Periodicity — A-Level Chemistry Revision
Revise Periodicity for A-Level Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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Go to Group 2What is Periodicity?
Periodicity refers to the repeating patterns of physical and chemical properties of elements across Period 3 of the periodic table. At A-Level, this involves explaining the trends in atomic radius, ionisation energy, electronegativity, and melting point. These trends are explained by changes in nuclear charge, electron shielding, and the structure and bonding of the elements, which change from metallic to giant covalent to simple molecular structures.
Board notes: All boards expect a detailed explanation of the trends in Period 3. AQA often asks for explanations of the specific drops in ionisation energy between groups 2/13 and 15/16. Edexcel may link periodicity to the reactions of Period 3 elements with oxygen and water. OCR questions often require students to sketch and label graphs of the periodic trends and explain the shapes of these graphs.
Step-by-step explanationWorked example
Explain the trend in first ionisation energy across Period 3 (Na to Ar). Step 1: State the general trend: First ionisation energy generally increases across Period 3. Step 2: Explain the reason: Across the period, the nuclear charge increases and the number of inner electron shells remains the same, so shielding is similar. This leads to a stronger attraction between the nucleus and the outer electron, requiring more energy to remove it. Step 3: Explain the anomalies: There is a dip between Mg (Group 2) and Al (Group 3) because the outer electron in Al is in a higher energy 3p orbital, which is further from the nucleus and easier to remove. There is another dip between P (Group 15) and S (Group 16) due to spin-pair repulsion in one of the 3p orbitals in sulfur.
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Common mistakes
- 1Confusing the trend in atomic radius with ionic radius. While atomic radius generally decreases across a period, the trend for ionic radius is more complex due to the formation of positive and negative ions.
- 2Incorrectly explaining the high melting point of silicon. Silicon has a very high melting point because it has a giant covalent structure, requiring a large amount of energy to break the strong covalent bonds, not because of strong intermolecular forces.
- 3Stating that the drop in ionisation energy between Group 15 and 16 (e.g., N to O) is due to increased shielding. The drop is actually due to electron-electron repulsion in the newly paired p-orbital, which makes the electron easier to remove.
Periodicity exam questions
Exam-style questions for Periodicity with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Periodicity
Core concept
Periodicity refers to the repeating patterns of physical and chemical properties of elements across Period 3 of the periodic table. At A-Level, this involves explaining the trends in atomic radius, io…
Frequently asked questions
Why does atomic radius decrease across a period?
As you move across a period, the number of protons in the nucleus increases, leading to a greater nuclear charge. This increased positive charge pulls the electrons in the same principal energy level closer to the nucleus, thus decreasing the atomic radius.
Why does sodium oxide form an alkaline solution while sulfur dioxide forms an acidic solution?
Sodium oxide is a basic oxide that reacts with water to form sodium hydroxide, a strong alkali. Sulfur dioxide is an acidic oxide that reacts with water to form sulfurous acid (H2SO3), a weak acid. This reflects the trend from basic to acidic oxides across a period.