9701 Chemistry · Topic 9 · AS Level
Chemical Periodicity Cheat Sheet — A Level Chemistry 9701
Periodicity is a memory topic with a logic underneath it — once you can explain why the melting point peaks at silicon and why aluminium oxide is amphoteric, the rest of Period 3 follows. These sheets set out every trend across the period along with the equations for the oxides, chlorides and hydroxides reacting with water, and the pH each produces.
✅ 28 worked sections
🎓 CAIE 9701 syllabus-mapped
⭐ 4.8 rated tutors
What’s on this cheat sheet
Chemical Periodicity — Period 3
01 · Atomic and ionic radius
Atomic radius falls across the period. Nuclear charge rises while electrons enter the same shell, so shielding is roughly constant and the outer electrons are pulled in more strongly.
Ionic radius drops sharply from Na⁺ to Si⁴⁺ (cations lose a shell), then jumps at P³⁻ and falls again to Cl⁻ (anions gain electrons and repel).
02 · Ionisation energy
Rises across the period — greater nuclear charge, smaller radius, similar shielding.
Two dips: Al < Mg because Al’s electron leaves a higher‑energy 3p; S < P because S has a paired 3p electron and pair repulsion makes it easier to remove.
03 · Melting point across period 3
| Element | Structure | m.p. |
|---|---|---|
| Na → Al | giant metallic | rises |
| Si | giant covalent | highest |
| P₄, S₈, Cl₂ | simple molecular | low |
| Ar | monatomic | lowest |
Na → Al rises because the cations carry more charge and are smaller, so more delocalised electrons and stronger metallic bonding. Among the molecular elements, S₈ > P₄ > Cl₂ because the larger molecule has stronger London forces.
04 · Reaction with oxygen
Na burns with a yellow flame → Na₂O. Mg burns with a brilliant white flame → MgO. Al forms a protective oxide layer. Si, P and S burn to SiO₂, P₄O₁₀ and SO₂.
Oxidation number of the element in its highest oxide equals its group number.
05 · Reaction with water
Na — vigorous with cold water, floats and fizzes: 2Na + 2H₂O → 2NaOH + H₂, pH ~13.
Mg — very slow in cold water (pH ~10), but rapid with steam: Mg + H₂O → MgO + H₂.
Al, Si — no reaction; Al is protected by its oxide layer.
06 · Oxides
| Oxide | Bonding | In water |
|---|---|---|
| Na₂O | ionic | pH 13, basic |
| MgO | ionic | pH 9, sparingly soluble |
| Al₂O₃ | ionic w/ covalent character | insoluble, amphoteric |
| SiO₂ | giant covalent | insoluble, weakly acidic |
| P₄O₁₀ | simple covalent | pH 2, acidic |
| SO₂ / SO₃ | simple covalent | pH 1–2, acidic |
Basic → amphoteric → acidic across the period, tracking the change from metallic to non‑metallic character.
07 · Amphoteric Al₂O₃
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
As an acid:
Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄
08 · Electronegativity across period 3
Rises from Na to Cl — the nucleus is more highly charged and the atom smaller, so the shared pair is attracted more strongly.
This drives the change in bonding: Δχ with oxygen or chlorine falls across the period, so the compounds shift from ionic to covalent.
09 · Equations to know
2Mg + O₂ → 2MgO
Si + O₂ → SiO₂
P₄ + 5O₂ → P₄O₁₀
S + O₂ → SO₂
Na₂O + H₂O → 2NaOH
MgO + H₂O → Mg(OH)₂
P₄O₁₀ + 6H₂O → 4H₃PO₄
SO₃ + H₂O → H₂SO₄
10 · Trends at a glance
| Property | Na → Ar |
|---|---|
| atomic radius | decreases |
| first IE | increases (dips at Al, S) |
| electronegativity | increases |
| melting point | peaks at Si |
| conductivity | high Na–Al, then none |
| oxide character | basic → amphoteric → acidic |
All of it follows from two things: rising nuclear charge across the period, and the change in structure from giant metallic through giant covalent to simple molecular.
11 · Worked example — explaining the melting points
Al³⁺ is smaller and carries three times the charge, and each atom releases three delocalised electrons, so the metallic bonding is much stronger.
Why does Si melt highest?
Giant covalent — melting breaks strong covalent bonds throughout the lattice.
Why is S₈ higher than P₄?
More electrons per molecule, so stronger London forces.
12 · Ionic radii across period 3
| Ion | Charge | Size |
|---|---|---|
| Na⁺, Mg²⁺, Al³⁺ | +1 → +3 | falls sharply |
| P³⁻, S²⁻, Cl⁻ | −3 → −1 | falls |
| anions vs cations | — | anions much larger |
Cations have lost their outer shell so are smaller than the parent atom; anions have gained electrons, so repulsion expands the shell. Across an isoelectronic series the radius falls as nuclear charge rises.
Marks lost here
— Saying shielding increases across a period.
— Explaining the Si melting point by London forces rather than a giant covalent network.
— Forgetting that MgO is only sparingly soluble, so its pH is ~9, not 13.
Chlorides, Hydroxides and Group Trends
13 · Chlorides of period 3
| Chloride | Bonding | With water |
|---|---|---|
| NaCl | ionic | dissolves, pH 7 |
| MgCl₂ | ionic | dissolves, pH ~6.5 |
| Al₂Cl₆ | covalent dimer | hydrolyses, pH ~3 |
| SiCl₄ | simple covalent | hydrolyses fully, pH ~2 |
| PCl₅ | simple covalent | hydrolyses fully, pH ~2 |
Hydrolysis produces steamy white fumes of HCl. Ionic chlorides simply dissolve; covalent ones react with water.
14 · Hydrolysis equations
PCl₅ + 4H₂O → H₃PO₄ + 5HCl
PCl₃ + 3H₂O → H₃PO₃ + 3HCl
[Al(H₂O)₆]³⁺ ⇌ [Al(H₂O)₅(OH)]²⁺ + H⁺
— the small, highly charged Al³⁺ polarises the water it hydrates, releasing H⁺ and giving an acidic solution.
15 · Hydroxides
NaOH — strongly basic, very soluble. Mg(OH)₂ — sparingly soluble, weakly basic. Al(OH)₃ — insoluble and amphoteric: dissolves in both acid and excess NaOH.
Al(OH)₃ + 3H⁺ → Al³⁺ + 3H₂O and Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻.
16 · Down a group
| Property | Trend ↓ |
|---|---|
| atomic radius | increases — more shells |
| ionisation energy | decreases |
| electronegativity | decreases |
| metallic character | increases |
Shielding and distance from the nucleus outweigh the rising nuclear charge.
17 · Periodicity of the elements’ oxidation states
Highest oxidation state = group number: Na +1, Mg +2, Al +3, Si +4, P +5, S +6, Cl +7.
Non‑metals also form negative states (P −3, S −2, Cl −1). Metals form only positive ones.
18 · Oxides with acid and alkali
MgO + 2HCl → MgCl₂ + H₂O
Acidic oxides + alkali
SiO₂ + 2NaOH → Na₂SiO₃ + H₂O
SO₂ + 2NaOH → Na₂SO₃ + H₂O
P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O
Amphoteric
Al₂O₃ reacts with both, as in block 09.
19 · Sulfur oxides and acid rain
SO₂ from burning sulfur‑containing fuels dissolves to give sulfurous acid, and oxidises in the air to SO₃, which gives sulfuric acid: SO₃ + H₂O → H₂SO₄.
The result damages limestone buildings, acidifies lakes and leaches nutrients from soil. Flue gases are scrubbed with CaO or CaCO₃: CaO + SO₂ → CaSO₃.
20 · Predicting an unfamiliar element
Use position rather than memory. An element in Group 2 of period 4 will have a larger radius and lower first ionisation energy than Ca’s neighbours above it, form a 2+ ion, and give a basic oxide and an ionic chloride that simply dissolves.
Always justify with the same three ideas: nuclear charge, shielding and distance.
21 · Observations to quote
| Test | Observation |
|---|---|
| Na + cold water | melts, floats, fizzes, moves |
| Mg + steam | bright white flame, white solid |
| SiCl₄ + water | steamy fumes, white precipitate |
| PCl₅ + water | violent, steamy HCl fumes |
| NaCl + water | dissolves, no visible change |
Describe what is seen — fizzing, fumes, a precipitate, a flame colour — not what you infer.
22 · Why bonding changes across the period
Δ electronegativity with chlorine falls steadily from Na to P, so the chlorides shift from ionic through polar covalent to simple covalent — and their behaviour with water changes with them.
The same logic explains the oxides. Al sits at the crossover, which is why Al₂O₃ and AlCl₃ both show intermediate character.
23 · Worked example — identify the chloride
Fumes → covalent, hydrolysed
Liquid at r.t.p. → not PCl₅ (solid)
White precipitate of SiO₂ → SiCl₄
SiCl₄ + 2H₂O → SiO₂ + 4HCl
24 · Group trends for comparison
Group 2 — reactivity increases down the group as ionisation energies fall; hydroxide solubility increases, sulfate solubility decreases.
Group 17 — oxidising power decreases down the group, so a halogen displaces any halide below it. Volatility decreases as London forces grow.
25 · Answer structure for a trend question
State the trend, then explain it with all three factors: nuclear charge, shielding and distance from the nucleus, and say which one dominates.
For a structure question the order is: type of structure → type of bonding → strength of the forces broken → the property asked about. Two marks are usually one for each half.
Definitions to quote
Amphoteric — reacts with both acids and bases to form a salt and water.
Electronegativity — the ability of an atom to attract a shared pair of electrons in a covalent bond.
First ionisation energy — the energy needed to remove one mole of electrons from one mole of gaseous atoms.
Marks lost here
— Writing “dissolves” for SiCl₄ and PCl₅; they hydrolyse, and the observation is steamy HCl fumes.
— Giving pH 7 for MgCl₂ solution without noting it is slightly acidic.
— Quoting only one of the two Al(OH)₃ equations when asked to prove it is amphoteric.
Get all 25 topic cheat sheets for 9701 Chemistry
One free PDF pack, AS + A Level, straight to WhatsApp.
Chemical Periodicity — Frequently Asked Questions
Why does the melting point peak at silicon in Period 3?
Sodium, magnesium and aluminium are metallic lattices of increasing charge density, so melting point rises. Silicon is a giant covalent lattice needing strong covalent bonds to be broken, giving the maximum. After silicon the elements are simple molecular, so only weak intermolecular forces are overcome and the melting point falls sharply.
Why is aluminium oxide amphoteric?
The bonding in Al₂O₃ is intermediate between ionic and covalent, so it reacts as a base with acids and as an acid with alkalis. Sodium and magnesium oxides are ionic and basic; phosphorus and sulfur oxides are covalent and acidic.
What pH do the Period 3 chlorides give in water?
Sodium chloride simply dissolves, giving pH 7. Magnesium chloride hydrolyses slightly to about pH 6.5. Aluminium, silicon and phosphorus chlorides hydrolyse extensively, releasing HCl fumes and giving pH values of roughly 3, 2 and 2.
