C3.6 Structure of Simple Molecules

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: why substances made of small molecules have low melting and boiling points and do not conduct, and how polymers compare. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the difference between covalent bonds and intermolecular forces
• Explain why substances made of small molecules have low melting and boiling points
• Explain how the size of molecules affects intermolecular forces and boiling point
• Explain why small molecules do not conduct electricity
• Explain why polymers are solids at room temperature, and describe the limitations of models used to show molecules

Key words

• Simple molecular substance: A substance made of small molecules, e.g. water, oxygen, methane.
• Intermolecular forces: Weak forces of attraction between molecules.
• Polymer: A substance made of very large molecules: long chains of repeating units joined by covalent bonds.
• Repeat unit: The small section of a polymer that repeats along the chain, shown in brackets with an n.

Two kinds of attraction

In a substance made of small molecules, there are two very different kinds of attraction:

• covalent bonds inside each molecule: very strong
• intermolecular forces between molecules: weak

Key fact: When a simple molecular substance melts or boils, the intermolecular forces are overcome. The covalent bonds do not break. Water vapour is still made of H₂O molecules.

Properties of small molecules

• low melting and boiling points; usually gases or liquids at room temperature; Explanation only weak intermolecular forces need to be overcome, which takes little energy
• do not conduct electricity (as solid, liquid or gas); Explanation molecules have no overall electric charge, and there are no free electrons or ions to carry charge

• fluorine, F₂; Melting point (°C) −220; Boiling point (°C) −188; State at 25 °C gas
• chlorine, Cl₂; Melting point (°C) −101; Boiling point (°C) −34; State at 25 °C gas
• bromine, Br₂; Melting point (°C) −7; Boiling point (°C) 59; State at 25 °C liquid
• iodine, I₂; Melting point (°C) 114; Boiling point (°C) 184; State at 25 °C solid

Bigger molecules, stronger forces

The intermolecular forces get stronger as molecules get bigger. More energy is needed to overcome them, so the melting and boiling points rise. You can see this down Group 7 above, and in the alkanes below.

Worked example: using the graph

In a freezer at −20 °C, propane (3 carbons, bp −42 °C) is above its boiling point, so it is a gas. Butane (4 carbons, bp −1 °C) is below its boiling point, so it is a liquid.
Rise from 3 to 4 carbons = −1 − (−42) = 41 °C. Rise from 4 to 5 carbons = 36 − (−1) = 37 °C.

Answer: propane is a gas and butane a liquid at −20 °C; each extra carbon raises the boiling point, by a little less each time

Polymers

Polymers such as poly(ethene) have very large molecules. Thousands of atoms are joined in long chains by strong covalent bonds. Because the molecules are so large, the intermolecular forces between the chains are relatively strong, so polymers are solids at room temperature.

Worked example: counting atoms in a polymer

One repeat unit of poly(ethene) has 2 C atoms and 4 H atoms. A chain has n = 8000 repeat units.
Carbon atoms = 2 × 8000 = 16 000; hydrogen atoms = 4 × 8000 = 32 000.

Answer: 16 000 C and 32 000 H atoms

Note: Polymers still do not conduct electricity: like small molecules, they have no free charged particles.

Models and their limitations

Chemists use several models for the same molecule. Each one is useful, but none of them shows everything.

• dot-and-cross diagram; Limitation shows electrons as dots and crosses, but all electrons are identical; shows electrons fixed in place, but they are constantly moving; does not show the shape
• 2D displayed formula / 2D ball-and-stick; Limitation shows which atoms are bonded, but not the true 3D shape or bond angles
• 3D ball-and-stick (model kit); Limitation shows the shape, but real bonds are not rigid sticks: molecules vibrate, bend and twist; atoms are not solid balls far apart
• model of a giant structure; Limitation shows only a tiny part of a structure with billions of particles, but you can still use it to find the ratio of particles and so the formula

Note: Simple molecular substances do not conduct, even when liquid or dissolved, because the molecules are neutral. Exception: some molecules, such as hydrogen chloride, react with water to form ions, so their solutions (e.g. hydrochloric acid) do conduct.

Common mistakes

• Saying covalent bonds break when a simple molecular substance melts or boils. Only the intermolecular forces are overcome.
• Saying small molecules have weak covalent bonds. The covalent bonds are strong; the forces between molecules are weak.
• Saying a substance conducts because it is a liquid. Liquid water and other small molecules have no free charged particles.
• Writing “intermolecular bonds” in the exam. Use “intermolecular forces”.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows chlorine molecules. A points to a covalent bond inside a molecule; B points to the forces between molecules. What is overcome when liquid chlorine boils? (Choose from: B only, A only, both A and B, neither A nor B.)

Answer: B only

Working: Only the weak intermolecular forces (B) are overcome. The strong covalent bonds (A) stay intact, so chlorine gas is still Cl₂.

Question 2. Use the graph to estimate the boiling point of hexane, which has 6 carbon atoms.

Answer: 69 °C

Working: From 4 to 5 carbons the rise is 37 °C, so from 5 to 6 it is a little less, about 30–35 °C: 36 + 33 ≈ 69 °C. The real value is 69 °C.

Question 3. Why do substances made of small molecules not conduct electricity? (Choose from: The molecules have no overall electric charge, The covalent bonds are too strong, The molecules are too far apart, They have low melting points.)

Answer: The molecules have no overall electric charge

Working: There are no ions or delocalised electrons – the molecules are neutral, so nothing charged can move.

Related lessons

• Previous lesson: C3.5 Covalent Bonding
• Next lesson: C3.7 Giant Covalent Structures
• In this chapter: C3.1 States of Matter
• In this chapter: C3.2 Atoms Into Ions
• In this chapter: C3.3 Ionic Bonding

Continue in the full course

This lesson is section C3.6 (AGCSC2006) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.