Cambridge O Level Chemistry · Syllabus 5070 · The Periodic Table
Halogen
What is Halogen?
An element of Group VII of the Periodic Table. Halogens are non-metals whose atoms have seven electrons in the outer shell, and each exists as a diatomic molecule of two atoms joined by a covalent bond, written Cl2, Br2 and I2. Each gains one electron to form an ion with a charge of one minus.
This definition is part of the The Periodic Table chapter in Cambridge O Level Chemistry.
Halogen in context
Down Group I the alkali metals become more reactive and lower-melting as the single outer electron is lost more easily, while down Group VII the halogens become less reactive as it becomes harder to gain the extra electron needed to fill the outer shell. A more reactive halogen displaces the halide ion of a less reactive one from solution. Transition elements, between Groups II and III, show variable oxidation numbers rather than one fixed charge, and Group VIII noble gases are unreactive because their outer shell is already full.
Common mistakes with Halogen
- 6 · Halogen and halide used interchangeably Defect Writing “the chlorine in sodium chloride”, or writing \(\mathrm{Cl}\) for the element and \(\mathrm{Cl_2^{-}}\) for the ion. Fix The halogen is the free element, a diatomic molecule: \(\mathrm{Cl_2}\), \(\mathrm{Br_2}\), \(\mathrm{I_2}\). The halide is the single negative ion inside a compound: \(\mathrm{Cl^{-}}\), \(\mathrm{Br^{-}}\), \(\mathrm{I^{-}}\). They have different colours, different reactivity and different formulae.
- 7 · Displacement run the wrong way Defect Predicting that iodine displaces chloride, usually justified by “iodine is bigger”. Fix Only a more reactive halogen displaces a less reactive halide, and reactivity decreases down Group VII, so the order is \(\mathrm{Cl_2 > Br_2 > I_2}\). Iodine is the least reactive of the three and displaces nothing from the other two. “No reaction” is a complete and correct answer worth full marks.
- Error 12 · “Chlorine is a green liquid; bromine is a brown gas.” Defect States shuffled between elements. Chemistry At r.t.p. the three states run gas, liquid, solid going down the group: chlorine is a pale yellow-green gas, bromine a red-brown liquid, iodine a grey-black solid. Repair Fix the order gas → liquid → solid to the order Cl → Br → I, and the rest follows. Transfer Which halogen is a liquid at r.t.p.? Bromine.
- Error 13 · Writing \(\mathrm{Cl + KBr \rightarrow KCl + Br}\) Defect Free halogens written as single atoms. Chemistry Halogen atoms have seven outer electrons and pair up to complete their shells, so the elements are diatomic: \(\mathrm{Cl_2}\), \(\mathrm{Br_2}\), \(\mathrm{I_2}\). Repair \(\mathrm{Cl_2 + 2KBr \rightarrow 2KCl + Br_2}\). Note that the subscript forces the \(2\) in front of \(\mathrm{KBr}\). Transfer Formula of the element bromine? \(\mathrm{Br_2}\). Of the bromide ion? \(\mathrm{Br^{-}}\).
- Error 14 · “Potassium bromide solution is red-brown.” Defect The colour of the element attributed to its ion. Chemistry Halide ions in solution are colourless. Red-brown is the free element bromine. This matters because the colourless starting solution is what makes a displacement visible. Repair Halide solution \(=\) colourless. Halogen in solution \(=\) coloured. Transfer What colour is aqueous potassium iodide? Colourless.
- Error 15 · “Iodine displaces chloride because iodine atoms are bigger.” Defect Displacement run backwards, on an irrelevant reason. Chemistry Displacement needs the added halogen to be the more reactive, and reactivity decreases down Group VII. Iodine is the least reactive of the three and displaces neither of the others. Atomic size is not a Topic 8 explanation. Repair Compare positions first: only a halogen above the halide can displace it. Transfer Does bromine displace chloride? No. Does bromine displace iodide? Yes.
Examiner tips on Halogen
- The diagonal is worth noticing. Three of the six non-reactions are the “same element” cases along the diagonal, and they are non-reactions for a different reason from the other three. If a question asks why nothing happened, say which of the two reasons applies: either the added halogen was less reactive, or it was the same element.
- A three-part observation sentence that always works. “The [starting colour] solution of [named halide] turns [final colour], because [named halogen] has been displaced and is present as [formula, aqueous].” If you cannot fill in the first blank, the question has not given you enough to answer — and saying so is worth more than a guess.
- Two directions, one comparison. Whenever a question asks about two halogens, there is only ever one fact in play: which is higher in the group. From it come the reactivity order, the direction of any displacement, the density comparison, and often the state. Find that one fact first and the rest of the answer writes itself.
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