Deduce & Write Ionic Compound Formulas
Chemistry: Cambridge International GCSE Supplement, topic 3.1.4
Deduce and Write Ionic Compound Formulas
Ionic compounds are neutral. Formulas are deduced by balancing ion charges or counting diagrammed ions. Write balanced symbol equations with state symbols and ionic equations.
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Understanding ionic compounds
- Ionic compounds form from positive and negative ions.
- Ions combine in ratios, making the compound neutral overall.
- Chemists use charges on ions to find the formula.
- The formula shows the simplest ratio of ions present.
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Deduce formulas from ion charges
- Positive and negative ion charges balance to zero.
- Example: sodium ion (Na+) and chloride ion (Cl-) form NaCl.
- For magnesium ion (Mg²+) and chloride ion (Cl-), formula is MgCl₂.
- Balance charges so that total positive equals total negative.
Deduce formulas from diagrams
- Models show ions in ratios indicating formula.
- Count the positive and negative ions shown.
- For example, 2 potassium ions and 1 oxide ion give K2O.
- Use the count to write the formula accurately.
Writing symbol equations
- Symbol equations show reactants and products with formulas.
- Include state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
- Aqueous means the substance is dissolved in water.
- Example: 2Na(s) + Cl2(g) → 2NaCl(s).
- Use coefficients to balance atoms and charge.
Writing ionic equations
- Ionic equations show ions that change during reactions.
- Spectator ions, which do not change, are omitted.
- For example:
- Full equation: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
- Ionic equation: Ag+(aq) + Cl-(aq) → AgCl(s)
- Ionic equations clarify the actual chemical change.
Test yourself on Deduce & Write Ionic Compound Formulas
When deducing the formula of an ionic compound from ion charges, what must be true about the total charges?