AQA A Level Chemistry 3.2.1 Periodicity Exam Questions

About AQA A Level Chemistry 3.2.1 Periodicity

Periodicity opens AQA A Level inorganic chemistry. You need to classify elements into s, p, d and f blocks by electron configuration, and explain the trends across Period 3 (sodium to argon) in atomic radius, first ionisation energy and melting point. The explanations draw directly on 3.1.1 Atomic Structure and 3.1.3 Bonding, so this topic is really about applying ideas you already know. It prepares you for 3.2.4 on Period 3 elements and their oxides. Examiners usually ask you to explain one trend with reference to nuclear charge and shielding, or to explain melting points by naming the structure and the bonds or forces overcome.

What you need to know

• Blocks An element is in the s, p, d or f block according to the sub-level its highest energy electron occupies. Group 1 and 2 are s block, Groups 3 to 0 are p block, and the transition elements are d 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 There is a general increase across Period 3 due to increasing nuclear charge and similar shielding. There is a dip at aluminium (outer electron in the higher energy 3p sub-level) and at sulfur (repulsion between paired 3p electrons).
• Melting point: metals Melting point rises from sodium to magnesium to aluminium. The metal ions have a greater charge and are smaller, and there are more delocalised electrons per atom, so metallic bonding is stronger.
• Melting point: silicon Silicon has the highest melting point in Period 3 because it is macromolecular, and many strong covalent bonds must be broken.
• Melting point: simple molecules P4, S8 and Cl2 are simple molecular and Ar is monatomic, so only van der Waals forces are overcome. S8 has the highest of these because it is the largest molecule with the most electrons, followed by P4, Cl2 and Ar.

Common mistakes in 3.2.1 Periodicity

• Saying shielding increases across a period. Electrons are added to the same shell, so shielding is similar while nuclear charge increases.
• Describing silicon's high melting point in terms of intermolecular forces. Silicon is macromolecular and covalent bonds are broken.
• Explaining the melting points of sulfur and phosphorus as breaking covalent bonds. Only van der Waals forces between molecules are overcome.
• Forgetting to mention the number of delocalised electrons and ion charge when explaining why aluminium melts higher than sodium.
• Confusing atomic radius with ionic radius. The Period 3 trend asked for is usually atomic radius.

Try three 3.2.1 Periodicity exam-style questions