Track oxidation numbers across Period 3 compounds
Relate outer-shell electron counts to oxidation numbers in the specified Period 3 oxides and chlorides.
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 oxidation number is usually assigned to oxygen in these Period 3 oxides?
What you will learn
- Calculate oxidation numbers in specified oxides and chlorides
- Describe the progression across the period
- Connect common oxidation numbers to outer-shell electrons
Oxides show increasing positive oxidation numbers
For the specified oxides, Na₂O has Na +1; MgO has Mg +2; Al₂O₃ has Al +3; P₄O₁₀ has P +5; SO₂ has S +4; and SO₃ has S +6. Oxygen is assigned −2 in these compounds, and the oxidation numbers sum to zero for each neutral formula.
The sequence reflects increasing numbers of valence electrons across Period 3. Some non-metals use more than one positive oxidation number: sulfur is +4 in SO₂ and +6 in SO₃. Do not infer that every oxidation number must rise by exactly one from every adjacent compound.
Chloride oxidation numbers follow valence counts
The specified chlorides have Na +1 in NaCl, Mg +2 in MgCl₂, Al +3 in AlCl₃, Si +4 in SiCl₄ and P +5 in PCl₅. Chlorine is −1 in these compounds, so charge balance gives the central element's oxidation number.
This is an oxidation-number pattern, not a claim that every bond is ionic. For covalent chlorides, oxidation numbers are formal bookkeeping values based on assigning bonding electrons to the more electronegative atom.
Pause and explain
What is silicon's oxidation number in SiCl₄?
Worked example
Find the oxidation numbers of sulfur in SO₂ and SO₃ and explain why they differ.
Show the worked solution
- Each O is −2.
- Neutral SO₂ gives S + 2(−2)=0, so S=+4.
- Neutral SO₃ gives S + 3(−2)=0, so S=+6.
Answer Sulfur is +4 in SO₂ and +6 in SO₃ because the neutral formulas contain two and three oxygen atoms respectively.
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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.
Calculate the oxidation number of phosphorus in P₄O₁₀.
Give me a hint
Use oxygen at −2 and a zero total for the neutral molecule.
Compare my reasoning
- Ten oxygen atoms contribute −20.
- The four P atoms together contribute +20.
- Each P has oxidation number +5.
Phosphorus is +5 in P₄O₁₀.
Look for these in your work
- Oxygen contribution calculated
- Four phosphorus atoms included
- Oxidation number found
Give the central-element oxidation numbers in MgCl₂, AlCl₃ and PCl₅.
Give me a hint
Assign each chlorine −1 and balance the neutral formula.
Compare my reasoning
- MgCl₂ requires Mg +2.
- AlCl₃ requires Al +3.
- PCl₅ requires P +5.
Mg = +2, Al = +3 and P = +5.
Look for these in your work
- All three values correct
- Chlorine subscript included
- Neutral totals checked
A learner claims that because Al is +3 in AlCl₃, the compound must be entirely ionic. Explain why that conclusion is not valid.
Give me a hint
An oxidation number is a formal assignment, not a direct structure measurement.
Compare my reasoning
- Oxidation numbers assign bonding electrons for accounting.
- They can be used for covalent as well as ionic compounds.
- Bonding type needs evidence about structure, properties and electronegativity.
The +3 oxidation number is a formal bookkeeping value and does not by itself prove ionic bonding; use structural and physical evidence.
Look for these in your work
- Formal nature explained
- Ionic inference rejected
- Appropriate evidence named
Common mistakes
- Forgetting to multiply oxygen's oxidation number by its subscript
- Using the chloride ion charge for the whole chloride molecule
- Assuming an oxidation number proves an ionic structure
- Saying sulfur has only one possible oxidation number
What can you explain now?
What oxidation number is usually assigned to oxygen in these Period 3 oxides?
Compare with the explanation
−2
Use oxygen at −2 and set the neutral compound total to zero.
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Make it stick
How do formula subscripts and oxidation-number rules reveal the Period 3 valence progression?
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.