How do the properties and reactivity of Groups 1, 7 and 0 change down each group?
Group 1 (alkali metals), Group 7 (halogens) and Group 0 (noble gases); their properties, reactions and the trends in reactivity down each group, explained by electronic structure.
A focused answer to AQA GCSE Chemistry 4.1.3, covering the alkali metals, the halogens and the noble gases, their characteristic reactions, displacement in Group 7, and how the reactivity trends down Groups 1 and 7 are explained by electronic structure.
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What this dot point is asking
AQA wants you to describe the properties and reactions of the alkali metals (Group 1), the halogens (Group 7) and the noble gases (Group 0), explain the reactivity trends down Groups 1 and 7 using electronic structure, and predict the outcome of halogen displacement reactions. The same idea, distance from the nucleus and shielding, explains both group trends, but in opposite directions because metals lose electrons and halogens gain them.
Group 1: the alkali metals
The alkali metals (lithium, sodium, potassium and below) are soft and have low densities. They each have one outer electron, which they lose to form a ion.
Reactivity increases down Group 1. As you go down, the outer electron is in a shell further from the nucleus and is shielded by more inner shells, so the attraction holding it is weaker and it is lost more easily, making the element more reactive. Potassium reacts more violently than sodium, which reacts more violently than lithium.
Group 7: the halogens
The halogens (fluorine, chlorine, bromine, iodine) are non-metals that exist as diatomic molecules such as . They each have seven outer electrons and form ions (or covalent bonds). Their colour deepens and their physical state changes from gas to liquid to solid down the group.
A more reactive halogen displaces a less reactive halogen from a solution of its salt. For example, chlorine displaces bromine from potassium bromide because chlorine is more reactive.
Group 0: the noble gases
The noble gases (helium, neon, argon and below) are unreactive because they have full outer shells of electrons (helium has two, the rest have eight). With a stable arrangement they have no tendency to gain, lose or share electrons. Their boiling points increase down the group as the atoms get larger and the intermolecular forces between them increase.
Try this
Q1. Explain why reactivity increases down Group 1. [2 marks]
- Cue. The outer electron is further from the nucleus and more shielded, so it is lost more easily.
Q2. Predict whether chlorine will displace iodine from potassium iodide, and explain. [2 marks]
- Cue. Yes; chlorine is more reactive than iodine, so it displaces it.
Q3. Explain why the noble gases are unreactive. [1 mark]
- Cue. They have full outer shells, so they do not gain, lose or share electrons.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksExplain why the reactivity of the alkali metals increases down Group 1, and contrast this with the trend in reactivity of the halogens down Group 7. Refer to electronic structure in your answer.Show worked answer →
A 4-mark question testing both group trends and the underlying reason.
Group 1 (2 marks): reactivity increases down the group because, going down, the outer electron is in a shell further from the nucleus and shielded by more inner shells, so the attraction holding it is weaker and it is lost more easily. Losing the outer electron is what alkali metals do when they react, so easier loss means greater reactivity.
Group 7 (2 marks): reactivity decreases down the group because, going down, the outer shell is further from the nucleus and more shielded, so the atom attracts an incoming electron less strongly and gains an electron less easily; gaining an electron is what halogens do when they react.
Markers reward the distance and shielding argument applied correctly to losing (Group 1) versus gaining (Group 7) an electron.
AQA 20213 marksChlorine water is added to a solution of potassium iodide and the mixture turns brown. Write the word equation for the reaction, name the type of reaction, and explain why it happens in terms of reactivity.Show worked answer →
A 3-mark halogen displacement question.
Word equation (1 mark): chlorine + potassium iodide gives potassium chloride + iodine. Type (1 mark): a displacement reaction. Explanation (1 mark): chlorine is more reactive than iodine (reactivity decreases down Group 7), so the more reactive chlorine displaces the less reactive iodine from its salt; the brown colour is the iodine released.
Markers reward the more-reactive-displaces-less-reactive reasoning and identifying iodine as the brown product.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)