A student investigates three elements from the same group of the periodic table: lithium, sodium, and potassium. They react each element with cold water and observe what happens. Explain why the reactivity of these elements increases going down the group, and describe the trend in the properties of the elements that accounts for this pattern.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lithium, sodium, and potassium are all in Group 1 of the periodic table. When placed in cold water, lithium reacts slowly, sodium reacts more vigorously, and potassium reacts most vigorously, sometimes catching fire.
Model answer (5 marks)
The elements are in the same group, so they all have one valence electron.
Going down the group the atoms have more electron shells; the valence electron is further from the nucleus.
The attractive force between the nucleus and the valence electron therefore decreases, so the valence electron is lost more easily.
Because the valence electron is lost more easily, the element reacts more readily with water, giving the observed increase in reactivity.
The melting point of the Group 1 elements decreases down the group, while the density generally increases.
Going down the group the atoms have more electron shells; the valence electron is further from the nucleus.
The attractive force between the nucleus and the valence electron therefore decreases, so the valence electron is lost more easily.
Because the valence electron is lost more easily, the element reacts more readily with water, giving the observed increase in reactivity.
The melting point of the Group 1 elements decreases down the group, while the density generally increases.
Examiner tips
- Use the command word ‘explain’ to show cause and effect; link the trend in reactivity to the distance of the valence electron from the nucleus.
- Mention the change in melting point and density as additional properties that trend down the group.
- Keep the answer concise – 5 marks, so one sentence per point is enough.
- Use UK spelling (e.g. ‘reactive’).
Common mistakes
- Confusing the trend in melting point (students say it increases instead of decreases).
- Forgetting to state that the elements have one valence electron or that the outer electron is further from the nucleus.
Mark scheme (5 marks)
- The elements are in the same group so they have the same number of electrons in their outer shell (one outer electron)
- Going down the group, the atoms have more electron shells / the outer electron is further from the nucleus
- The attractive force between the nucleus and the outer electron decreases going down the group (the outer electron is lost more easily)
- Because the outer electron is lost more easily, the element is more reactive / reacts more readily with water
- The melting point / density of the Group 1 elements decreases / increases going down the group (melting point decreases; density generally increases)
Key terms in this question
group · reactivity · periodic table
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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