1.6.3Principles of Chemistry
Formulae for Ionic Compounds — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 13 parts Lesson 36 of 50
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5The balancing principle
Overall neutrality
- Overall neutrality An ionic compound has no overall charge,
- so the total positive charge from the cations must equal the total negative charge from the anions.
Ratio of ions
- Ratio of ions The formula gives the simplest ratio of ions in the lattice, not the number in a molecule,
- because ionic compounds do not form molecules.
Order and names
- Order and names The metal (positive ion) is written first in both name and formula, for example sodium chloride, NaCl.
Part 2 of 5Method: swap and simplify
1
Write the symbol and charge of each ion, positive first.
2
Use the numbers (not the signs) of the charges as the subscript on the opposite ion, so the charges are swapped.
3
Cancel any common factor so the ratio is as simple as possible.
4
Put brackets around a compound ion if more than one is needed, for example Mg(OH)2. Omit the subscript 1.
5
Check by multiplying number of ions by charge on each side; the totals should be equal.
Part 3 of 5Worked examples
| Compound | Ions | Working | Formula |
|---|---|---|---|
| Sodium chloride | Na+, Cl− | 1 + and 1− balance | NaCl |
| Magnesium chloride | Mg2+, Cl− | One 2+ needs two 1− | MgCl2 |
| Aluminium oxide | Al3+, O2− | 2 × 3+ = 6+ and 3 × 2− = 6− | Al2O3 |
| Magnesium oxide | Mg2+, O2− | Swap gives Mg2O2, simplify | MgO |
| Calcium nitrate | Ca2+, NO3− | Two nitrates needed, use brackets | Ca(NO3)2 |
| Ammonium sulfate | NH4+, SO42− | Two ammonium needed, use brackets | (NH4)2SO4 |
Part 4 of 5Direct comparison method
1Total positive charge
- Total positive charge Find a common multiple of the two charges, called the lowest common multiple.
2Ions needed
- Ions needed Divide that multiple by each ion's charge to find how many of each are required.
3Write the formula
- Write the formula Use those numbers as subscripts.
- For Al3+ and O2− the multiple is 6, giving 2 Al and 3 O: Al2O3.
Part 5 of 5Naming and variable charges
Roman numerals
- Roman numerals Where a metal can form different ions, the name states the charge: iron(II) chloride is FeCl2 and iron(III) chloride is FeCl3.
Naming from the formula
- Naming from the formula Name the metal first, then the non-metal with an -ide ending, or the compound ion by its own name: CaCO3 is calcium carbonate.
Same-size charges
- Same-size charges When the charges are equal, such as Mg2+ and O2−, swapping leaves subscripts that must be simplified to give MgO, not Mg2O2.
Key terms
| Term | Meaning |
|---|---|
| Ionic compound | A compound made of positive and negative ions held together by electrostatic attraction in a lattice. |
| Formula | A combination of symbols and numbers that shows the ratio of atoms or ions in a substance. |
| Compound ion | A group of atoms with one overall charge, such as nitrate. |
| Subscript | A small number written below and after a symbol to show how many of that atom or ion are present. |
| Lattice | A regular, repeating three-dimensional arrangement of ions. |
Avoid these mistakes
1
Changing the charges themselves rather than the number of ions when balancing.
2
Forgetting brackets around a compound ion, writing MgOH2 instead of Mg(OH)2.
3
Not simplifying the ratio, giving Mg2O2 instead of MgO.
4
Writing the non-metal before the metal in a name or formula.
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