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Question: "Explain why potassium is more reactive than sodium." (6 marks)
Potassium has more electron shells than sodium — potassium has four shells, whereas sodium has three. [1] This means the outer electron in potassium is further from the nucleus than the outer electron in sodium. [1] There are also more inner electron shells in potassium, which provide greater shielding of the outer electron from the attractive force of the positive nucleus. [1] As a result, the electrostatic attraction between the nucleus and the outer electron is weaker in potassium than in sodium. [1] This means potassium loses its outer electron more easily than sodium. [1] Because potassium loses electrons more readily, it is a better reducing agent and reacts more vigorously with water and other substances, making it more reactive overall. [1]
Examiner note: Full marks require ALL four structural points: more shells, greater distance, more shielding, weaker attraction — then the link to easier electron loss and greater reactivity. Partial answers that only say "further from the nucleus" without mentioning shielding typically score 3-4 marks. Many students lose marks by stating the conclusion without explaining the mechanism.
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