Writing chemical formulae · रासायनिक सूत्र लिखना
Cross the valencies over — then simplify and add brackets.
Writing chemical formulae games
The criss-cross method
- Write the symbols: positive ion first (Al), negative ion second (O).
- Write their valencies: Al 3, O 2.
- Cross the valencies over as subscripts: Al₂O₃.
- Simplify if both numbers share a factor (Ca₂O₂ → CaO). Don't write 1.
- Put a polyatomic ion in brackets when it is taken more than once: Ca(OH)₂, (NH₄)₂SO₄, Al₂(SO₄)₃.
| Positive ion | Symbol | Valency |
|---|---|---|
| Sodium | Na | 1 |
| Potassium | K | 1 |
| Calcium | Ca | 2 |
| Magnesium | Mg | 2 |
| Aluminium | Al | 3 |
| Ammonium | NH₄ | 1 |
| Copper(II) | Cu | 2 |
| Zinc | Zn | 2 |
| Iron(III) | Fe | 3 |
| Hydrogen | H | 1 |
| Negative ion | Symbol | Valency |
|---|---|---|
| chloride | Cl | 1 |
| oxide | O | 2 |
| sulphate | SO₄ | 2 |
| nitrate | NO₃ | 1 |
| carbonate | CO₃ | 2 |
| hydroxide | OH | 1 |
| phosphate | PO₄ | 3 |
| sulphide | S | 2 |
Goes with NCERT Exploration Class 9 (2026–27), Ch 9 “Atomic Foundations of Matter” (formula writing). Every answer is made and checked by a program (total positive charge = total negative charge). See also the periodic table with valency.
A one-page chart to stick on the wall, and a worksheet to practise.
Valency table
Criss-cross method
- Write the symbols: positive ion first (Al), negative ion second (O).
- Write their valencies: Al 3, O 2.
- Cross the valencies over as subscripts: Al₂O₃.
- Simplify if both numbers share a factor (Ca₂O₂ → CaO). Don't write 1.
- Put a polyatomic ion in brackets when it is taken more than once: Ca(OH)₂, (NH₄)₂SO₄, Al₂(SO₄)₃.
| Positive ion | Symbol | Valency |
|---|---|---|
| Sodium | Na | 1 |
| Potassium | K | 1 |
| Calcium | Ca | 2 |
| Magnesium | Mg | 2 |
| Aluminium | Al | 3 |
| Ammonium | NH₄ | 1 |
| Copper(II) | Cu | 2 |
| Zinc | Zn | 2 |
| Iron(III) | Fe | 3 |
| Hydrogen | H | 1 |
| Negative ion | Symbol | Valency |
|---|---|---|
| chloride | Cl | 1 |
| oxide | O | 2 |
| sulphate | SO₄ | 2 |
| nitrate | NO₃ | 1 |
| carbonate | CO₃ | 2 |
| hydroxide | OH | 1 |
| phosphate | PO₄ | 3 |
| sulphide | S | 2 |
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Chemical formulae — worksheet
Name: ____________________ Class: ______ Date: __________
1. Write the formula: magnesium chloride ______ aluminium oxide ______ calcium carbonate ______
2. Write the formula: ammonium sulphate ______ sodium phosphate ______ zinc hydroxide ______
3. Name these: KNO₃ ______ CaO ______ CuSO₄ ______
4. Why is the formula of calcium oxide CaO and not Ca₂O₂? ______________________
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