What is the trend in atomic radius across Period 3 of the Periodic Table?

In Period 3, the atomic radius decreases from left to right across the period. This is because as you move across the period, the number of protons in the nucleus increases, leading to a greater nuclear charge. This increased attraction pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.

How does the electronegativity change across Period 3 elements?

Electronegativity increases across Period 3 from left to right. This is due to the increasing nuclear charge, which attracts the bonding pair of electrons more strongly. Elements like sodium have lower electronegativity, while chlorine has a higher electronegativity, reflecting its strong ability to attract electrons.

Why do the melting and boiling points vary across Period 3?

The melting and boiling points of Period 3 elements vary due to differences in their structures and bonding. Metals like sodium, magnesium, and aluminum have metallic bonding, which is strong and leads to higher melting and boiling points. Silicon, a metalloid, has a giant covalent structure, resulting in very high melting and boiling points. Non-metals like phosphorus, sulfur, and chlorine have molecular structures with weaker van der Waals forces, leading to lower melting and boiling points.

What is the trend in ionization energy across Period 3?

Ionization energy generally increases across Period 3. This is because the increasing nuclear charge across the period results in a stronger attraction between the nucleus and the outer electrons, making it more difficult to remove an electron. However, there are slight deviations due to electron configurations, such as the drop between magnesium and aluminum, and phosphorus and sulfur.

How do the chemical properties of Period 3 elements change across the period?

The chemical properties of Period 3 elements change from metallic to non-metallic as you move from left to right. Metals like sodium and magnesium react by losing electrons, while non-metals like chlorine gain electrons during reactions. This change is due to the increasing electronegativity and ionization energy across the period, which influences how these elements interact with other substances.

Why is "Wrong hydrolysis products for SiCl₄/PCl₅." a common mistake in Periodicity of chemical properties of the elements in Period 3?

Why it happens: Guessing the products. How to avoid it: SiCl₄ → SiO₂ + 4HCl; PCl₅ → H₃PO₄ + 5HCl; AlCl₃ → Al(OH)₃ + 3HCl. Source: 9701 Examiner Reports 2022-2024

Why is "Showing only one reaction to prove amphoteric behaviour." a common mistake in Periodicity of chemical properties of the elements in Period 3?

Why it happens: Forgetting it must do both. How to avoid it: Give reactions with both an acid and a base.

Why is "Saying NaCl solution is acidic." a common mistake in Periodicity of chemical properties of the elements in Period 3?

Why it happens: Treating it like the covalent chlorides. How to avoid it: NaCl is ionic and dissolves to give a neutral (pH 7) solution.

Why is "Assuming every oxide changes the pH of water." a common mistake in Periodicity of chemical properties of the elements in Period 3?

Why it happens: Expecting all to react. How to avoid it: SiO₂ and Al₂O₃ are insoluble — they are acidic/amphoteric but don't dissolve to change pH.

The Periodic Table: chemical periodicity(AS-Level Inorganic Chemistry)Cambridge International A Level Chemistry 9701

Periodicity of chemical properties of the elements in Period 3

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Periodicity of Chemical Properties of Period 3 — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

Reactions of the Period 3 elements with oxygen, chlorine and water; the acid–base behaviour of their oxides and hydroxides (including amphoteric aluminium); and the hydrolysis of the chlorides — all explained by the change from ionic to covalent bonding across the period.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

9.2.1 — Describe, and write equations for, the reactions of the elements with oxygen, chlorine and water (Na and Mg only).
9.2.2 — State and explain the variation in oxidation number of the oxides and chlorides in terms of outer-shell electrons.
9.2.3 — Describe, and write equations for, the reactions of the oxides with water and the likely pHs.
9.2.4 — Describe, explain and write equations for the acid/base behaviour of the oxides and hydroxides, including amphoteric behaviour (NaOH only).
9.2.5 — Describe, explain and write equations for the reactions of the chlorides with water and the likely pHs.
9.2.6–9.2.7 — Explain the trends in terms of bonding and electronegativity, and suggest the type of bonding present.

Reactions of the elements with oxygen and chlorine

Each element combines with O₂ and Cl₂ in its characteristic ratio.

With oxygen:

$4\text{Na} + \text{O}_2 \rightarrow 2\text{Na}_2\text{O}$
$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$ (brilliant white flame)
$4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3$
$\text{Si} + \text{O}_2 \rightarrow \text{SiO}_2$
$\text{P}_4 + 5\text{O}_2 \rightarrow \text{P}_4\text{O}_{10}$
$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$

With chlorine:

$2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}$
$\text{Mg} + \text{Cl}_2 \rightarrow \text{MgCl}_2$
$2\text{Al} + 3\text{Cl}_2 \rightarrow 2\text{AlCl}_3$
$\text{Si} + 2\text{Cl}_2 \rightarrow \text{SiCl}_4$
$2\text{P} + 5\text{Cl}_2 \rightarrow 2\text{PCl}_5$

With water (Na and Mg only):

$2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$ (vigorous; floats and fizzes).
$\text{Mg} + 2\text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2$ (very slow with cold water; with steam: $\text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2$ ).

Oxides: Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂.
Chlorides: NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅.
Water: Na vigorous (NaOH); Mg slow / steam → MgO.

Oxidation numbers of the oxides and chlorides

The element's oxidation number rises across the period, matching its number of outer electrons.

The oxidation number of the Period 3 element in its highest oxide/chloride increases across the period, because it uses all of its outer-shell electrons in bonding:

Element	Outer e⁻	Oxide	Ox. no.	Chloride	Ox. no.
Na	1	Na₂O	+1	NaCl	+1
Mg	2	MgO	+2	MgCl₂	+2
Al	3	Al₂O₃	+3	AlCl₃	+3
Si	4	SiO₂	+4	SiCl₄	+4
P	5	P₄O₁₀	+5	PCl₅	+5
S	6	SO₃	+6	—	—

So the maximum oxidation number = the group number / number of outer-shell electrons.

Ox. number rises +1 → +6 across the period.
Equals the number of outer-shell electrons used.

Oxides with water and their pH

Metal oxides → alkaline; Al₂O₃/SiO₂ insoluble (≈7); non-metal oxides → acidic.

Oxide	Reaction with water	Solution pH
Na₂O	$\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}$	~13–14 (strongly alkaline)
MgO	$\text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2$ (sparingly soluble)	~9–10 (weakly alkaline)
Al₂O₃	insoluble — no reaction	~7
SiO₂	insoluble — no reaction	~7
P₄O₁₀	$\text{P}_4\text{O}_{10} + 6\text{H}_2\text{O} \rightarrow 4\text{H}_3\text{PO}_4$	~1–2 (acidic)
SO₂	$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3$	~2–3 (acidic)
SO₃	$\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$	~0–1 (strongly acidic)

The pattern: basic (Na₂O, MgO) → amphoteric/insoluble (Al₂O₃, SiO₂) → acidic (P₄O₁₀, SO₂, SO₃) as you cross the period.

Na₂O strongly alkaline; MgO weakly alkaline.
Al₂O₃ and SiO₂ insoluble (pH ~7).
P₄O₁₀, SO₂, SO₃ acidic.

Acid–base behaviour and amphoteric aluminium

Basic oxides react with acids; acidic oxides with bases; Al₂O₃ and Al(OH)₃ do both.

Basic oxides/hydroxides (Na₂O, MgO, NaOH, Mg(OH)₂) react with acids: $\text{MgO} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O}$

Acidic oxides (P₄O₁₀, SO₂, SO₃) react with bases: $\text{SO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_3 + \text{H}_2\text{O}$

Amphoteric — Al₂O₃ and Al(OH)₃ react with both acids and bases:

With acid: $\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$
With base (NaOH): $\text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4$ (sodium aluminate)
$\text{Al(OH)}_3 + 3\text{HCl} \rightarrow \text{AlCl}_3 + 3\text{H}_2\text{O}$ and $\text{Al(OH)}_3 + \text{NaOH} \rightarrow \text{NaAl(OH)}_4$

Hydroxide summary: NaOH (strong base) → Mg(OH)₂ (weak base, sparingly soluble) → Al(OH)₃ (amphoteric).

Basic oxides + acid → salt + water.
Acidic oxides + base → salt + water.
Al₂O₃ / Al(OH)₃ amphoteric (react with both).

Chlorides with water and the bonding trend

Ionic NaCl just dissolves (neutral); covalent chlorides hydrolyse to acidic solutions, fuming HCl.

Chloride	Reaction with water	pH
NaCl	dissolves (ionic) — just dissociates	~7 (neutral)
MgCl₂	dissolves, slight hydrolysis	~6 (slightly acidic)
AlCl₃	$\text{AlCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + 3\text{HCl}$	~3 (acidic, HCl fumes)
SiCl₄	$\text{SiCl}_4 + 2\text{H}_2\text{O} \rightarrow \text{SiO}_2 + 4\text{HCl}$	~2 (acidic, fumes)
PCl₅	$\text{PCl}_5 + 4\text{H}_2\text{O} \rightarrow \text{H}_3\text{PO}_4 + 5\text{HCl}$	~2 (acidic, fumes)

Explaining the trend (bonding and electronegativity):

NaCl, MgCl₂ are ionic — they simply dissolve (NaCl gives a neutral solution).
Across the period the electronegativity difference falls, so chlorides become increasingly covalent (AlCl₃ has covalent character; SiCl₄, PCl₅ are simple covalent molecules).
Covalent chlorides hydrolyse: water attacks them, releasing HCl (steamy fumes) and giving an acidic solution.

NaCl neutral (ionic, dissolves); MgCl₂ slightly acidic.
AlCl₃, SiCl₄, PCl₅ hydrolyse → acidic + HCl fumes.
Ionic → covalent across the period drives the change.

How it’s examined

This is one of the most equation-heavy AS topics — Paper 2 demands correct equations for oxides/chlorides with water, the amphoteric reactions of aluminium, and the pH trend explained by bonding. Examiner reports flag wrong hydrolysis products, forgetting HCl fumes, and not linking the chloride trend to ionic→covalent bonding.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 9.2; 9701 Examiner Reports 2022-2024; 9701/22 & 9701/42 May/Jun 2024 question papers and mark schemes. Last reviewed 2026-05-20.

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Step-by-step worked examples — Periodicity of chemical properties of the elements in Period 3

Step-by-step solutions to past-paper-style questions on periodicity of chemical properties of the elements in period 3, written exactly the way a tutor would explain them at the board.

1Classifying the Period 3 oxides (2 marks)
Getting started• oxides
Question
Classify Na₂O, Al₂O₃, SiO₂ and SO₃ as basic, amphoteric or acidic.
Step-by-step solution
1. Step 1
  Metal oxides on the left are basic: Na₂O.
2. Step 2
  Al₂O₃ is amphoteric (the crossover point).
3. Step 3
  Non-metal oxides on the right are acidic: SiO₂, SO₃.
Answer
Na₂O basic; Al₂O₃ amphoteric; SiO₂ and SO₃ acidic.
Examiner tip
Acid–base character shifts basic → amphoteric → acidic across the period (metal → non-metal).
2Elements with oxygen and chlorine (2 marks)
Getting started• equations
Question
Write balanced equations for (a) sodium burning in oxygen and (b) aluminium reacting with chlorine.
Step-by-step solution
1. Step 1
  (a) Sodium oxide.
  $4\text{Na} + \text{O}_2 \rightarrow 2\text{Na}_2\text{O}$
2. Step 2
  (b) Aluminium chloride.
  $2\text{Al} + 3\text{Cl}_2 \rightarrow 2\text{AlCl}_3$
Answer
4Na + O₂ → 2Na₂O; 2Al + 3Cl₂ → 2AlCl₃.
3Oxides with water (3 marks)
Building confidence• oxides
Question
Write equations for the reactions of Na₂O and SO₃ with water and state the approximate pH of each solution.
Step-by-step solution
1. Step 1
  Na₂O (basic oxide).
  $\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}\quad(\text{pH} \approx 14)$
2. Step 2
  SO₃ (acidic oxide).
  $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4\quad(\text{pH} \approx 0\text{–}1)$
Answer
Na₂O → NaOH (pH ~14); SO₃ → H₂SO₄ (pH ~0–1).
Examiner tip
Note SiO₂ and Al₂O₃ are insoluble, so they don't change the pH of water despite their acid/base nature.
4Amphoteric aluminium oxide (4 marks)
Building confidence• amphoteric
Question
Explain what is meant by amphoteric, and write equations showing Al₂O₃ reacting with HCl and with NaOH.
Step-by-step solution
1. Step 1
  Amphoteric: reacts with both acids and bases.
2. Step 2
  With acid:
  $\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$
3. Step 3
  With base (NaOH):
  $\text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4$
Answer
Amphoteric = reacts with both. Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄.
Examiner tip
Both equations are needed to demonstrate amphoteric behaviour; the aluminate is NaAl(OH)₄.
5Chloride hydrolysis trend (4 marks)
Stretch• chlorides
Question
Explain why NaCl dissolves to give a neutral solution but SiCl₄ reacts with water to give an acidic one. Include an equation for SiCl₄.
Step-by-step solution
1. Step 1
  NaCl is ionic — it simply dissociates into Na⁺ and Cl⁻ (neutral, pH 7).
2. Step 2
  SiCl₄ is a covalent molecule that is hydrolysed by water.
3. Step 3
  Equation:
  $\text{SiCl}_4 + 2\text{H}_2\text{O} \rightarrow \text{SiO}_2 + 4\text{HCl}$
4. Step 4
  HCl produced makes the solution acidic (pH ~2).
Answer
Ionic NaCl just dissolves (neutral); covalent SiCl₄ hydrolyses to give HCl (acidic).
6Acid–base trend of the oxides (4 marks)
Stretch• oxides, trend
Question
Explain how the acid–base character of the Period 3 oxides changes from Na₂O to SO₃, linking it to bonding. (4)
Step-by-step solution
1. Step 1
  Left (Na₂O, MgO): ionic, basic — they react with acids and give alkaline solutions.
2. Step 2
  Middle (Al₂O₃): amphoteric — bonding has both ionic and covalent character.
3. Step 3
  Right (SiO₂, P₄O₁₀, SO₃): covalent, acidic — they react with bases / give acidic solutions.
4. Step 4
  Overall: basic → amphoteric → acidic, mirroring the metal → non-metal change across the period.
Answer
Basic (ionic metal oxides) → amphoteric (Al₂O₃) → acidic (covalent non-metal oxides) across the period.
Examiner tip
Tie the acid–base trend to the ionic→covalent (metal→non-metal) change in bonding.

Key Formulae — Periodicity of chemical properties of the elements in Period 3

The formulae you need to memorise for periodicity of chemical properties of the elements in period 3 on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

Highest oxidation state = group number
$\text{ON in highest oxide} = \text{group number}$
$ON$
oxidation number of the Period 3 element
When to use
Predicting highest-oxide formulae: Na₂O (+1), MgO (+2), Al₂O₃ (+3), SiO₂ (+4), P₄O₁₀ (+5), SO₃ (+6), Cl₂O₇ (+7).

Key Definitions and Keywords — Periodicity of chemical properties of the elements in Period 3

Definitions to memorise and the exact keywords mark schemes credit for periodicity of chemical properties of the elements in period 3 answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Amphoteric
Examiner keyword
Able to react with both acids and bases (e.g. Al₂O₃ and Al(OH)₃).
Basic / acidic oxide
Examiner keyword
Basic oxides (metal oxides) react with acids; acidic oxides (non-metal oxides) react with bases.
Hydrolysis (of a chloride)
Examiner keyword
The reaction of a covalent chloride with water to produce an acidic solution and HCl fumes.
Aluminate ion
Examiner keyword
The Al(OH)₄⁻ ion formed when Al₂O₃ / Al(OH)₃ reacts with alkali (shows amphoteric behaviour).

Common Mistakes and Misconceptions — Periodicity of chemical properties of the elements in Period 3

The traps other students keep falling into on periodicity of chemical properties of the elements in period 3 questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Wrong hydrolysis products for SiCl₄/PCl₅.
9701 Examiner Reports 2022-2024
Why it happens
Guessing the products.
How to avoid it
SiCl₄ → SiO₂ + 4HCl; PCl₅ → H₃PO₄ + 5HCl; AlCl₃ → Al(OH)₃ + 3HCl.
✕Showing only one reaction to prove amphoteric behaviour.
Why it happens
Forgetting it must do both.
How to avoid it
Give reactions with both an acid and a base.
✕Saying NaCl solution is acidic.
Why it happens
Treating it like the covalent chlorides.
How to avoid it
NaCl is ionic and dissolves to give a neutral (pH 7) solution.
✕Assuming every oxide changes the pH of water.
Why it happens
Expecting all to react.
How to avoid it
SiO₂ and Al₂O₃ are insoluble — they are acidic/amphoteric but don't dissolve to change pH.

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