AQA A-Level Chemistry: Periodicity (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.2.1 Periodicity of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.2.1.1 Classification and 3.2.1.2 Physical properties of Period 3 elements. All of it is AS content, so nothing here is A-level only. The topic is examined on AS Paper 1 and A-level Paper 1, and A-level Paper 3 can test any content.

Learn the ideas first in the periodicity study guide and the periodicity revision notes. The course hub is AQA A-Level Chemistry, and the printable checklist lists every specification point. Questions on why the first ionisation energies of aluminium and sulfur dip are in the atomic structure practice questions, so they are not repeated here. No question needs a calculator.

Questions

1. Which element is in the d block of the Periodic Table?

• A gallium (Z = 31)
• B zinc (Z = 30)
• C calcium (Z = 20)
• D helium (Z = 2)

[1]

2. State the property of an atom that determines the position of its element in the Periodic Table. [1]

3. An atom of element E has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p².

(a) State the block of the Periodic Table that contains E. [1] (b) State the period and the group that contain E. [2]

4. For each element, state its block and explain your answer.

(a) helium, which is placed in Group 0 [2] (b) the element with proton number 24 [2]

5. Describe and explain the trend in atomic radius of the elements from sodium to chlorine. [4]

6. Explain why the first ionisation energy of chlorine is greater than that of silicon. [3]

7. (a) Write an equation, with state symbols, for the first ionisation energy of aluminium. [1] (b) State which element in Period 3 has the highest first ionisation energy. [1]

8. Explain why magnesium has a higher melting point than sodium. [3]

9. Explain why sulfur has a higher melting point than chlorine. [3]

10. Explain why the melting points of silicon, phosphorus and argon decrease in the order Si > P > Ar. Refer to the structure of each element. [6]

11. X, Y and Z are elements in Period 3.

• X forms 3+ ions and conducts electricity when solid.
• Y has the highest melting point in Period 3.
• Z exists as diatomic molecules.

(a) Identify X, Y and Z. [3] (b) Explain why Y has a higher melting point than X. [2] (c) State the block that contains all three elements. [1]

12. A student writes: “Across Period 3, melting point increases because atomic radius decreases and nuclear charge increases.”

Evaluate this statement. [6]

Answers

1. B [1]. Zinc’s last electron in energy order goes into 3d (3d¹⁰ 4s²). Gallium is p block (4p¹), calcium is s block and helium is s block. [1] Examiner insight: Gallium is the tempting wrong answer: 3d¹⁰ appears in its configuration, but its last electron goes into 4p.

2. Its proton number (atomic number) [1]. [1] Examiner insight: “Mass number” or “relative atomic mass” gets no credit; the table is ordered by proton number.

3. (a) p block, because the last electron is added to the 4p sub-shell [1]. (b) Period 4 [1]; Group 4 (14), since it has four outer electrons, 4s² 4p² [1]. [3] Examiner insight: In (b) period and group are separate marks; giving “Group 2” from the 4s² alone loses the group mark.

4. (a) s block [1]; its configuration is 1s², so its highest-energy electron is in an s sub-shell [1]. (b) d block [1]; the configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ (chromium), and the element sits in the block where the 3d sub-shell is being filled [1]. [4] Examiner insight: The block alone is worth only one mark in each part; the second mark needs the sub-shell reason, not a restatement of the answer.

5. Atomic radius decreases from Na to Cl [1]. Nuclear charge increases, because each element has one more proton [1]. The outer electrons are in the same (third) shell, so shielding is similar [1]. The outer electrons are attracted more strongly and pulled closer to the nucleus [1]. [4] Examiner insight: “More shells” or “more shielding” contradicts the same-shell point and can cost the shielding mark.

6. Chlorine has more protons (17 against 14), so a greater nuclear charge [1]. In both atoms the outer electron is removed from the third shell, so shielding is similar [1]. Chlorine’s atomic radius is smaller, so its outer electron is attracted more strongly and more energy is needed to remove it [1]. [3] Examiner insight: Each mark is for a separate factor; a long answer about nuclear charge alone scores one mark at most.

7. (a) Al(g) → Al⁺(g) + e⁻ [1]. (b) Argon [1]. [2] Examiner insight: The equation mark needs (g) on the aluminium atom and the ion; Al(s) on the left, or no state symbols, does not score.

8. Both are giant metallic lattices with metallic bonding [1]. Magnesium forms Mg²⁺ ions, which have a greater charge (and are smaller) than Na⁺ ions, and each Mg atom provides two delocalised electrons compared with one for Na [1]. So the attraction between the positive ions and the delocalised electrons is stronger in magnesium, and more energy is needed to overcome it [1]. [3] Examiner insight: Answers that mention “ionic bonds” or “intermolecular forces” in a metal lose the structure mark, even if the rest is right.

9. Both are simple molecular, made of S₈ and Cl₂ molecules [1]. S₈ has more electrons (128) than Cl₂ (34), so the van der Waals forces between S₈ molecules are stronger [1]. More energy is needed to overcome these forces, so sulfur has the higher melting point [1]. [3] Examiner insight: Saying that S–S or Cl–Cl covalent bonds break on melting does not earn the second mark here, because it shows the wrong idea of what is overcome.

10. Silicon is macromolecular (giant covalent) [1]. To melt it, many strong covalent bonds must be broken, which needs a lot of energy, so its melting point is very high [1]. Phosphorus is simple molecular, made of P₄ molecules [1]. When it melts only the weak van der Waals forces between molecules are overcome; the P–P covalent bonds are not broken [1]. Argon exists as single atoms, each with 18 electrons, compared with 60 electrons in a P₄ molecule [1]. So the van der Waals forces between argon atoms are weaker than those between P₄ molecules, less energy is needed to overcome them, and argon has the lowest melting point [1]. [6] Examiner insight: In a six-mark explanation each element needs its structure named and what is broken or overcome; leaving argon out caps the answer well below full marks.

11. (a) X is aluminium [1]; Y is silicon [1]; Z is chlorine [1]. (b) Silicon is macromolecular, so melting means breaking many strong covalent bonds [1]; this needs more energy than overcoming the metallic bonding between Al³⁺ ions and delocalised electrons in aluminium [1]. (c) p block [1]. [6] Examiner insight: In (a) each identification is marked on its own, so a wrong X does not lose the marks for a correct Y and Z.

12. Award one mark for each of these points.

• Melting point does increase from Na to Al [1], but the main reason is stronger metallic bonding: higher ion charge and more delocalised electrons per atom, not radius alone [1].
• Silicon has the highest melting point because it is macromolecular and covalent bonds must be broken [1]; this is a change of structure, not a radius effect.
• From P onwards the melting point is much lower even though radius keeps falling and nuclear charge keeps rising [1], because P, S and Cl are simple molecular and only van der Waals forces between molecules are overcome [1].
• Conclusion: the statement is incorrect for the period as a whole; melting point depends on structure and bonding, and peaks at silicon rather than rising throughout [1].

[6] Examiner insight: “Evaluate” needs a clear judgement at the end; listing correct facts without saying whether the statement holds usually loses the final mark.

Where marks are usually lost

• Explaining Period 3 trends with “more shells” or “more shielding”.
• Giving nuclear charge as the only factor for radius or ionisation energy and leaving out similar shielding.
• Saying covalent bonds break when P₄, S₈ or Cl₂ melts.
• Describing silicon as having “strong intermolecular forces”; it has covalent bonds throughout.
• Writing metallic bonding as attraction between atoms, or between ions and ions.
• Comparing van der Waals forces by size of molecule without linking to the number of electrons.
• Classifying helium as p block or gallium as d block.
• Missing state symbols in ionisation energy equations.
• Ending an “evaluate” answer without a judgement.

Next steps

• Periodicity revision notes
• Periodicity study guide
• Atomic structure practice questions
• Bonding
• AQA A-Level Chemistry exam preparation
• AQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.2.1 Periodicity (3.2.1.1 Classification and 3.2.1.2 Physical properties of Period 3 elements).