Explaining the trend in the reactivity of the halogens
2 marks · EasyA chlorine atom can react to form a chloride ion. Calculate the total number of electrons in this chloride ion. Explain your answer using the electronic configuration.
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18 electrons. Chlorine has 17 electrons as an atom and gains 1 electron to form a chloride ion (Cl−).
Also accept: Chloride ion has 18 electrons.
Common mistakes: Stating 17 electrons; Saying the atom does not gain electrons
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Marking points
M1 · 1 mark · States or explains that chlorine atom has 17 electrons and gains one electron to form Cl−.
A1 · 1 mark · Gives the correct total number of electrons as 18. (follow-through)
Worked solution
A chlorine atom has 17 protons and 17 electrons. To form a chloride ion (Cl−), it gains 1 electron.
Total electrons in Cl− = 17 + 1 = 18.