Writing formulae using valency: Class 9 practice
Ions combine so the total positive and negative charges balance. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Common ions and valencies
The idea behind writing formulae using valency
Ions combine so the total positive and negative charges balance. Criss-cross the valencies and simplify: Al³⁺ and O²⁻ give Al₂O₃. Polyatomic ions keep brackets when more than one is needed.
Step-by-step method
- Write each ion with its valency.
- Criss-cross the valency numbers as subscripts.
- Simplify the ratio; use brackets for polyatomic ions.
Worked example 1 (Basic)
Write the formula of the compound formed by Al (valency 3) and Cl (valency 1).
- Criss-cross: Al gets 1, Cl gets 3
- Already simplest
- Formula: AlCl₃
- Answer: AlCl₃
Worked example 2 (Standard)
Write the formula of the compound formed by Al (valency 3) and CO₃ (valency 2).
- Criss-cross: Al gets 2, CO₃ gets 3
- Already simplest
- Formula: Al₂(CO₃)₃
- Answer: Al₂(CO₃)₃
Worked example 3 (Challenge)
Write the formula of the compound formed by Na (valency 1) and NO₃ (valency 1).
- Criss-cross: Na gets 1, NO₃ gets 1
- Already simplest
- Formula: NaNO₃
- Answer: NaNO₃
Common mistakes and misconceptions
- Not simplifying (Ca₂O₂)
- Dropping brackets: CaOH₂ instead of Ca(OH)₂
Practice questions
- (Basic) Write the formula of the compound formed by Na (valency 1) and S (valency 2).
- (Basic) Write the formula of the compound formed by Al (valency 3) and O (valency 2).
- (Standard) Write the formula of the compound formed by Na (valency 1) and SO₄ (valency 2).
- (Standard) Write the formula of the compound formed by Al (valency 3) and SO₄ (valency 2).
- (Challenge) Write the formula of the compound formed by Fe (valency 3) and Cl (valency 1).
- (Challenge) Write the formula of the compound formed by Ca (valency 2) and Cl (valency 1).
Answer key with full working
- Q1: Criss-cross: Na gets 2, S gets 1 → Already simplest → Formula: Na₂S → Answer: Na₂S
- Q2: Criss-cross: Al gets 2, O gets 3 → Already simplest → Formula: Al₂O₃ → Answer: Al₂O₃
- Q3: Criss-cross: Na gets 2, SO₄ gets 1 → Already simplest → Formula: Na₂SO₄ → Answer: Na₂SO₄
- Q4: Criss-cross: Al gets 2, SO₄ gets 3 → Already simplest → Formula: Al₂(SO₄)₃ → Answer: Al₂(SO₄)₃
- Q5: Criss-cross: Fe gets 1, Cl gets 3 → Already simplest → Formula: FeCl₃ → Answer: FeCl₃
- Q6: Criss-cross: Ca gets 1, Cl gets 2 → Already simplest → Formula: CaCl₂ → Answer: CaCl₂
Syllabus note
This skill is listed under "Atoms and Molecules" for Class 9 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is writing formulae using valency in?
"Atoms and Molecules", Class 9 Chemistry (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.