Write Period 3 reactions with oxygen and chlorine
Predict and balance the specified Period 3 oxide and chloride products.
Section 9 · The Periodic Table: chemical periodicity
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Period 3 trends are qualitative unless supplied data are given. Equations show the stated products and conditions; observations and pH depend on concentration and experimental conditions. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, balanced equations and ionic charges.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the formula of elemental chlorine in a reaction equation?
What you will learn
- Write balanced oxide-forming equations
- Write balanced chloride-forming equations
- Respect the products and reagent conditions specified by the syllabus
Use the syllabus product set for reactions with oxygen
The specified Period 3 products are Na₂O, MgO, Al₂O₃, P₄O₁₀ and SO₂. Balanced equations are: 4Na + O₂ → 2Na₂O; 2Mg + O₂ → 2MgO; 4Al + 3O₂ → 2Al₂O₃; P₄ + 5O₂ → P₄O₁₀; S + O₂ → SO₂.
For phosphorus, use the P₄ molecular form shown. The sulfur product in this syllabus outcome is SO₂. In actual burning, product distributions can depend on oxygen supply; use the stated target product when constructing the examination equation.
Write the specified chlorides from their elements
The products listed are NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅. Balanced equations include: 2Na + Cl₂ → 2NaCl; Mg + Cl₂ → MgCl₂; 2Al + 3Cl₂ → 2AlCl₃; Si + 2Cl₂ → SiCl₄; P₄ + 10Cl₂ → 4PCl₅ (with excess chlorine for PCl₅).
Balance atoms using coefficients; never alter a product formula to force a balance. Check each element separately and use the stated product conditions where the element can form more than one chloride.
Pause and explain
How should P₄ + Cl₂ be balanced to give PCl₅?
Worked example
Write balanced equations for aluminium reacting with oxygen to form Al₂O₃ and with chlorine to form AlCl₃.
Show the worked solution
- For the oxide, balance Al and O atoms: 4Al + 3O₂ → 2Al₂O₃.
- For the chloride, balance Al and Cl atoms: 2Al + 3Cl₂ → 2AlCl₃.
- Check each element on both sides.
Answer 4Al + 3O₂ → 2Al₂O₃; 2Al + 3Cl₂ → 2AlCl₃.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Balance the equation for sodium reacting with oxygen to form Na₂O.
Give me a hint
Start with the oxygen molecule, then match sodium atoms.
Compare my reasoning
- O₂ contains two O atoms, so form 2Na₂O.
- That product contains four Na atoms.
- Use 4Na + O₂ → 2Na₂O.
4Na + O₂ → 2Na₂O.
Look for these in your work
- Specified product retained
- Na atoms balanced
- O atoms balanced
Write the balanced equation for P₄ reacting with excess chlorine to form PCl₅.
Give me a hint
Keep the P₄ molecule and count chlorine atoms on four PCl₅ units.
Compare my reasoning
- Four PCl₅ units contain four P and 20 Cl atoms.
- Ten Cl₂ molecules supply 20 Cl atoms.
- P₄ + 10Cl₂ → 4PCl₅.
P₄ + 10Cl₂ → 4PCl₅.
Look for these in your work
- Molecular phosphorus written
- Chlorine coefficient correct
- Atoms checked
A learner writes S + 3/2 O₂ → SO₃ for the syllabus reaction. Correct the target and equation, then state one condition that can affect real sulfur oxide products.
Give me a hint
Use the specified product for this outcome and integer coefficients.
Compare my reasoning
- The syllabus specifies SO₂ for sulfur burning.
- Write S + O₂ → SO₂.
- Oxygen supply can affect the products formed in real combustion.
Use S + O₂ → SO₂ for the specified product. In actual combustion, oxygen supply can influence whether more highly oxidised sulfur products form.
Look for these in your work
- Product corrected
- Equation balanced with integers
- Condition acknowledged
Common mistakes
- Writing PCl₃ when PCl₅ is specified
- Using sulfur trioxide instead of the specified sulfur dioxide product
- Changing subscripts instead of balancing coefficients
- Forgetting that elemental chlorine is Cl₂
What can you explain now?
What is the formula of elemental chlorine in a reaction equation?
Compare with the explanation
Cl₂
Chlorine is a diatomic element in its standard molecular form.
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Make it stick
How do you keep product formulas fixed while balancing a Period 3 element reaction?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.