for a crucial difference between molecular substances and others. Molecular substances don't conduct electricity, regardless of their size. This catches many students off guard.

Electric current is moving charge. For electricity to flow, you need charged particles that can move freely. Molecular substances are made of neutral molecules - no overall charge, no mobile charges, no conductivity.

Compare this with metals (mobile electrons) or ionic solutions (mobile ions). These conduct electricity because they have charged particles that can move. Molecules, even huge ones, remain electrically neutral.

5. Weak vs Strong - The Key Comparison

Here's the fundamental concept that explains everything: intermolecular forces are weak compared to covalent bonds. This single fact explains most properties of molecular substances.

Worked example: Comparing Bond Strengths

Explain why water boils at 100°C but doesn't decompose into hydrogen and oxygen.

1. Boiling breaks intermolecular forces between $H_2O$ molecules — The molecules separate but stay intact
2. Intermolecular forces: ~10 kJ/mol Covalent O-H bonds: ~460 kJ/mol — Covalent bonds are about 46 times stronger
3. 100°C provides enough energy to break weak intermolecular forces — But not nearly enough to break strong covalent bonds
4. Water molecules separate intact during boiling — Decomposition would require much higher temperatures

Answer: Boiling breaks weak intermolecular forces (10 kJ/mol) but leaves strong covalent bonds (460 kJ/mol) intact, so water molecules separate without decomposing.

This strength difference explains why molecular substances have relatively low melting and boiling points, why they don't conduct electricity, and why they often exist as gases or liquids at room temperature.

6. Exam Success Strategy

Exam questions on this topic follow predictable patterns. Examiners love testing whether you can distinguish between intermolecular forces and covalent bonds, and whether you understand the size-property relationship.

Common exam trap: questions asking why molecular substances don't conduct electricity. Many students write about 'no free electrons' - this is incomplete. The full answer is that molecules have no overall charge, so there are no mobile charged particles of any type.

Exam precision note: In How Molecule Size Affects Properties and Electrical Conductivity - Properties of small molecules - How bonding and structure are related to the properties of substances, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Small simple covalent molecules do not conduct electricity because they have no free ions and no delocalised electrons. As molecule size increases, boiling point often rises because the intermolecular forces become stronger, but this does not change the fact that the substances still lack mobile charge carriers.