Sodium and potassium are both elements in Group 1 of the periodic table. When a small piece of potassium is placed on cold water it reacts more vigorously than sodium does under the same conditions. Explain why potassium reacts more vigorously with cold water than sodium does.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Group 1 elements all react with cold water to produce a metal hydroxide and hydrogen gas. The reactions become more vigorous as you move down the group.
Model answer (5 marks)
Potassium has a larger atomic radius than sodium because it has one more electron shell, so its outer 4s electron is further from the nucleus.
The further distance means the nuclear attraction on that electron is weaker.
A weaker attraction makes the outer electron easier to remove.
In Group 1 elements the reaction with water involves loss of the single outer electron to give a 1+ ion.
Therefore, as you move down the group the outer electron is lost more readily, so potassium reacts more vigorously with cold water than sodium.
The further distance means the nuclear attraction on that electron is weaker.
A weaker attraction makes the outer electron easier to remove.
In Group 1 elements the reaction with water involves loss of the single outer electron to give a 1+ ion.
Therefore, as you move down the group the outer electron is lost more readily, so potassium reacts more vigorously with cold water than sodium.
Examiner tips
- Use the chain of reasoning: more shells → weaker attraction → easier electron loss → more vigorous reaction.
- Show the link to Group 1 reactivity trend and the 1+ ion formation.
Common mistakes
- Confusing the number of shells with the principal quantum number only.
- Forgetting to mention the 1+ ion or the loss of the outer electron.
- Using vague terms like "more reactive" without explaining why.
Mark scheme (5 marks)
- Potassium has more electron shells than sodium / potassium's outer electron is in a shell further from the nucleus than sodium's
- The outer electron in potassium is less strongly attracted to / held by the nucleus than in sodium
- The outer electron in potassium is lost more easily than in sodium
- Elements in Group 1 react by losing their single outer electron to form a 1+ ion
- Reactivity increases down Group 1 because it becomes progressively easier to lose the outer electron as you go down the group
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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