8.3The Periodic Table
Group VII properties — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 10 parts Lesson 3 of 5 1 quiz
Lesson summary
8.3 Group VII properties — Notes — IGCSE Chemistry notes: Describe the physical properties and trends of the halogens, explain the decrease in reactivity down the group and use displacement reactions.Halogens: the poisonous Group VII non-metals fluorine, chlorine, bromine, iodine…
- Halogens: the poisonous Group VII non-metals fluorine, chlorine, bromine, iodine and astatine
- They exist as diatomic molecules: F2, Cl2, Br2, I2 and At2
- All have seven outer-shell electrons, and gain one electron to form halide ions such as Cl−
- Safety: fluorine is not used in schools, so experiments use chlorine, bromine and iodine
- Iodine: solid iodine is dark grey-black, iodine vapour is purple and aqueous iodine is brown
- Density increases
Part 1 of 8The halogens
1
Halogens: the poisonous Group VII non-metals fluorine, chlorine, bromine, iodine and astatine.
2
They exist as diatomic molecules: F2, Cl2, Br2, I2 and At2.
3
All have seven outer-shell electrons, and gain one electron to form halide ions such as Cl−.
4
Safety: fluorine is not used in schools, so experiments use chlorine, bromine and iodine.
Part 2 of 8Appearance at room temperature
| Halogen | State | Colour |
|---|---|---|
| Fluorine | Gas | Yellow |
| Chlorine | Gas | Pale yellow-green |
| Bromine | Liquid | Red-brown |
| Iodine | Solid | Grey-black |
| Astatine (predicted) | Solid | Black |
- Iodinesolid iodine is dark grey-black, iodine vapour is purple and aqueous iodine is brown.
Part 3 of 8Trends down the group
1
Density increases.
2
Melting and boiling points increase, so fluorine has the lowest and astatine the highest.
3
Colour becomes darker, from yellow fluorine to black astatine.
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Reactivity decreases, which is the opposite of the trend in Group I.
Part 4 of 8Why reactivity decreases down the group
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A halogen reacts by gaining one outer electron to fill its outer shell.
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The number of electron shells increases down the group.
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The atom is larger, with the outer shell further from the nucleus and more shielding from the inner shells.
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The nucleus attracts an incoming electron less, so it is harder to gain an electron.
Part 5 of 8Halogen displacement
Halogen displacement
A more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide,
- and a halogen only displaces the halides of halogens below it.
Redox: the more reactive halogen gains electrons (is reduced) and the halide ion of the less reactive halogen loses electrons (is oxidised).
Aqueous colours: chlorine is very pale green (looks colourless), bromine is orange (yellow when dilute) and iodine is brown.
| Halogen added | KCl(aq) | KBr(aq) | KI(aq) |
|---|---|---|---|
| Chlorine | No reaction | Orange (Br2 formed) | Brown (I2 formed) |
| Bromine | No reaction | No reaction | Brown (I2 formed) |
| Iodine | No reaction | No reaction | No reaction |
Halogen displacement
Method
- Place 5 cm3 of each potassium halide solution (KCl, KBr, KI) in separate test tubes.
- Add a few drops of chlorine water to each and shake.
- Record colour changes against a white background, where orange or yellow shows bromine and brown shows iodine.
- Repeat with fresh halide solutions using bromine water, then iodine solution.
- A colour change shows displacement, so the added halogen is more reactive.
Order of reactivity: chlorine is more reactive than bromine, which is more reactive than iodine.
Part 6 of 8Displacement equations
Displacement equations
Chlorine and bromide: 2KBr(aq) + Cl2(aq) → 2KCl(aq) + Br2(aq), with ionic form 2Br− + Cl2 → 2Cl− + Br2.
Chlorine and iodide: Cl2 + 2I− → 2Cl− + I2.
Bromine and iodide: MgI2(aq) + Br2(aq) → MgBr2(aq) + I2(aq), with ionic form 2I− + Br2 → 2Br− + I2.
Part 7 of 8Predicting halogen properties
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The melting point, boiling point, physical state, density or colour of some halogens can be used to predict others.
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Fluorine is at the top of Group VII, so it is a pale yellow gas with the lowest melting and boiling points.
3
Astatine is at the bottom, so it is a black solid with the highest melting and boiling points.
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Using data: put the elements in order of their position in Group VII, then look for general patterns.
Part 8 of 8More from the specification
More from the specification
Periodic
- Describing properties that repeat at regular intervals along the table
Noble gases
- The Group 0 elements, which are unreactive monatomic gases
Trends in Physical State and Colour of Halogens
- Down Group VII, halogens darken in colour and increase in melting and boiling points: fluorine is a pale yellow gas, chlorine is a pale yellow-green gas, bromine is a red-brown liquid,
- and iodine is a dark grey-black solid.
Decreasing Halogen Reactivity Down Group VII
- Reactivity decreases down Group VII because larger atomic radii and increased electron shielding make it harder for incoming electrons to be attracted into the outer shell.
Halogen Displacement Reactions in Aqueous Solution
- A more reactive halogen displaces a less reactive halide ion from its aqueous salt solution (e.g. Cl2 + 2KBr → 2KCl + Br2, turning the solution orange).
Observation
- What is seen, heard, smelt or measured
Transition elements
- The central block of metals between Group II and Group III.
Alkali metals
- The Group I metals, which form alkaline solutions with water.
Key terms
| Term | Meaning |
|---|---|
| Halogen | a Group VII non-metal with seven outer electrons. |
| Diatomic | existing as molecules of two atoms. |
| Halide | a negative ion formed when a halogen gains one electron. |
| Displacement | a reaction in which a more reactive element takes the place of a less reactive one in a compound. |
| Shielding | the reduction in attraction to the nucleus caused by inner electron shells. |
Avoid these mistakes
1
Saying halogens get more reactive down the group: reactivity decreases.
2
Saying chlorine gas is greenit is pale yellow-green.
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Saying iodine is purpleit is a grey-black solid that forms a purple vapour (brown in solution).
4
Saying chlorine displaces bromine from bromine: it displaces bromine from a bromide.
5
Writing the halogens as single atomsthey are diatomic.
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