# 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.

> OCR A-Level Chemistry B: Salters (H433) — 3.1 The periodic table and periodicity · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. The elements are in the same group so they have the same number of electrons in their outer shell (one outer electron)
2. Going down the group, the atoms have more electron shells / the outer electron is further from the nucleus
3. The attractive force between the nucleus and the outer electron decreases going down the group (the outer electron is lost more easily)
4. Because the outer electron is lost more easily, the element is more reactive / reacts more readily with water
5. The melting point / density of the Group 1 elements decreases / increases going down the group (melting point decreases; density generally increases)

## Key terms

- [group](https://www.gradenine.co.uk/glossary/group)
- [reactivity](https://www.gradenine.co.uk/glossary/reactivity)
- [periodic table](https://www.gradenine.co.uk/glossary/periodic-table)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-investigates-three-elements-from-3e97aba1) · Published by Druglandscape Ltd.