Explaining the trend in the reactivity of the halogens
2 marks · EasyExplain, in terms of electronic configuration, why bromine is less reactive than chlorine.
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Bromine’s outer shell is further from the nucleus than chlorine’s, so the added electron is less strongly attracted. Therefore, bromine is less reactive.
Must show: Bromine's outer shell is further from the nucleus, Weaker attraction for the added electron, Less reactive
Also accept: Bromine has more shells than chlorine, so the nucleus attracts the added electron less strongly.
Common mistakes: Bromine has fewer electrons (incorrect).; Bromine has higher nuclear charge so is more reactive (incorrect reasoning).
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Marking points
M1 · 1 mark · Mentions that bromine's outer shell is further from the nucleus (or bigger atomic radius/more electron shells).
A1 · 1 mark · Links weaker attraction to the lower reactivity of bromine.
Worked solution
Bromine's outer shell is further from the nucleus than chlorine’s. The added electron is less strongly attracted, so bromine is less reactive.