Mini-Lesson

Periodicity & Period 3 Elements

This mini-lesson covers AQA 3.2.1 Periodicity and 3.2.4 Period 3 elements and their oxides: s, p, d and f block classification, the trends in atomic radius, first ionisation energy and melting point across Na–Ar, the reactions of Na and Mg with water, and the oxides of Na–S and their reactions with water, acids and bases.

Work through each screen, answer the questions as you go (some are wordy, most are calculations) and collect ⭐ stars. Everything here is A-level standard — the maths is done properly, not skipped. Press Start when you're ready.

Periodicity · classification and trends

Blocks, atomic radius and ionisation energy

An element belongs to the s, p, d or f block according to the sub-shell that its highest-energy electron occupies — which is exactly why the Periodic Table has the shape it does.

Across Period 3 (Na → Ar):

Atomic radius decreases. Each element has one more proton but the extra electron enters the same shell, so shielding is essentially unchanged. The greater nuclear charge pulls the shell in.
First ionisation energy generally increases — same reasoning. But there are two dips:
- Al < Mg: aluminium’s outer electron is in a 3p orbital, which is higher in energy and slightly shielded by the 3s pair.
- S < P: in sulfur, two electrons are paired in one 3p orbital and repel each other, so one is easier to remove.

Melting point across Period 3 — driven entirely by structure and bonding: Na < Mg < Al (giant metallic: more delocalised electrons and smaller ions → stronger bonding); Si has by far the highest (giant covalent — strong covalent bonds must be broken); then a collapse to P₄, S₈, Cl₂, Ar, which are simple molecular with only weak van der Waals forces (S₈ > P₄ > Cl₂ > Ar, in order of the number of electrons).

Quick check

?Why does silicon have the highest melting point in Period 3?

Periodicity · reactions with water

Na and Mg with water

Sodium reacts vigorously with cold water, fizzing and melting into a ball:

2Na + 2H₂O → 2NaOH + H₂gives a strongly alkaline solution, pH about 13–14

Magnesium reacts only very slowly with cold water, giving sparingly soluble magnesium hydroxide and a much weaker alkali (pH about 9):

Mg + 2H₂O → Mg(OH)₂ + H₂very slow — a few bubbles over several days

But magnesium reacts rapidly with steam, burning with a brilliant white flame to give the oxide, not the hydroxide:

Mg + H₂O(g) → MgO + H₂note the different product — a favourite exam distinction

Calculate

Your turn

10.0250 mol of Na₂O is dissolved in water and made up to 1.00 dm³. Na₂O + H₂O → 2NaOH. Calculate the concentration of NaOH formed.

mol dm⁻³

Hint: 1 mol Na₂O gives 2 mol NaOH, so n(NaOH) = 0.0500 mol; c = n ÷ V.

Calculate

Your turn

2Calculate the pH of that NaOH solution. (K_w = 1.0 × 10⁻¹⁴ mol² dm⁻⁶.) Give your answer to 2 decimal places.

Hint: [OH⁻] = 0.0500, so [H⁺] = 1.0 × 10⁻¹⁴ ÷ 0.0500 = 2.0 × 10⁻¹³; pH = −log[H⁺].

Periodicity · the oxides

The Period 3 oxides and their reactions with water

The Period 3 elements burn in oxygen to give Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ and SO₃. Their behaviour changes systematically as the bonding changes from ionic to giant covalent to simple molecular.

Na₂O — ionic; dissolves readily: Na₂O + H₂O → 2NaOH, pH 13–14.
MgO — ionic but with a much higher lattice enthalpy, so only slightly soluble: MgO + H₂O → Mg(OH)₂, pH about 9.
Al₂O₃ — ionic with significant covalent character; insoluble, and amphoteric — it reacts with both acids and bases.
SiO₂ — giant covalent; insoluble in water (pH 7), but it is an acidic oxide because it reacts with hot concentrated NaOH.
P₄O₁₀ — simple molecular; reacts violently: P₄O₁₀ + 6H₂O → 4H₃PO₄, pH about 0–1.
SO₂ / SO₃ — simple molecular: SO₂ + H₂O → H₂SO₃ (pH ≈ 3) and SO₃ + H₂O → H₂SO₄ (pH ≈ 0–1).

Melting points of the oxides follow the same structural logic: Na₂O, MgO and Al₂O₃ are giant (high, and MgO is highest because of its 2+/2− charges); SiO₂ is giant covalent (high); P₄O₁₀, SO₂ and SO₃ are simple molecular (low).

Sort it

Sort each Period 3 oxide or hydroxide

Tap a compound, then tap whether it is basic, amphoteric or acidic.

🟩 Basic

🟪 Amphoteric

🟦 Acidic

Periodicity · amphoteric behaviour

Al₂O₃: the amphoteric oxide

Aluminium oxide is amphoteric — it behaves as a base with acids and as an acid with bases. This is the classic Period 3 demonstration that the metal–non-metal boundary is a gradual change, not a sharp line.

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂Oacting as a BASE (check: Al 2, O 3, Cl 6, H 6 on both sides ✓)

Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄acting as an ACID (check: Al 2, Na 2, O 8, H 8 on both sides ✓)

The acidic oxides likewise react with bases — for example SiO₂ + 2NaOH → Na₂SiO₃ + H₂O (Si 1, O 4, Na 2, H 2 on both sides ✓), and SO₂ + 2NaOH → Na₂SO₃ + H₂O.

Writing these equations is a common exam task. Work out the anion the acidic oxide forms (SiO₃²⁻, SO₃²⁻, SO₄²⁻, PO₄³⁻), then balance the sodium and finally the hydrogen with water.

Calculate

Your turn

3Calculate the oxidation state of phosphorus in P₄O₁₀.

Hint: 4x + 10(−2) = 0 (it is a neutral compound), so 4x = 20.

Calculate

Your turn

4Calculate the relative molecular mass, M_r, of P₄O₁₀. (P = 31.0, O = 16.0)

Hint: M_r = 4(31.0) + 10(16.0) = 124 + 160.

Match it

Match each oxide to the pH it gives with water

Tap an oxide on the left, then the pH of the resulting solution on the right.

Oxide added to water

Approximate pH

Quick check

?Why does MgO give a solution of only about pH 9, while Na₂O gives pH 13–14?

Quick check

?Which Period 3 oxide is insoluble in water but still classed as acidic?

Quick check

?Magnesium is heated strongly in steam. What is the magnesium-containing product?

Periodicity · exam traps

Explaining trends the way the mark scheme wants

Melting point must be explained by STRUCTURE AND BONDING — never by "the atoms get stronger". Say: giant metallic (and how many delocalised electrons), giant covalent (strong covalent bonds broken), or simple molecular (weak van der Waals broken).
The two ionisation-energy dips have DIFFERENT reasons. Al: the electron is in a 3p orbital, higher in energy. S: two electrons are paired in one 3p orbital and repel. Do not use the same explanation twice.
"Insoluble" is not the same as "neutral". SiO₂ gives pH 7 in water, but it is an acidic oxide because it reacts with hot concentrated NaOH.
Al₂O₃ is amphoteric — you must be able to write an equation with an acid and an equation with a base.

Atomic radius across the period: "the nuclear charge increases while the shielding stays essentially the same, so the outer electrons are pulled in more strongly." Both halves of that sentence are needed for the mark.

Quick check

?Which Period 3 oxide has the highest melting point?

Calculate

Your turn

5Calculate the relative formula mass of Al₂O₃. (Al = 27.0, O = 16.0)

Hint: M_r = 2(27.0) + 3(16.0).

Recap

The big ideas to know

Blocks: s, p, d or f — set by the sub-shell of the highest-energy electron

Atomic radius: decreases across a period: more nuclear charge, same shielding

First IE: rises across Period 3, with dips at Al (3p electron) and S (paired 3p electrons repel)

Melting point: Na < Mg < Al (metallic, rising) · Si is the highest (giant covalent) · then P₄, S₈, Cl₂, Ar are simple molecular and low

With water: Na reacts vigorously → NaOH, pH 13–14; Mg reacts very slowly → Mg(OH)₂, pH about 9 (but burns in steam → MgO)

Oxides: Na₂O and MgO are basic · Al₂O₃ is amphoteric · SiO₂, P₄O₁₀, SO₂ and SO₃ are acidic

That is the whole of AQA 3.2.1 and 3.2.4. Press Finish.

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