How do the properties of elements change within Groups 1, 7 and 0?
The properties and trends of the Group 1 alkali metals, the Group 7 halogens and the Group 0 noble gases, including reactivity trends and displacement reactions of the halogens.
A CCEA GCSE Chemistry answer on group trends, covering the properties and reactivity of the Group 1 alkali metals, the Group 7 halogens with their displacement reactions, and the unreactive Group 0 noble gases, and explaining each trend in terms of electron arrangement.
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What this dot point is asking
CCEA wants you to describe the characteristic properties of the Group 1 alkali metals, the Group 7 halogens and the Group 0 noble gases, state the reactivity trends down Groups 1 and 7, explain those trends using electron arrangement, and write the displacement reactions of the halogens.
Group 1: the alkali metals
Each alkali metal has one outer electron, which it loses to form a +1 ion. Going down the group the atom is larger, so the outer electron is further from the nucleus and shielded by more inner shells. It is therefore held less strongly and lost more easily, so the metal becomes more reactive down the group. This is why potassium reacts far more violently with water than lithium.
Group 7: the halogens
Halogen reactivity is about gaining an electron. Going down the group the outer shell is further from the nucleus and more shielded, so the atom attracts an extra electron less strongly. So the halogens become less reactive down the group, the opposite trend to Group 1.
Group 0: the noble gases
Their full outer shells (helium 2, neon 2,8, argon 2,8,8) make them stable. This inertness is useful: argon fills light bulbs to stop the filament burning, and helium fills balloons because it is light and does not react or burn.
Worked example
Examples in context
Example 1. Storing reactive metals. Sodium and potassium are stored under oil because they react quickly with water and oxygen in air. Their reactivity, which rises down Group 1, dictates how they must be handled in the lab, a direct application of the trend.
Example 2. Chlorine in water treatment. Chlorine is added to drinking water and swimming pools to kill bacteria, exploiting how readily this reactive halogen reacts. Its position near the top of Group 7 makes it reactive enough to be an effective disinfectant, showing how a group trend underlies a public-health use.
Try this
Q1. State and explain the trend in reactivity down Group 7. [2 marks]
- Cue. Reactivity decreases; larger atom, more shielding, so an extra electron is gained less easily.
Q2. Explain why the noble gases are unreactive. [1 mark]
- Cue. They have full outer shells, so they do not need to gain, lose or share electrons.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20194 marksDescribe and explain the trend in reactivity of the Group 1 alkali metals as you go down the group.Show worked answer →
Markers want the trend stated and explained in terms of the outer electron.
Reactivity increases down Group 1: lithium reacts steadily with water, sodium more vigorously, and potassium so vigorously that the hydrogen produced ignites.
The explanation is about losing the single outer electron. Going down the group, atoms get larger and the outer electron is in a shell further from the nucleus and is shielded by more inner shells. So the outer electron is held less strongly and is lost more easily, making the reaction faster and the metal more reactive.
Markers reward the trend (more reactive down the group) plus the explanation: bigger atom, more shielding, outer electron lost more easily.
CCEA 20213 marksChlorine is added to a solution of potassium bromide. Describe what is observed and write the ionic equation for the reaction.Show worked answer →
The marks are for the observation and a correct equation.
Chlorine is more reactive than bromine because it is higher in Group 7, so chlorine displaces bromine from the solution. The colourless solution turns orange or brown as bromine is formed.
The reaction is a displacement of bromide ions by chlorine:
Markers reward the colour change to orange/brown (bromine released) and a balanced ionic equation showing chlorine displacing bromide.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)