Explaining the trend in the reactivity of the halogens
4 marks · NormalThis question is about electrons and halogen reactivity.
State the charge on the ion formed when a chlorine atom gains an electron.
Explain, in terms of electron arrangement, why chlorine is more reactive than bromine.
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(a) 1−
(b) Chlorine has a smaller atom than bromine, so when it gains an electron, the added electron is closer to the nucleus and more strongly attracted. This means chlorine forms Cl⁻ ions more easily than bromine forms Br⁻ ions, making chlorine more reactive. · must show Chlorine atom is smaller than bromine atom, Electron added is closer to nucleus in chlorine, Stronger attraction for new electron in chlorine, Chlorine forms ion more easily/more reactive
Common mistakes: Wrong charge (e.g., Cl2− or forgetting minus sign); Saying bromine is more reactive than chlorine
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Marking points
B1 (a) · 1 mark · States that the charge is 1− (or −1)
M1 (b) · 1 mark · States or implies chlorine has a smaller atom than bromine
M2 (b) · 1 mark · Mentions the added electron is closer to the nucleus and feels stronger attraction in chlorine
A1 (b) · 1 mark · Correctly relates this to chlorine being more reactive (forms Cl⁻ ions more easily / takes electrons more readily)
Worked solution
a) The charge is 1−. b) Chlorine atoms are smaller than bromine atoms, so when a chlorine atom gains an electron the new electron is added to a shell closer to the nucleus. This means the attraction from the nucleus is stronger, making it easier for chlorine to gain an electron and form a Cl⁻ ion. Therefore, chlorine is more reactive than bromine.