AQA · GCSE · Chemistry
Course version: 8462 · 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.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Group 7 halogens exist as diatomic molecules. Their relative molecular mass increases as you descend from fluorine to iodine because each atom has more protons and neutrons. As the molecules get larger, the intermolecular forces between them become stronger. More energy is therefore needed to overcome these intermolecular forces, so melting point and boiling point rise steadily down the group. For example, fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid. The trend in melting and boiling points is explained entirely by the strength of the intermolecular forces, not by the covalent bonds within the molecules.
What earns the marks
- Group 7 elements are diatomic molecules, so relative molecular mass increases down the group as atomic number increases.
- Larger molecules have stronger intermolecular forces between them.
- Stronger intermolecular forces require more energy to overcome, so melting point and boiling point increase down the group.
- The trend is a physical property trend explained by intermolecular forces, not by the strength of the covalent bonds within each molecule.
- State symbols at room temperature change from gas to liquid to solid: fluorine and chlorine are gases, bromine is a liquid, iodine is a solid.
Common mistakes and how to put them right
Saying that melting point increases because the covalent bonds get stronger.
Put it right: Intermolecular forces between molecules get stronger, while covalent bonds within molecules do not break during melting.
Confusing relative molecular mass with relative atomic mass.
Put it right: Halogens are diatomic, so relative molecular mass is twice the relative atomic mass for each element.
Claiming that all halogens are gases at room temperature.
Put it right: Bromine is a liquid and iodine is a solid under standard conditions.
Exam tips
- Link each property to the structure and bonding explanation: larger molecules, stronger intermolecular forces, more energy needed to melt or boil.
- Use the correct comparison language, such as higher, lower, stronger and weaker, rather than vague words like bigger or more.
- When asked to predict a trend, refer to the position in Group 7 and the change in intermolecular forces rather than memorising colours alone.
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. Their relative molecular mass increases as you descend from fluorine to iodine because each atom has more protons and neutrons. As the molecules get larger, the intermolecular forces between them become stronger. More energy is therefore needed to overcome these intermolecular forces, so melting point and boiling point rise steadily down the group. For example, fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid. The trend in melting and boiling points is explained entirely by the strength of the intermolecular forces, not by the covalent bonds within the molecules. A strong answer earns marks for these points. Group 7 elements are diatomic molecules, so relative molecular mass increases down the group as atomic number increases. Larger molecules have stronger intermolecular forces between them. Stronger intermolecular forces require more energy to overcome, so melting point and boiling point increase down the group. The trend is a physical property trend explained by intermolecular forces, not by the strength of the covalent bonds within each molecule. A common slip: Saying that melting point increases because the covalent bonds get stronger. Instead, intermolecular forces between molecules get stronger, while covalent bonds within molecules do not break during melting. Exam tip: Link each property to the structure and bonding explanation: larger molecules, stronger intermolecular forces, more energy needed to melt or boil.
Key words
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Diatomic molecules
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Intermolecular forces
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Group 7 trends
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Structure and properties
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See every definition
- Diatomic molecules
- Group 7 halogens exist as diatomic molecules, so relative molecular mass is twice the relative atomic mass for each element.
- Intermolecular forces
- As the molecules get larger, the forces between them become stronger, and more energy is needed to overcome them.
- Group 7 trends
- The further down the group an element is, the higher its relative molecular mass, melting point and boiling point.
- Structure and properties
- The trend in melting and boiling points is explained by the strength of the intermolecular forces, not by the covalent bonds within the molecules.
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