OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 18

✅ Correct Answer Requirements

• Show a general upward trend from B through C, N, O, F to Ne (ignoring the dip at O).
• Ensure the final point for Ne is lower than the peak for He (He is around 2400 kJ mol⁻¹, Ne is around 2080 kJ mol⁻¹).
• Show the characteristic Period 2 drops: a dip at B (vs Be) and a dip at O (vs N). The graph already shows the dip at B; you must correctly position O lower than N, but higher than C, and F higher than O.

❌ Common Errors

• Drawing Ne higher than He. Remember that Helium has the highest first ionisation energy in the entire periodic table because its electron is closest to the nucleus with zero shielding.
• Failing to show the drop in ionisation energy between Nitrogen and Oxygen (due to electron-electron repulsion in the 2p orbital).

📐 Step-by-Step Calculation

1. Identify the value: The first ionisation energy of Lithium is given on the graph as approximately 500 kJ mol⁻¹ .
2. Convert per mole to per atom: Divide by the Avogadro constant ( 6.02 × 10²³ atoms per mole).
3. Calculate: 500 / (6.02 × 10²³) = 8.3056 × 10⁻²² kJ .
4. Apply formatting rules: Round to 2 significant figures and write in standard form: 8.3 × 10⁻²² kJ .

🧠 Exam Technique & Guidance

• Acceptable Range: Examiners allowed answers based on readings from 500 up to 550 kJ mol⁻¹ (giving 8.3 × 10⁻²² to 9.1 × 10⁻²² kJ ).
• Strict Formatting: If you fail to use standard form or round to the wrong number of significant figures, the mark is withheld even if the math is correct.

💡 Key Knowledge: He vs Li (2 Marks)

• Distance/Shielding: He outer electrons are in a lower energy / closer shell (1s shell vs 2s shell) with fewer inner shells shielding the nuclear charge.
• Attraction: Outer electrons in He experience greater nuclear attraction from the nucleus.

💡 Key Knowledge: Be vs Li (2 Marks)

• Nuclear Charge: Be has a greater nuclear charge (more protons: 4 vs 3).
• Shielding/Radius: Both elements have outer electrons in the same shell (2s) with similar shielding, but Be has a smaller atomic radius, resulting in greater nuclear attraction.

✅ Correct Answer

• Sub-shells: The outer electron in Be is in the 2s sub-shell, whereas the outer electron in B is in the 2p sub-shell.
• Energy Levels: The B(2p) sub-shell is at a higher energy level than the Be(2s) sub-shell, making it easier to remove.

❌ Common Errors & Forbidden Terms

• DO NOT use the word "shell" when discussing s and p — you must refer to sub-shells or orbitals. Calling them shells loses the mark.
• DO NOT mention "2s shielding 2p" as the primary reason for the drop between Be and B; the main reason is the higher energy level of the 2p sub-shell.
• Avoid vague statements like "less energy to remove" without linking it to the sub-shell energy level or orbital distance.