Halogens: appearance, molecules and group trends
Keep the elements' room-temperature appearances separate from their halide ions and solution colours.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Distinguish atoms, diatomic molecules and ions.
- Read group position and distinguish density from physical state.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which formula represents a neutral chlorine molecule?
What you will learn
- Describe chlorine, bromine and iodine as diatomic non-metals with their required r.t.p. appearances.
- Use increasing density and decreasing reactivity down Group VII to make supported predictions.
Recall element formula, colour and state together
The required Group VII halogens are diatomic non-metals: chlorine Cl₂, bromine Br₂ and iodine I₂. At room temperature and pressure, chlorine is a pale yellow-green gas, bromine a red-brown liquid and iodine a grey-black solid. The formula's 2 counts two atoms per molecule; it does not mean the molecule has a −2 ionic charge.
Chloride Cl⁻, bromide Br⁻ and iodide I⁻ are the halide ions and are different species from the elements. Iodine's grey-black solid appearance must not be substituted with the brown aqueous iodine observation or a vapour colour. State and surrounding medium affect the observation, so identify what material and conditions a question describes. Halogen handling is supervised laboratory work.
Their reactivity trend runs opposite to Group I
Down Group VII in the stated comparison, density increases and reactivity decreases. Chlorine is more reactive than bromine, which is more reactive than iodine. This differs from Group I's increasing reactivity down its metal group. The halogens share seven outer electrons in the syllabus main-group model and commonly form −1 halide ions.
Use supplied information and position to predict another Group VII element's property qualitatively, rather than assigning an exact density from one remembered value. A question about state must use stated melting and boiling information or an explicitly given trend; group membership alone does not provide every missing temperature. The displacement lesson tests the chemical reactivity ordering through reactions with halide ions.
Pause and explain
Which is the required iodine appearance at r.t.p.?
Worked example
State iodine's formula and r.t.p. appearance, then compare its reactivity with bromine.
Show the worked solution
- The elemental formula is I₂ because iodine is diatomic in this comparison.
- At r.t.p. it is a grey-black solid, not a brown liquid.
- Iodine lies below bromine in Group VII and is less reactive in the stated trend.
Answer I₂; grey-black solid at r.t.p.; less reactive than bromine.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give the formulas and required r.t.p. appearances of chlorine, bromine and iodine.
Give me a hint
Pair each diatomic formula with its own state and colour.
Compare my reasoning
- Cl₂: pale yellow-green gas.
- Br₂: red-brown liquid.
- I₂: grey-black solid.
Cl₂ pale yellow-green gas; Br₂ red-brown liquid; I₂ grey-black solid.
Look for these in your work
- Each molecule has two atoms.
- All colours and states match their elements.
Order chlorine, bromine and iodine from most to least reactive and state the density direction down the group.
Give me a hint
Reactivity decreases while density increases in this stated comparison.
Compare my reasoning
- Chlorine is most reactive, followed by bromine.
- Iodine is least reactive of the three.
- Density increases down their listed Group VII order.
Reactivity: chlorine > bromine > iodine. Density increases down the group.
Look for these in your work
- Both directions are distinguished.
- The ordering uses the given group position.
A student labels brown aqueous iodine as iodine's grey-black r.t.p. solid appearance and calls iodide I₂⁻. Correct the species and observation distinctions.
Give me a hint
A solid element, dissolved iodine and iodide ions are not one observation or formula.
Compare my reasoning
- Elemental iodine is I₂ and is a grey-black solid at r.t.p.
- Iodine in a suitable aqueous observation can appear brown; that describes a different medium.
- The simple iodide ion is I⁻, not the neutral element I₂ or a guessed I₂⁻ formula.
I₂ is the grey-black r.t.p. solid; brown describes the stated iodine solution; iodide is I⁻.
Look for these in your work
- Species and medium are both named.
- Subscript and ionic charge are kept separate.
Common mistakes
- Calling elemental chlorine Cl⁻ rather than Cl₂.
- Giving iodine's solution colour as its r.t.p. solid appearance.
- Applying Group I's increasing-reactivity direction to the halogens.
- Calling a diatomic subscript an ionic charge.
What can you explain now?
State bromine's formula and appearance at r.t.p.
Compare with the explanation
Br₂; red-brown liquid.
Bromine is the liquid of the three required examples; chlorine is gas and iodine solid under the stated conditions.
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Make it stick
Recall all three formulas and r.t.p. appearances, then explain the reactivity direction and why a halide ion is different from its parent element.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.