Formulae for Ionic Compounds
2 guided lessons · 18 checks · 1. Principles of Chemistry → Ionic Bonding
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Direct comparison
Every ionic compound is electrically neutral overall: the total positive charge carried by its metal ions exactly balances the total negative charge carried by its non-metal ions. Reading the two charges side by side is therefore enough to work out how many of each ion the formula needs. This direct comparison method starts from the charges alone and finishes with the formula.
What you know already
From Ionic Bonding you already know the common ions and the charges they carry, from Group 1 metals giving 1+ ions to the sulfate and nitrate groups. This note turns that charge table into a working method: combining any positive and negative ion so the charges cancel exactly, giving the formula of the ionic compound they form.
Key points of understanding
- Ionic compounds carry no overall charge: the positive and negative charges on their ions cancel exactly.
- This neutrality belongs to the compound as a whole and is not the same statement as a neutral atom.
- In the direct comparison method, the two ion charges are written side by side and checked for cancellation.
- One sulfate ion, carrying 2-, exactly cancels the 2+ charge on the iron(II) ion, so the formula is FeSO4.
- When the charges are equal in size but opposite in sign, the formula needs one of each ion.
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1.Direct comparison
Every ionic compound is electrically neutral overall: the total positive charge carried by its metal ions exactly balances the total negative charge carried by its non-metal ions. Reading the two charges side by side is therefore enough to work out how many of each ion the formula needs. This direct comparison method starts from the charges alone and finishes with the formula.
Key points
- Ionic compounds carry no overall charge: the positive and negative charges on their ions cancel exactly.
- This neutrality belongs to the compound as a whole and is not the same statement as a neutral atom.
- In the direct comparison method, the two ion charges are written side by side and checked for cancellation.
- One sulfate ion, carrying 2-, exactly cancels the 2+ charge on the iron(II) ion, so the formula is FeSO4.
- When the charges are equal in size but opposite in sign, the formula needs one of each ion.
Guided content from the SME authoring corpus — note rn_J7ZcTWyRxRgWQ45R, pinned at fa1f302050ba · promoted SyllabAI/syllabai-resources e967e75f1339
