AQA Chemistry 5.2.2.4 · Model
Properties of small molecules
Chlorine is a gas, bromine a liquid and iodine a solid, yet every one of them is a pair of atoms held by one covalent bond. What sets the difference?
Combined Science · Foundation tier
Start here
A kettle of water. A bubble of steam.
Water boils at 100 °C. Splitting water into hydrogen and oxygen needs an electric current running through it for minutes, or a temperature of thousands of degrees. Both processes start with the same H₂O molecules.
Commit first
What is inside a bubble of steam?
A small molecule is a fixed group of atoms joined by strong covalent bonds: H₂O, CH₄, Cl₂. Between one molecule and the next there are only weak intermolecular forces. So a small-molecule substance has two kinds of attraction in it, one strong and one weak, and only the weak one decides when it melts or boils. Heat some molecules and see which gives way.
The two-forces model
Pick a molecule. Turn up the heat.
0 of 5 molecules heated
20 °C · liquid
water · melts 0 °C · boils 100 °C
Liquid: some intermolecular forces have been overcome; molecules slide past each other.
The bigger the molecule, the stronger the intermolecular forces between neighbours, and the more energy it takes to separate them. That is the whole pattern down Group 7, and it is why most small-molecule substances are gases or liquids at room temperature while a few large ones are solids. Small-molecule substances also do not conduct electricity: a molecule has no overall charge, and there are no free electrons or ions.
Rank it
Lowest boiling point to highest.
The molecules are drawn roughly to scale. Tap them in order from the lowest boiling point to the highest. Tap a placed one to take it back.
Command words in this lesson
- Explain
- Name the weak forces between molecules. Say the covalent bonds do not break.
- Suggest
- Use the size pattern on a substance you have not met.
- Use the data
- Quote the numbers from the question in your answer.
Key fact
Melting or boiling a small-molecule substance overcomes the weak forces between molecules. The strong covalent bonds inside each molecule stay intact.
Examiner tip
Key note · AQA 5.2.2.4
Properties of small molecules
- Small molecules: a fixed number of atoms joined by strong covalent bonds.
- Between molecules there are only weak intermolecular forces.
- Melting and boiling overcome the intermolecular forces. The covalent bonds stay intact.
- So small-molecule substances have low melting and boiling points: mostly gases or liquids at room temperature.
- Bigger molecules, stronger intermolecular forces, higher melting and boiling points.
- They do not conduct: molecules have no overall charge.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask Mr Badmus AI
Mixing up bonds and intermolecular forces?
Melting and boiling points are rounded data-book values. The model draws covalent bonds as thick lines and intermolecular forces as dotted lines; intermolecular forces are not bonds and do not join particular pairs of molecules. Iodine sublimes when heated gently in an open tube, but at atmospheric pressure it does melt at 114 °C. Water boils far higher than its small size suggests: its molecules have an extra, stronger kind of intermolecular force, which GCSE does not require.