Calcium chloride
2 marks · EasyExplain why two chlorine atoms are needed to react with one calcium atom to form calcium chloride.
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Also accept: Each Cl atom needs one electron; Ca has two to give; so two Cl needed for each Ca.
Common mistakes: Thinks Ca gives only one electron.; Thinks one Cl is enough.
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Marking points
M1 · 1 mark · Recognises calcium must lose two electrons (or that Ca forms a 2+ ion).
M1 · 1 mark · Recognises that each chlorine atom gains only one electron (or that Cl forms a 1- ion).
Worked solution
A calcium atom needs to lose two electrons; each chlorine atom can only accept one electron, so two chlorine atoms are needed to accept the two electrons from one calcium atom.