Explain Period 3 chemical trends with bonding and electronegativity
Connect oxidation numbers, oxide acid/base behavior and chloride hydrolysis to changing bonding and electronegativity.
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 broad oxide acid/base trend occurs across Period 3?
What you will learn
- Explain oxide trends across Period 3
- Explain chloride behavior from bonding and electronegativity
- Use charge density and covalent character appropriately
Oxides change from basic to acidic across the period
Across Period 3, electronegativity generally increases and the difference between the element and oxygen tends to decrease. Bonding changes from predominantly ionic in Na₂O and MgO, through amphoteric Al₂O₃, to covalent non-metal oxides such as SiO₂, P₄O₁₀, SO₂ and SO₃.
The ionic metal oxides contain oxide ions that react with H⁺, so they are basic. Covalent non-metal oxides form acidic solutions or react with bases. Al₂O₃ is amphoteric and reacts with both acid and aqueous sodium hydroxide. The change is a trend, not a sharp universal boundary between all compounds.
Chloride hydrolysis reflects bonding and cation polarisation
NaCl and MgCl₂ are modelled mainly as ionic chlorides; their aqueous behavior involves hydrated ions. Al³⁺ has higher charge density than Mg²⁺ and hydrolyses coordinated water more strongly, making AlCl₃(aq) more acidic at comparable concentration.
SiCl₄ and PCl₅ are covalent chlorides that react with water by hydrolysis, producing acidic products. Increasing electronegativity across the period and changing element–chlorine bonding help explain the change from ionic salt dissolution to covalent chloride hydrolysis. Use observations, equations and conditions together rather than inferring a full structure from a single pH reading.
Pause and explain
Why does SO₃ form an acidic solution?
Worked example
Explain why Na₂O is basic and SO₃ is acidic, linking the answer to bonding and water behavior.
Show the worked solution
- Na₂O is predominantly ionic and contains oxide ions.
- Oxide ions react with water to form NaOH and with acids to form salt and water.
- SO₃ is a covalent non-metal oxide that forms H₂SO₄ in water.
Answer Na₂O contains basic oxide ions and forms alkaline NaOH in water; covalent SO₃ forms sulfuric acid, so it is an acidic oxide.
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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.
Explain the change from basic Na₂O/MgO to acidic SO₂/SO₃ using bonding and reaction behavior.
Give me a hint
Compare the ionic metal oxides with the covalent non-metal oxides.
Compare my reasoning
- Na₂O and MgO are predominantly ionic metal oxides.
- Their oxide ions neutralise acid; Na₂O also forms hydroxide in water.
- SO₂ and SO₃ are covalent non-metal oxides that form acidic solutions.
The ionic metal oxides are basic, whereas covalent SO₂ and SO₃ form acidic solutions; the trend reflects increasing non-metal character and electronegativity across Period 3.
Look for these in your work
- Bonding contrast made
- Acid/base behavior linked
- Trend explained
At the same concentration, explain why AlCl₃(aq) is more acidic than MgCl₂(aq).
Give me a hint
Compare the polarising power of hydrated 3+ and 2+ cations.
Compare my reasoning
- Both hydrated cations can hydrolyse water.
- Al³⁺ has greater charge density than Mg²⁺.
- It polarises coordinated water more strongly and produces more H₃O⁺.
Al³⁺ has higher charge density and hydrolyses water more strongly, so AlCl₃(aq) is more acidic than MgCl₂(aq) at comparable concentration.
Look for these in your work
- Both cations considered
- Charge density compared
- Hydrolysis linked to acidity
A student infers that an oxide is covalent solely because its aqueous mixture is acidic. Explain what additional evidence and reasoning are needed.
Give me a hint
Use bonding, electronegativity, reaction equations and physical observations together.
Compare my reasoning
- Acidic behavior is evidence of an acidic oxide but is not a complete structure determination.
- Compare metal/non-metal identity and electronegativity with its reaction products.
- Use physical properties such as melting point or conductivity where suitable, while stating conditions.
Acidity alone does not establish the full bonding type. Combine reaction behavior with periodic position, electronegativity and relevant physical evidence.
Look for these in your work
- Inference limit stated
- Relevant periodic reasoning used
- Multiple evidence types identified
Common mistakes
- Saying electronegativity alone determines every product
- Treating Al₂O₃ as only acidic or only basic
- Assuming ionic/covalent labels can be inferred from pH alone
- Ignoring that concentration affects pH
What can you explain now?
What broad oxide acid/base trend occurs across Period 3?
Compare with the explanation
Basic metal oxides, amphoteric Al₂O₃, then acidic non-metal oxides
The trend follows changing bonding and element electronegativity.
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Make it stick
How do electronegativity, bonding and water reactions combine to explain Period 3 chemical trends?
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.