Writing simple formulae

你需要知道的

• Ions form charges by gaining or losing electrons: Group 1 forms +1 ions, Group 2 +2, oxygen −2 and the halogens −1.
• The charges in an ionic compound must cancel to zero, so magnesium fluoride is MgF2 and aluminium oxide is Al2O3.
• Ionic formulae are written in the simplest ratio, so calcium sulfide is CaS rather than Ca2S2.
• Transition metals can form more than one ion, so a Roman numeral shows the charge, e.g. iron(II) chloride is FeCl2 and iron(III) chloride is FeCl3.

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