AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Atomic structure and the periodic table › The periodic table
Practise Group 7. 8 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. Displacement and 'explain the trend' questions are very common.
Key facts
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)
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
diagram
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.
Each halogen atom gains one electron (red) to fill its outer shell.
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.
How to answer each type of question
Predict and explain a displacement result
2 to 3 marksGrade 5
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?
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(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)
Describe the compounds halogens form
2 marksGrade 5
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.
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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).
Don’t lose marks
Naming the ion or compound with '-ine': NaBr is sodium bromide, not sodium bromine.
Writing Cl instead of Cl2 for chlorine.
Saying halogen atoms lose electrons. They gain one electron.
Explaining the trend as for Group 1 ('the electron is lost more easily'). For the halogens it is about gaining an electron.
Saying iodine displaces chlorine or bromine.
More tips
Memory tricks
Group 7 is the opposite of Group 1: the halogens get less reactive down the group.
A halogen can displace only the halogens below it in the group.
The colours get darker down the group: pale yellow (F2), pale green (Cl2), red-brown (Br2), grey-black (I2).
Halogen to halide: change '-ine' to '-ide'.
Exam technique
In displacement questions, first decide which halogen is more reactive, then say whether a reaction happens.
Explain the trend in three steps: distance of the outer shell → strength of attraction → how easily an electron is gained.
For a colour change, give the colour the solution turns (orange or brown), not the colour of the halogen you added.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
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.
Grade 4
Describe the trends down Group 7Relative molecular mass, melting point and boiling point increase; reactivity decreases.
Grade 5
Describe the compounds halogens formIonic compounds with metals (halide ions, 1−); covalent molecules with non-metals, e.g. HCl.
Grade 5
Predict whether a displacement reaction happensA more reactive halogen displaces a less reactive one from a solution of its salt.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Chlorine, bromine and iodine are Group 7 elements. What name is given to the elements in Group 7?
the halogens
Iodine reacts with hydrogen to form hydrogen iodide, HI. Write a balanced symbol equation for this reaction.
H2 + I2 → 2HI
The halogens are the elements in Group 7. Give the charge on a halide ion.
1−
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Chlorine, bromine and iodine are Group 7 elements.
(a) What name is given to the elements in Group 7?[1]
(b) The molecules of Group 7 elements are made of pairs of atoms. Give the formula of a bromine molecule.[1]
(c) Which Group 7 element is a grey-black solid at room temperature? Tick (✓) one box.[1]
Bromine
Chlorine
Fluorine
Iodine
(d) Sodium reacts with chlorine. Name the product of the reaction.[1]
(e) How many electrons are in the outer shell of an atom of a Group 7 element?[1]
Show the answer and mark scheme
(a)Answer: the halogens
halogens
(b)Answer: Br2
Br2
(c)Answer: Iodine
(d)Answer: sodium chloride
sodium chloride
(e)Answer: 7
7
Question 2Medium6 marks
Astatine, At, is below iodine in Group 7.
(a) Predict whether astatine would react with sodium iodide solution. Explain your answer.[2]
(b) Iodine reacts with hydrogen to form hydrogen iodide, HI. Write a balanced symbol equation for this reaction.[1]
(c) Hydrogen iodide is a covalent compound. Sodium iodide is an ionic compound. Explain this difference in terms of the elements that react.[2]
(d) Predict the formula of the compound formed when calcium reacts with astatine.[1]
Show the answer and mark scheme
(a)Answer: No reaction, because astatine is less reactive than iodine.
no (reaction)
astatine is less reactive than iodine (as it is lower in Group 7)
(b)Answer: H2 + I2 → 2HI
H2 + I2 → 2HI
(c)Answer: Hydrogen and iodine are both non-metals, so they share electrons; sodium is a metal, so it transfers an electron to iodine and ions form.
hydrogen and iodine are both non-metals so they share (pairs of) electrons
sodium is a metal so an electron is transferred (from sodium to iodine) / ions are formed