Group 7 (AQA GCSE Chemistry Combined Science): Revision Notes
Group 7
What are the halogens?
The elements in Group 7 are called halogens. All halogen atoms have seven electrons in their outer shell. This means they all behave in similar ways when they react.
The five halogens in Group 7 are:
- Fluorine (F)
- Chlorine (Cl)
- Bromine (Br)
- Iodine (I)
- Astatine (At)
Properties of Group 7 elements
All halogens share these important properties:
- They are non-metals
- They form molecules made of pairs of atoms (like , )
- They react with metals to make ionic compounds called halides
- In ionic compounds, halogen ions have a -1 charge (like , )
- They react with other non-metals to make covalent compounds
Trends down Group 7
As you move down Group 7, several properties change in predictable ways:
Reactivity
- Fluorine is the most reactive halogen
- Astatine is the least reactive halogen
- Reactivity decreases as you go down the group
- This trend is opposite to Group 1 metals
The decreasing reactivity down Group 7 is the opposite pattern to Group 1 metals, where reactivity increases down the group.
Melting and boiling points
- These increase as you go down the group
- Fluorine has the lowest melting and boiling points
- Astatine has the highest melting and boiling points
Molecular mass
- This increases as you go down the group
- Fluorine molecules are the lightest
- Astatine molecules are the heaviest
Displacement reactions
A displacement reaction happens when a more reactive halogen pushes out (displaces) a less reactive halogen from its compound.
How displacement works
The key principle is that more reactive halogens can displace less reactive ones from solutions, but less reactive halogens cannot displace more reactive ones.
Results of mixing halogens with halide solutions:
- Chlorine can displace bromine and iodine (turns solutions orange or brown)
- Bromine can displace iodine (turns solution brown)
- Iodine cannot displace any other halogens
Worked Example: Chlorine displacing Bromine
When chlorine is added to potassium bromide solution:
Word equation: chlorine + potassium bromide → potassium chloride + bromine
Chemical equation:
This works because chlorine is more reactive than bromine.
Key Points to Remember:
- Halogens have 7 electrons in their outer shell
- They are non-metals that form molecules with pairs of atoms
- Reactivity decreases down the group (opposite to Group 1)
- Melting/boiling points increase down the group
- More reactive halogens can displace less reactive ones from solutions
- Halogen ions always have a -1 charge in ionic compounds