OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.2.1 Periodicity Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.2.1 Periodicity: what you need to know, common mistakes and exam-style questions with answers. Covers the s, p, d and f blocks and trends in atomic radius, ionisation energy and melting point across Period 3.
Practise 3.2.1 Periodicity with past papers
Once you have learnt this topic, find the 3.2.1 Periodicity questions in real OxfordAQA International A Level Chemistry past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
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About OxfordAQA International A Level Chemistry 3.2.1 Periodicity
Periodicity is the first inorganic chemistry topic and is examined in Unit 1. You need to classify elements into s, p, d and f blocks based on their electron configurations, and describe and explain the trends across Period 3 (sodium to argon) in atomic radius, first ionisation energy and melting point in terms of structure and bonding. It links closely to atomic structure and bonding in Unit 1 and to the reactions of Period 3 elements and their oxides and chlorides in 3.2.4 at A2. Examiners commonly ask you to explain the melting points of silicon and the simple molecular elements, or the decrease in atomic radius across the period.
What you need to know
- Blocks Elements are classified as s, p, d or f block according to the sub-shell containing their highest energy electron. Sodium and magnesium are s block; aluminium to argon are p block.
- Atomic radius Atomic radius decreases across Period 3 because nuclear charge increases while electrons are added to the same shell with similar shielding, so outer electrons are pulled closer.
- First ionisation energy First ionisation energy generally increases across Period 3 because of increasing nuclear charge and decreasing radius. There are dips at aluminium and sulfur, explained by sub-shells and electron pair repulsion.
- Melting points of metals Melting point rises from sodium to aluminium because metallic bonding gets stronger: ions have higher charge, are smaller and there are more delocalised electrons.
- Silicon and non-metals Silicon has the highest melting point because it is macromolecular, with many strong covalent bonds to break. Phosphorus (P4), sulfur (S8), chlorine (Cl2) and argon have low melting points because only van der Waals forces are overcome; S8 is highest of these as it is the largest molecule.
Common mistakes in 3.2.1 Periodicity
- Saying covalent bonds break when sulfur or phosphorus melts. Only intermolecular forces are overcome.
- Explaining the decrease in atomic radius by increased shielding. Shielding stays roughly the same; nuclear charge increases.
- Ignoring molecule size for P4, S8 and Cl2. Larger molecules have more electrons and stronger van der Waals forces.
- Calling silicon a giant ionic structure. It is macromolecular (giant covalent).
Try three 3.2.1 Periodicity exam-style questions
1. Explain why silicon has a much higher melting point than phosphorus. (3 marks)
Show the mark scheme answer
Silicon is macromolecular with many strong covalent bonds; phosphorus is simple molecular (P4); only weak van der Waals forces between P4 molecules need to be overcome.
2. Explain the trend in atomic radius across Period 3. (2 marks)
Show the mark scheme answer
Nuclear charge / number of protons increases; electrons in the same shell with similar shielding are attracted more strongly, so radius decreases.
3. Explain why sulfur has a higher melting point than chlorine. (2 marks)
Show the mark scheme answer
Sulfur exists as S8 molecules, which are larger than Cl2 molecules with more electrons; so van der Waals forces between molecules are stronger.
3.2.1 Periodicity FAQs
What does OxfordAQA International A Level Chemistry 3.2.1 Periodicity cover?
It covers the s, p, d and f blocks and trends in atomic radius, ionisation energy and melting point across Period 3.
Which unit is 3.2.1 Periodicity examined in?
It is examined in Unit 1, the first International AS paper, which covers inorganic and physical chemistry. The Unit 5 practical and synoptic paper at A2 can also assess it.
How do I explain melting points across Period 3?
Identify the structure first: metallic for sodium to aluminium, macromolecular for silicon and simple molecular for phosphorus, sulfur, chlorine and argon. Then explain which forces or bonds must be overcome and how strong they are.
