AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
In Group 7, the further down the group an element is the higher its…
Full specification wording
Group 7: In Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point.
Group 7 halogens exist as diatomic molecules, so their relative molecular mass increases as you go down the group because each atom has more protons and neutrons.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 3 of 7
Study notes
From the checked teaching notes for this specification point.
The idea
Group 7 halogens exist as diatomic molecules, so their relative molecular mass increases as you go down the group because each atom has more protons and neutrons. For example, fluorine is F₂ (Mr = 38), chlorine is Cl₂ (Mr = 71) and bromine is Br₂ (Mr = 160). Larger molecules have more electrons and stronger London dispersion forces between molecules, so more energy is needed to overcome these intermolecular forces. This explains why melting point and boiling point increase down the group: fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid. The trend is assessed by describing the state and trend down the group and linking it to intermolecular forces, not by memorising exact temperatures.
What earns the marks
- Halogens are diatomic molecules, so relative molecular mass increases down the group as atoms gain more protons and neutrons.
- Larger molecules have more electrons, leading to stronger London dispersion forces between molecules.
- Stronger intermolecular forces require more energy to overcome, so melting point and boiling point increase down the group.
- The physical states at room temperature change from gas (F₂, Cl₂) to liquid (Br₂) to solid (I₂), consistent with increasing melting and boiling points.
- The trend applies to the elements in Group 7 and is explained by intermolecular forces, not by covalent bond strength within each molecule.
Common mistakes and how to put them right
Thinking that relative molecular mass increases because atoms get bigger without linking it to more protons and neutrons.
Put it right by stating that each successive halogen atom has more nucleons, so the diatomic molecule has greater Mr.
Confusing intermolecular forces with covalent bonds.
Put it right by explaining that London dispersion forces between molecules are overcome during melting and boiling, not the covalent bonds inside each molecule.
Assuming all halogens are gases at room temperature.
Put it right by recalling that bromine is a liquid and iodine is a solid, which supports the increasing melting and boiling point trend.
Exam tips
- When describing the trend, name the halogens in order and state the change in state from gas to liquid to solid.
- Use the phrase London dispersion forces or intermolecular forces, and link stronger forces to more energy needed for melting or boiling.
- If asked to explain, compare two named halogens, such as chlorine and bromine, to show the link between Mr, forces and boiling point.
Try the lesson
The start of the Group 7: In Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point. lesson, made from checked teaching notes for this specification point.
Have a go first
Which halogen has the highest boiling point?
Listen
Group 7 halogens exist as diatomic molecules, so their relative molecular mass increases as you go down the group because each atom has more protons and neutrons. For example, fluorine is F₂ (Mr = 38), chlorine is Cl₂ (Mr = 71) and bromine is Br₂ (Mr = 160). Larger molecules have more electrons and stronger London dispersion forces between molecules, so more energy is needed to overcome these intermolecular forces. This explains why melting point and boiling point increase down the group: fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid. The trend is assessed by describing the state and trend down the group and linking it to intermolecular forces, not by memorising exact temperatures. A strong answer earns marks for these points. Halogens are diatomic molecules, so relative molecular mass increases down the group as atoms gain more protons and neutrons. Larger molecules have more electrons, leading to stronger London dispersion forces between molecules. Stronger intermolecular forces require more energy to overcome, so melting point and boiling point increase down the group. The physical states at room temperature change from gas (F₂, Cl₂) to liquid (Br₂) to solid (I₂), consistent with increasing melting and boiling points. A common slip: Thinking that relative molecular mass increases because atoms get bigger without linking it to more protons and neutrons. Instead, stating that each successive halogen atom has more nucleons, so the diatomic molecule has greater Mr. Exam tip: When describing the trend, name the halogens in order and state the change in state from gas to liquid to solid.
See it
Put it in order
Put these steps in the order they happen.
- 1. Stronger London dispersion forces between molecules
- 2. State at room temperature changes from gas to liquid to solid
- 3. More energy needed to overcome the intermolecular forces
- 4. Further down Group 7
- 5. More protons and neutrons in each atom
- 6. Higher melting point and boiling point
- 7. Higher relative molecular mass of the diatomic molecule
- 8. Larger molecules have more electrons
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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