About Periodic Table periodicity in A Level Chemistry
Periodic Table periodicity is one of the 18 main topics in our A Level Chemistry (Cambridge (CAIE) 9701) question bank, and about 6% of its real questions come from it. Tayyari Karo has 38 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2, Paper 4; 29 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 20 hard, 15 medium, 3 easy.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
Try 6 real questions
These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2026 · Paper 1 · Periodic Table periodicity
In which row are both statements correct? (A: electronegativity of Na lower than S; pH of dissolved Na2O lower than pH of dissolved SO2. B: first ionisation energy of Mg lower than Na; m.p. of MgO higher than Na2O. C: m.p. of Al higher than Na; m.p. of AlCl3 lower than NaCl. D: atomic radius of Si higher than Na; m.p. of SiCl4 lower than NaCl.)
- Row A
- Row B
- Row C
- Row D
Show answer and explanation
Answer: C. Row C
Al has stronger metallic bonding than Na, and AlCl3 is covalent/molecular so melts lower than ionic NaCl. In the other rows one statement is false (Na2O solution has higher pH than SO2; Mg IE is higher than Na; Si radius is smaller than Na).
October/November 2025 · Paper 1 · Periodic Table periodicity
The atomic radii and ionic radii for three Period 3 elements are: X (atomic 0.118 nm, ionic 0.053 nm), Y (atomic 0.099 nm, ionic 0.180 nm), Z (atomic 0.160 nm, ionic 0.072 nm). Using this data, which statement is correct?
- Element X has lower electrical conductivity than element Y.
- Element Y has a higher melting point than element X.
- Element Y and element Z react to form an ionic compound.
- Element Z forms ionic compounds by gaining electrons.
Show answer and explanation
Answer: C. Element Y and element Z react to form an ionic compound.
X (ionic radius smaller than atomic) forms a cation, e.g. Al; Y (ionic radius larger than atomic) forms an anion, e.g. Cl; Z (ionic radius smaller than atomic) forms a cation, e.g. Na. Y (a non-metal, e.g. Cl) and Z (a metal, e.g. Na) react to form an ionic compound.
May/June 2024 · Paper 1 · Periodic Table periodicity
Which statement is correct?
- The atomic radius of silicon is larger than that of aluminium.
- The boiling point of chlorine is higher than that of silicon.
- The first ionisation energy of sulfur is greater than that of phosphorus.
- The electrical conductivity of magnesium is greater than that of sodium.
Show answer and explanation
Answer: D. The electrical conductivity of magnesium is greater than that of sodium.
Mg has two delocalised electrons per atom (vs one for Na) and a smaller, more closely packed metallic lattice, giving magnesium a higher electrical conductivity than sodium.
October/November 2024 · Paper 1 · Periodic Table periodicity
Q is a semi-conductor. The chloride of Q reacts with water to form white fumes and an acidic solution. Which Period 3 element is Q?
- magnesium
- aluminium
- silicon
- phosphorus
Show answer and explanation
Answer: C. silicon
Silicon is the Period 3 semi-conductor; SiCl4 hydrolyses readily in moist air/water to give white fumes of HCl and an acidic solution (plus SiO2).
May/June 2023 · Paper 1 · Periodic Table periodicity
Element X has the second largest atomic radius in its period. An atom of X has three occupied electron shells only. The oxide of X is shaken with water. What could be the pH of the resulting solution?
- 5
- 7
- 9
- 14
Show answer and explanation
Answer: C. 9
X (three shells, second-largest radius in Period 3) is magnesium; MgO is a basic oxide that gives a weakly alkaline solution (pH 9) with water.
October/November 2022 · Paper 1 · Periodic Table periodicity
Which row describes the relative sizes of the ionic radii of Na+, Mg2+ and S2⁻, from smallest to largest?
A: Na+, Mg2+, S2−
B: Mg2+, Na+, S2−
C: S2−, Na+, Mg2+
D: S2−, Mg2+, Na+
- Row A
- Row B
- Row C
- Row D
Show answer and explanation
Answer: B. Row B
Mg2+ and Na+ are isoelectronic with Mg2+ having more protons so it is smaller than Na+; S2− has an extra electron shell filled and is much larger, giving Mg2+ < Na+ < S2−.
