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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?
- 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
The charges on the ions are not shown in the formula of the compound.
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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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.
