AQA GCSE Combined Science (8464), Higher tier · Chemistry › Atomic structure and the periodic table › The periodic table
Practise Group 7. 14 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The halogens in Group 7: their properties and trends, the compounds they form with metals and non-metals, why they get less reactive down the group, and displacement reactions. Higher tier students may also be asked for ionic equations. Displacement and 'explain the trend' questions are very common.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Recall the halogens and their statesAt room temperature chlorine is a green gas, bromine a red-brown liquid and iodine a grey-black solid.
4
Describe the trends down Group 7Relative molecular mass, melting point and boiling point increase; reactivity decreases.
5
Describe the compounds halogens formIonic compounds with metals (halide ions, 1−); covalent molecules with non-metals, e.g. HCl.
5
Predict whether a displacement reaction happensA more reactive halogen displaces a less reactive one from a solution of its salt.
6
Write equations for displacement reactionse.g. Cl2 + 2NaBr → 2NaCl + Br2, with the colour change.
7
Explain why reactivity decreases down Group 7The outer shell is further from the nucleus, so an electron is attracted less strongly and gained less easily.
8
Write ionic equations for displacement (Higher tier)Leave out the spectator metal ions, e.g. Cl2 + 2I− → 2Cl− + I2.
Notes
The halogens
Group 7 contains fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At).
They are non-metals and consist of molecules made of pairs of atoms: F2, Cl2, Br2, I2.
They all have seven electrons in their outer shell, so they have similar reactions. In reactions with metals each atom gains one electron to form a halide ion with a 1− charge, e.g. Cl−.
At room temperature: chlorine is a pale green gas, bromine is a red-brown liquid and iodine is a grey-black solid.
Trends down the group
Going down the group, the relative molecular mass, melting point and boiling point all increase.
Going down the group, reactivity decreases.
Why: the atoms have more shells, so the outer shell is further from the nucleus. The attraction for an extra electron is weaker, so it is harder for the atom to gain an electron.
Compounds of the halogens
With metals, halogens form ionic compounds (metal halides) containing halide ions with a 1− charge: 2Na + Cl2 → 2NaCl.
With non-metals, halogens form covalent compounds made of molecules, by sharing electrons: H2 + Cl2 → 2HCl (hydrogen chloride).
Chlorine or bromine added to potassium iodide solution turns it brown as iodine forms.
There is no reaction if the halogen added is less reactive than the one in the salt, e.g. iodine and potassium bromide.
Ionic equation (Higher tier): the potassium ions are spectator ions, so Cl2 + 2Br− → 2Cl− + Br2. grade 8+
Cheatsheet
Group 7 = halogens: seven outer electrons; molecules of two atoms (Cl2)
Down the group: relative molecular mass, melting point and boiling point increase
Down the group: reactivity decreases
Reason: outer shell further from nucleus → weaker attraction → harder to gain an electron
With metals: ionic halides (1− ions); with non-metals: covalent molecules
A more reactive halogen displaces a less reactive one from a solution of its salt
Cl2 + 2KBr → 2KCl + Br2 (colourless → orange)
Ionic (HT): Cl2 + 2I− → 2Cl− + I2grade 8+
How to answer each type of question
Predict and explain a displacement result
2 to 3 marks5
Decide which halogen is more reactive (the one higher up the group).
A reaction happens only if the added halogen is more reactive than the one in the salt.
Give the colour change: orange for bromine, brown for iodine.
Example. A student adds bromine water to potassium chloride solution and to potassium iodide solution. (a) Which solution reacts? Explain your answer. (b) What colour does the solution that reacts turn?
Show the model answer
(a) Potassium iodide (1). Bromine is more reactive than iodine, so it displaces it, but bromine is less reactive than chlorine (1). (b) Brown (1)
Write a balanced equation for a displacement reaction
2 marks6
The halogens are X2; the salts are metal halides such as NaBr or KI.
The two halogens swap places.
Balance: you need 2 of the salt for each halogen molecule.
Example. Write a balanced symbol equation for the reaction between chlorine and sodium iodide solution.
Compare the number of shells and the distance of the outer shell from the nucleus.
Compare the attraction for an extra electron.
Say which atom gains an electron more easily.
Example. Explain why chlorine is more reactive than bromine.
Show the model answer
A chlorine atom has fewer shells, so its outer shell is closer to the nucleus (1). There is a stronger attraction between the nucleus and an electron coming into the outer shell (1). So chlorine gains an electron more easily (1).
Describe the compounds halogens form
2 marks5
With a metal: ionic compound, containing halide ions (1−).
With a non-metal: covalent compound, made of molecules.
Example. Chlorine reacts with sodium and with hydrogen. Compare the compounds formed in these two reactions.
Show the model answer
With sodium, chlorine forms an ionic compound, sodium chloride, which contains Cl− ions (1). With hydrogen, chlorine forms a covalent compound, hydrogen chloride, made of molecules (1).
Write the ionic equation for a displacement reaction (Higher tier only)
2 marks8
Write the halide ions from the salt and leave out the metal ions (spectator ions).
Balance so that the charges are the same on both sides.
Example. Bromine reacts with potassium iodide solution. Write the ionic equation for this reaction.
(a) Write the ionic equation for the reaction of chlorine with bromide ions.[2]
(b) Complete the half equation for chlorine. Cl2 + ....e− → ....Cl−[1]
(c) Which species is oxidised in this reaction? Explain your answer.[2]
(d) Explain why the reactivity of the halogens decreases down Group 7.[3]
Show the answer and mark scheme
(a)Answer: Cl2 + 2Br− → 2Cl− + Br2
Cl2 + Br− → Cl− + Br2 (correct species)
balanced: Cl2 + 2Br− → 2Cl− + Br2
(b)Answer: Cl2 + 2e− → 2Cl−
Cl2 + 2e− → 2Cl−
(c)Answer: Bromide ions, because they lose electrons.
bromide ions / Br−
(because they) lose electrons
(d)Answer: The atoms get larger, so the outer shell is further from the nucleus; there is weaker attraction for an extra electron, so an electron is gained less easily.
atoms get larger / outer shell is further from the nucleus / more shells
(so there is) weaker attraction (from the nucleus) for the electron being gained