(b) Group 7 (halogens) – chlorine, bromine and iodine
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
2.5Halogen colours and statesKnow the colours, physical states (at room temperature) and trends in physical properties of these elements2.6Predicting halogen propertiesUse knowledge of trends in Group 7 to predict the properties of other halogens2.7Halogen displacement reactionsUnderstand how displacement reactions involving halogens and halides provide evidence for the trend in reactivity in Group 728C Group 7 electronic configurationsExplain the trend in reactivity in Group 7 in terms of electronic configurations
Compare the colours and states of Group 7 elements
At room temperature, chlorine, bromine and iodine show a clear physical trend down Group 7: their colours become darker and their melting and boiling points increase.
| Element | Colour | Physical state at room temperature |
|---|---|---|
| chlorine, ClX2 | pale green | gas |
| bromine, BrX2 | red-brown | liquid |
| iodine, IX2 | dark grey | solid |
The rising melting and boiling points explain the state sequence gas → liquid → solid at the same room temperature. Each element consists of diatomic molecules, so the elemental formula is XX2, not a single atom or a halide ion.
State the colour of the element in the named state. Solid iodine is dark grey; purple describes iodine vapour, and an aqueous bromine solution may appear orange-brown rather than matching the pure liquid exactly.
Predict properties of unfamiliar halogens
Predict an unfamiliar halogen by extending the Group 7 trends in the correct direction and keeping the shared diatomic formula XX2.
| Position | Physical prediction | Reactivity prediction |
|---|---|---|
| above chlorine, such as fluorine | paler, lower melting and boiling points; gas at room temperature | more reactive |
| below iodine, such as astatine | darker, higher melting and boiling points; solid at room temperature | less reactive |
The trends support fluorine as a pale yellow gas and astatine as a dark grey or black solid. Their molecular formulae are FX2 and AtX2. Displacement evidence establishes the reactivity direction chlorine > bromine > iodine, which can then be extended upward or downward.
A trend supports a range or comparison, not an invented exact value. Predict 'higher boiling point than iodine' or 'less reactive than iodine' unless numerical data are supplied; do not guess a precise boiling point or shade.
Use displacement to rank halogen reactivity
A more reactive halogen displaces a less reactive halogen from a solution containing its halide ions. A less reactive halogen cannot displace a more reactive one.
| Halogen added | Chloride ions | Bromide ions | Iodide ions |
|---|---|---|---|
| chlorine | no reaction (same element) | reaction | reaction |
| bromine | no reaction | no reaction (same element) | reaction |
| iodine | no reaction | no reaction | no reaction (same element) |
\ce{Cl2 + 2Br^- -> 2Cl^- + Br2}\qquad\ce{Br2 + 2I^- -> 2Br^- + I2}
Chlorine displaces bromide and iodide, while bromine displaces only iodide. Iodine displaces neither chloride nor bromide. This evidence gives the order chlorine > bromine > iodine, so reactivity decreases down Group 7. For example, chlorine added to colourless potassium bromide solution forms orange bromine.
Keep the species distinct: chlorine is the halogen ClX2, whereas chloride is the ion ClX−. A halogen cannot displace itself, and a colour change is evidence only after the product halogen has been identified.
Explain why Group 7 reactivity decreases down the group
Every Group 7 atom has seven outer-shell electrons and reacts by gaining one electron to form a −1 halide ion. Reactivity therefore depends on how strongly the atom attracts an incoming electron.
| Atom | Electronic configuration | Occupied shells | Relative reactivity |
|---|---|---|---|
| fluorine | 2,7 | 2 | highest |
| chlorine | 2,8,7 | 3 | lower |
| bromine | 4 occupied shells, 7 outer electrons | 4 | lower again |
| iodine | 5 occupied shells, 7 outer electrons | 5 | lowest of these four |
Down the group, atoms have more occupied shells. The outer shell is farther from the nucleus and inner electrons provide more shielding, so the nucleus attracts an incoming electron less strongly. Gaining that electron becomes harder, so reactivity decreases down Group 7.
Do not apply the Group 1 explanation here: halogens gain an electron rather than lose one. Nuclear charge increases down the group, but the increased distance and shielding outweigh it for the incoming electron.