Halogens: diatomic non-metals
Group VII = the halogens. Every halogen atom has 7 outer-shell electrons and readily gains one more, so all are non-metals. Each exists as a diatomic molecule -- two atoms of one element joined covalently: , , . Down the group (chlorine -> bromine -> iodine) density increases and reactivity decreases: chlorine is the most reactive of the three, iodine the least. Fluorine and astatine are also Group VII.
Appearance at r.t.p.: gas, liquid, solid
Learn the three appearances at room temperature and pressure (r.t.p.) as one fixed set: chlorine is a pale yellow-green gas, bromine a red-brown liquid, iodine a grey-black solid. The three different states are themselves evidence of the density trend -- particles sit progressively closer together from gas to liquid to solid as you go down the group. Colour and state are both examinable, so store them as a pair rather than deducing one from the other.
(Extended) Displacement of halide ions
A halogen higher in Group VII displaces the halide ion of one lower down from a solution of its salt: chlorine displaces both bromide and iodide; bromine displaces iodide only; iodine displaces neither. Why: the more reactive halogen gains electrons more readily, so its atoms take an electron from the less reactive halide ion. The halogen added is reduced and the halide ion oxidised, so halogen displacement is always a redox reaction.
Drawn from real examiner reports.
Halogen molecule ≠ halide ion
A free halogen is a neutral diatomic MOLECULE (, , ). A halide ion is a SINGLE atom that has gained one electron, carrying a 1- charge (, , ), as in a salt like . Examiners flagged bromide as "Br" (no charge) or "Br2+" (subscript kept, charge invented) instead of .
Flagged Jun 2022 P32 Q8aii
Chlorine and iodine are not liquids
Candidates have stated that BOTH chlorine and iodine are liquids at r.t.p. Only bromine is. Chlorine boils at about -34 degC, far below room temperature, so it is a gas; iodine melts at about 114 degC, far above room temperature, so it is a solid. Learn the states in fixed order down the group -- gas, liquid, solid -- instead of guessing from the name.
Flagged Jun 2022 P32 Q8ai; Nov 2022 P11 Q21; Jun 2023 P43 Q5; Nov 2023 P42 Q5b
Reactivity ≠ boiling point
Bromine has a LOWER boiling point than iodine. Candidates wrongly explained this by calling bromine "more reactive" -- but reactivity is a CHEMICAL property (how readily an atom gains an electron) and boiling point a PHYSICAL one. The real reason: bromine molecules are smaller, so intermolecular forces between them are weaker and need less energy to overcome.
Flagged Jun 2022 P32 Q8ai; Nov 2022 P11 Q21; Jun 2023 P43 Q5; Nov 2023 P42 Q5b
Iodine is grey-black, not purple
At r.t.p. iodine is a grey-black solid. "Iodine is purple" is a common wrong answer: iodine only gives a purple vapour when it is heated and sublimes, which is not its appearance at r.t.p. Aqueous iodine formed in a displacement reaction appears brown in solution, not purple either. Give the colour that the conditions stated in the question call for.
Group VII ≠ Group I trend direction
Reactivity decreases down Group VII (the halogens) but increases down Group I (the alkali metals). The two trends run in OPPOSITE directions, so "reactivity always increases down a group" is wrong and costs the mark on any halogen question. Density, by contrast, increases down both groups -- check which property the question names before quoting a direction.
(Extended) Displacement runs one way only
Candidates reverse the direction, writing iodine as displacing bromide. Displacement works only when the added halogen is the more reactive one -- higher in the group than the halide already in solution. So bromine water added to potassium chloride gives no reaction and no colour change, and iodine displaces neither chloride nor bromide. "No reaction" is creditable.
Give colour AND state, one mark each
An "appearance at r.t.p." question awards a mark per halogen and expects BOTH the colour and the physical state. "Iodine is dark" without "solid", or "bromine is a liquid" without "red-brown", typically loses that mark. Three halogens, 3 marks, six pieces of information.
Match the command word
"State" wants the fact alone (the colour and state, or the trend). "Describe" wants what happens (the colour change seen). "Explain" wants the reason: relative reactivity and electron gain. Restating the observation for an "explain" is true but off-question, and scores nothing.
(Extended) Three-step displacement check
(1) Locate both halogens in Group VII. (2) The one HIGHER in the group is more reactive, since reactivity decreases downwards. (3) If the halogen added is the more reactive one, it displaces the halide ion and you name the products; if not, answer "no reaction".
(Extended) Predicting a Group VII element
For an unfamiliar halogen like astatine, extend the trend the named three set: density keeps increasing, reactivity keeps decreasing, colour darkens, state stays solid below iodine. Justify each prediction by naming the trend -- a bare "higher"/"lower" earns no reasoning mark.
Cambridge 0654 spec reference: Section C8 "The Periodic Table", sub-topic C8.3 "Group VII properties". Core: describe the halogens chlorine, bromine and iodine as diatomic non-metals with general trends down the group (increasing density, decreasing reactivity); state their appearance at room temperature and pressure (r.t.p.). Extended (Supplement): describe and explain the displacement reactions of halogens with other halide ions; predict the properties of other Group VII elements, given information about the elements.
| Term | Mark-scheme-precise meaning |
|---|---|
| Halogen | A Group VII element; a diatomic non-metal (7 outer-shell electrons) |
| Diatomic molecule | A molecule made of exactly two atoms of the same element, e.g. |
| Halide ion | A halogen atom that has gained one electron, giving it a single 1- charge, e.g. , , |
| Displacement reaction (halogens) | A reaction in which a more reactive halogen takes the place of a less reactive halogen in a compound, forming the less reactive halogen as a free element and the halide ion of the more reactive halogen |
| r.t.p. | Room temperature and pressure -- the reference condition for stating a substance's physical state |
Group VII trends (going DOWN the group: chlorine -> bromine -> iodine):
Appearance at r.t.p.:
| Halogen | Formula | State at r.t.p. | Colour |
|---|---|---|---|
| Chlorine | Gas | Pale yellow-green | |
| Bromine | Liquid | Red-brown | |
| Iodine | Solid | Grey-black |
Define "diatomic molecule", and give the formula of the three named halogens.
Magnesium forms an ion with a 2+ charge (). Chlorine forms a halide ion with a 1- charge ().
Use this information to write the formula of magnesium chloride. (1 mark)
