AQA A-Level Chemistry Paper 3, 2019: Question 14

✅ Correct Answer

B — Helium (He)

💡 Key Knowledge

• First Ionisation Energy Definition: The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions:
  X(g) → X⁺(g) + e⁻
• The Three Core Factors:
  1. Atomic radius: Smaller radius = outer electrons closer to the nucleus = stronger attraction.
  2. Nuclear charge: More protons = greater positive charge attracting outer electrons.
  3. Shielding: Fewer internal electron shells = less repulsion and stronger effective nuclear pull.
• Why Helium has the highest of ALL elements: Its electron configuration is 1s² . The electrons occupy the shell closest to the nucleus (minimal atomic radius), experience zero inner-shell shielding, and are held by a nucleus with 2 protons (double that of hydrogen).

📐 Step-by-Step Element Comparison

1. Compare H ( 1s¹ ) vs He ( 1s² ):
   Both have electrons in the 1s orbital with no inner shielding shells. However, He has 2 protons compared to H’s 1 proton. Greater nuclear charge pulls the electrons closer, making He’s atomic radius smaller and the attraction significantly stronger (He = 2372 kJ mol⁻¹ vs H = 1312 kJ mol⁻¹).
2. Eliminate Period 2 Elements (Li and Ne):
   Both Li ( 1s² 2s¹ ) and Ne ( 1s² 2s² 2p⁶ ) remove an electron from the second principal energy level ( n = 2 ).
3. Account for Shielding:
   Outer electrons in Li and Ne experience shielding from the full inner 1s² core, and are located further from the nucleus. Even though Ne has 10 protons, the increased distance and shielding mean its first ionisation energy (2081 kJ mol⁻¹) is lower than Helium's (2372 kJ mol⁻¹).

❌ Common Errors & Misconceptions

• Choosing Neon (Ne): A very common error. Students see that Ne has a high nuclear charge (10 protons) and a stable full octet, forgetting that Ne’s outer electrons are in the n = 2 shell and shielded by the inner 1s² shell.
• Choosing Hydrogen (H): Confusing the fact that hydrogen is the smallest atom with it having the strongest hold on its electron, forgetting that helium has twice the nuclear charge with no additional shielding.
• Choosing Lithium (Li): Forgetting that lithium starts a new period; its outer electron is far from the nucleus and shielded, giving it one of the lowest ionisation energies among these choices (520 kJ mol⁻¹).