The Periodic Table (AQA GCSE Chemistry): Revision Notes
Group 7
What are halogens?
Group 7 elements are known as halogens. All halogen atoms have seven electrons in their outer shell. This gives them similar chemical properties and reactions.
The main halogens you need to know are:
- Fluorine (F) - most reactive
- Chlorine (Cl)
- Bromine (Br)
- Iodine (I)
- Astatine (At) - least reactive
The similar electron configuration in the outer shell (7 electrons) is what gives all halogens their characteristic properties and explains why they behave in predictable ways as a group.
Properties of halogens
Halogens share several important characteristics that make them unique among the elements.
Physical structure:
- They are non-metals
- They exist as molecules made of pairs of atoms (, , , )
Chemical behaviour:
- They react with metals to form ionic compounds
- When they react, they form halide ions with a charge of -1 (like , , )
- They react with other non-metals to form simple molecular compounds
Trends down Group 7
As you go down Group 7, there are clear patterns that help predict how these elements will behave.
Reactivity: Gets less reactive going down the group
- Fluorine is most reactive
- Astatine is least reactive
- This is opposite to Group 1!
Melting and boiling points: Get higher going down the group
- Fluorine has lowest melting/boiling points
- Astatine has highest melting/boiling points
Molecular mass: Gets higher going down the group
- Each element has more protons and electrons than the one above
Key Difference from Group 1: Remember that halogens get LESS reactive as you go down the group, which is the opposite trend to Group 1 metals that get MORE reactive going down.
Displacement reactions
A displacement reaction happens when a more reactive halogen pushes out a less reactive halogen from its compound.
The Golden Rule of Halogen Displacement: More reactive halogens can displace less reactive ones from solutions of their salts. This means:
- Chlorine can displace both bromine and iodine
- Bromine can displace iodine only
- Iodine cannot displace any others
When chlorine water is added to potassium bromide solution, you can observe a clear displacement reaction:
Word equation: Chlorine + potassium bromide → potassium chloride + bromine
Chemical equation:
You can see this happen because the solution changes colour - bromine forms and turns the solution orange.
Worked Example: Chlorine and Potassium Iodide Reaction
When chlorine is bubbled through a colourless solution of potassium iodide, a brown colour appears.
Step 1: Identify the reactivity
- Chlorine is more reactive than iodine (chlorine is higher up Group 7)
Step 2: Predict what happens
- The chlorine will displace the iodide ions from the solution
- This forms iodine molecules
Step 3: Observe the result
- Iodine forms, which causes the brown colour to appear
- This confirms that displacement has occurred
Chemical equation:
Key Points to Remember:
- Halogens are Group 7 elements with 7 electrons in their outer shell
- Reactivity decreases down the group (opposite to Group 1)
- Melting points and molecular mass increase down the group
- More reactive halogens displace less reactive ones from their salt solutions
- Displacement reactions show reactivity order - chlorine displaces bromine and iodine, bromine displaces iodine