Why do the Group 7 halogens get less reactive down the group, and what are displacement reactions?
Group 7 the halogens: their physical properties and trends, their reactions to form ions and compounds, the decreasing reactivity down the group, and halogen displacement reactions.
A focused answer to Edexcel GCSE Chemistry topic 6, covering the physical properties and trends of the Group 7 halogens, how they form 1- ions and covalent molecules, the decreasing reactivity down the group explained by electronic structure, and halogen displacement reactions with their ionic equations.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Edexcel wants you to describe the physical properties and trends of the Group 7 halogens, explain how they form ions and covalent molecules, describe and explain the decreasing reactivity down the group in terms of electronic structure, and write equations for halogen displacement reactions. The trend explanation and displacement equations are reliable marks.
Physical properties and trends
The halogens are reactive non-metals that exist as diatomic molecules (, , , ). Going down the group:
- They get darker in colour (chlorine pale green, bromine orange-brown, iodine grey-black).
- Their melting and boiling points increase, so at room temperature chlorine is a gas, bromine a liquid and iodine a solid.
All halogen atoms have seven electrons in their outer shell.
Forming ions and compounds
Halogens are one electron short of a full outer shell, so they are very reactive:
- With metals they gain one electron to form a ion (a halide), forming ionic compounds such as sodium chloride.
- With non-metals they share electrons to form covalent molecules such as hydrogen chloride, .
Because they all have seven outer electrons, the halogens have similar chemical properties and react in similar ways.
The reactivity trend and its explanation
Reactivity decreases down the group, the opposite of Group 1. The explanation is about how easily the atom gains an electron:
Displacement reactions
Because reactivity decreases down the group, chlorine displaces bromine and iodine, and bromine displaces iodine, but not the reverse. For example, chlorine added to potassium bromide solution turns it orange as bromine is released:
The ionic equation shows chlorine gaining electrons (reduced) and bromide ions losing them (oxidised), so it is a redox reaction.
Try this
Q1. State the number of electrons in the outer shell of a Group 7 atom. [1 mark]
- Cue. Seven.
Q2. State the trend in melting point down Group 7. [1 mark]
- Cue. The melting point increases down the group.
Q3. Write the ionic equation for chlorine displacing iodine from sodium iodide solution. [2 marks]
- Cue. .
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20184 marksChlorine is bubbled through potassium bromide solution. State what you would observe, write the ionic equation, and explain why this reaction happens.Show worked answer →
A 4-mark halogen displacement question.
The colourless solution turns orange (or brown) as bromine is formed (1 mark for the observation). The ionic equation is (1 mark). The reaction happens because chlorine is more reactive than bromine (1 mark), so chlorine displaces bromine from the potassium bromide, taking its place as the more reactive halogen (1 mark).
Markers reward the colour change, the balanced ionic equation, and the more-reactive-halogen explanation.
Edexcel 20213 marksExplain why the reactivity of the Group 7 halogens decreases down the group, in terms of electronic structure.Show worked answer →
A 3-mark trend-explanation question.
Halogens react by gaining one electron to complete their outer shell. Going down the group, the outer shell is further from the nucleus (1 mark) and is more shielded by inner shells, so the attraction for the incoming electron is weaker (1 mark). This means an electron is gained less easily down the group, so reactivity decreases (1 mark).
Markers reward "outer shell further from the nucleus", weaker attraction for the gained electron, and the conclusion that reactivity falls.
Related dot points
- Group 1 the alkali metals: their physical properties, their reactions with water and oxygen, the trend in reactivity down the group, and the explanation in terms of electronic structure.
A focused answer to Edexcel GCSE Chemistry topic 6, covering the physical properties of the Group 1 alkali metals, their reactions with water and oxygen, the increasing reactivity down the group, and how the trend is explained by the increasing distance of the outer electron from the nucleus.
- Group 0 the noble gases: their lack of reactivity explained by full outer shells, the trends in boiling point and density down the group, and their uses.
A focused answer to Edexcel GCSE Chemistry topic 6, covering why the Group 0 noble gases are unreactive in terms of their full outer shells, the trends in boiling point and density down the group, that they are monatomic gases, and their uses such as in lighting and inert atmospheres.
- The periodic table: how the elements are arranged by atomic number into groups and periods, the development of the table by Mendeleev, metals and non-metals, and electronic configurations of the first 20 elements.
A focused answer to Edexcel GCSE Chemistry topic 1, covering how the modern periodic table is arranged by atomic number, the work of Mendeleev, the meaning of groups and periods, the metal and non-metal divide, and how to write the electronic configurations of the first 20 elements and link them to group number.
- Covalent and metallic bonding: shared electron pairs, simple molecular and giant covalent structures (diamond, graphite, fullerenes, graphene), polymers, metallic bonding, and how each structure explains properties.
A focused answer to Edexcel GCSE Chemistry topic 1, covering covalent bonding and dot-and-cross diagrams, simple molecular substances, the giant covalent structures of diamond, graphite, fullerenes and graphene, simple polymers, metallic bonding with delocalised electrons, and how each structure explains melting point and conductivity.
- Reactions of metals: reactions with oxygen, water and dilute acids, using these to place metals in a reactivity series, displacement reactions, and recycling and life cycle considerations.
A focused answer to Edexcel GCSE Chemistry topic 4, covering how metals react with oxygen, water and dilute acids, using the observations to order metals in a reactivity series, displacement reactions of more reactive metals, writing ionic equations for displacement, and why metals are recycled.
Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)