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💡 Balance the charges
For ionic compounds, criss-cross the charge numbers, then simplify to the smallest whole-number ratio — the overall compound must be neutral.
How do you write an ionic compound’s formula?
Ionic Formula Steps
Cation first, then the anion Criss-cross the charge numbers Simplify to the smallest ratio Brackets for repeated polyatomic ions
- Write the cation first , then the anion.
- Write the charges under the symbols and criss-cross the numbers.
- Divide the subscripts by a common factor to get the simplest ratio (e.g. Mg₂O₂ → MgO).
- Use brackets for two or more of the same polyatomic ion, e.g. Mg(OH)₂.
Writing ionic formulae — worked steps
- Calcium chloride: Ca (2+) and Cl (1−) → CaCl₂ .
- Aluminium oxide: Al (3+) and O (2−) → Al₂O₃ .
- Magnesium oxide: Mg (2+) and O (2−) → Mg₂O₂ simplified to MgO .
- Calcium carbonate: Ca (2+) and CO₃ (2−) → CaCO₃ .
- Magnesium hydroxide: Mg (2+) and OH (1−) → Mg(OH)₂ .
- Aluminium hydroxide is Al(OH)₃ (not AlOH₃); aluminium sulfate is Al₂(SO₄)₃ .
ⓘ Note
- Ions carry charges, e.g. Na⁺ and Cl⁻.
- But these charges are not written in the compound’s formula.
- Sodium chloride is written NaCl, not Na⁺Cl⁻.
NCERT Question 3 — You want to design a
You want to design a new ionic compound, where the total positive charge is 6+ and the total negative charge is 6−. Which of the following combinations gives the correct number of ions? (i) 2 Al³⁺ and 3 Cl⁻ (ii) 3 Mg²⁺ and 1 PO₄³⁻ (iii) 2 Fe³⁺ and 3 O²⁻ (iv) 3 Ca²⁺ and 2 SO₄²⁻
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NCERT Question 5 — Write the chemical formulae for
Write the chemical formulae for the following compounds. (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide
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NCERT Question 6 — Write the formulae of the
Write the formulae of the compounds formed from the following pairs of ions. (i) Ca²⁺ and Br⁻ (ii) Al³⁺ and CO₃²⁻ (iii) K⁺ and SO₄²⁻ (iv) NH₄⁺ and Cl⁻
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NCERT Question 9 — Write the formulae for the
Write the formulae for the compounds formed by the reaction of: (i) Magnesium and nitrogen (ii) Lithium and nitrogen (iii) Sodium and sulfur (iv) Aluminium and oxygen
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NCERT Question 10 — Complete the Table 9.3 by
Complete the Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top. LiNO₃ is given as an example.
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| NO₃⁻ | SO₄²⁻ | PO₄³⁻ | |
|---|---|---|---|
| NH₄⁺ | |||
| Li⁺ | LiNO₃ | ||
| Al³⁺ | |||
| Cu²⁺ |
⏸▶ Pause and Ponder
- 16. Name the following: (i) CO₂ → carbon dioxide. (ii) NO₂ → nitrogen dioxide. (iii) SF₆ → sulfur hexafluoride. (iv) PCl₃ → phosphorus trichloride.
- 17. Write the formula for the following: (i) Sodium hydrogencarbonate → NaHCO₃ . (ii) Sulfur dioxide → SO₂ . (iii) Ferric chloride → FeCl₃ . (iv) Cuprous oxide → Cu₂O .
- 18. Write the formulae for the compounds formed from the following pairs of ions: (i) Fe³⁺ and OH⁻ → criss-cross 3 and 1 → Fe(OH)₃ . (ii) K⁺ and CO₃²⁻ → criss-cross 1 and 2 → K₂CO₃ .
✅ Test Yourself
-
What is the first step in writing a chemical formula?
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Write the symbols of the constituent elements (cation first for ionic). -
Write the formula of calcium chloride by criss-crossing.
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Ca is 2+, Cl is 1− → CaCl₂ . -
Why is Mg₂O₂ written simply as MgO?
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The subscripts are divided by the common factor 2. -
When are brackets used in a formula?
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When two or more polyatomic ions of the same type are present, e.g. Mg(OH)₂.
Important Definitions
- Formula unit — the simplest whole-number ratio of ions in an ionic compound.
- Brackets in a formula — used when two or more of the same polyatomic ion are present, e.g. Mg(OH)₂.
