Group VII Properties — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

Section 1 of 12The halogens

• Members
  • fluorine, chlorine, bromine, iodine and astatine, all poisonous non-metals.
• Diatomic
  • they exist as molecules of two atoms: F2, Cl₂, Br₂, I₂ and At2.
  • Outer shell
    • each has seven outer electrons.
  • Ion formation
    • they gain one electron to form halide ions, 1-.
  • Safety
    • fluorine is not used in schools, so work uses chlorine, bromine and iodine.

Section 2 of 12Physical state and colour

Halogen	State at 20 °C	Colour
Fluorine	Gas	Yellow
Chlorine	Gas	Pale yellow-green
Bromine	Liquid	Red-brown
Iodine	Solid	Grey-black
Astatine	Solid	Black

• Trend
  • density, melting point and boiling point increase down the group, and the colour becomes darker.
• Iodine forms
  • solid is dark grey-black, vapour is purple and aqueous solution is brown.

Section 3 of 12Trend in reactivity

• Direction
  • reactivity increases going up Group VII, the opposite of Group I.
• Reason 1
  • each reacting halogen must gain one electron.
  • Reason 2
    • higher in the group there are fewer shells, so the outer shell is closer to the nucleus.
  • Reason 3
    • the attraction for an extra electron is stronger, so the atom gains electrons more easily.

Section 4 of 12Predicting properties

• Melting and boiling points
  • increase down the group, so fluorine has the lowest and astatine the highest.
• Physical state
  • fluorine is a gas and astatine is a solid.
  • Colour
    • fluorine is lighter (yellow) and astatine is black.
• Method
  • put the elements in order of position in the group, then spot the pattern in the data.

Section 5 of 12Halogens in aqueous solution

Aqueous solution	Colour
Chlorine	Very pale green, usually appearing colourless as it is dilute
Bromine	Orange, yellow when diluted
Iodine	Brown

Section 6 of 12Displacement reactions

• Rule
  • a more reactive halogen displaces a less reactive halogen from a solution of its halide.
• Order
  • chlorine is most reactive of the three and iodine the least.
  • Result
    • the less reactive halogen ends up as the free element.
• Chlorine and potassium bromide
  • the solution turns orange as bromine forms.
• Equation
  • 2KBr(aq) + Cl₂(aq) → 2KCl(aq) + Br₂(aq).
• Bromine and magnesium iodide
  • the solution turns brown as iodine forms.
• Equation
  • MgI₂(aq) + Br₂(aq) → MgBr₂(aq) + I₂(aq).

Section 7 of 12Electronic structure of the first halogens

Halogen	Atomic number	Configuration	Shells
Fluorine	9	2,7	2
Chlorine	17	2,8,7	3
Bromine	35	2,8,18,7	4

• Pattern
  • each has seven electrons in the outer shell, so each forms a 1- ion.
• Chemical similarity
  • they react in similar ways because of the same outer shell.
  • Difference
    • the number of shells increases down the group.

Section 8 of 12Observations in a displacement experiment

1. Add a few drops of the halogen solution to the halide solution in a test tube.
2. Note any colour change after mixing.
3. Compare the colour with the table of halogen colours in solution.
4. A new colour shows the less reactive halogen has been released.
5. No colour change shows no reaction.

• Chlorine with iodide
  • a brown colour shows iodine formed.
• Bromine with chloride
  • no change, because bromine is less reactive than chlorine.
  • Conclusion
    • the order is chlorine, then bromine, then iodine.

Section 9 of 12Uses of the trend

• Prediction
  • astatine would be the least reactive halogen and fluorine the most reactive.
• Displacement
  • fluorine would displace chlorine, bromine and iodine from solutions of their halides.
  • Astatine
    • a solid that would not displace the others.
• Physical data
  • plot melting point or boiling point against position in the group to estimate missing values.

Section 10 of 12Worked examples

1. Predict the state and colour of astatine at room temperature. [2]2 marks

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2. Explain why chlorine is more reactive than bromine. [3]3 marks

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3. Write an equation for chlorine and potassium iodide and state the observation. [2]2 marks

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4. Explain why no reaction occurs when iodine is added to potassium bromide. [1]1 mark

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Section 11 of 12Key terms

Term	Meaning
Halogen	A Group VII non-metal with seven outer electrons.
Diatomic	Made of molecules containing two atoms.
Displacement reaction	A more reactive element takes the place of a less reactive one in a compound.
Halide ion	A negative ion formed when a halogen gains one electron.
Reactivity trend	Increases up Group VII.

Section 12 of 12Exam technique

1. Saying halogen reactivity increases down the group — it decreases.
2. Forgetting halogens are diatomic.
3. Saying bromine solution is brown — bromine is orange and iodine is brown.
4. Writing that a less reactive halogen displaces a more reactive one.