AQA A-Level Chemistry Paper 1, 2019: Question 3

✅ Correct Answer

Repeating pattern (or trend) of physical or chemical properties across different periods.

💡 Key Knowledge

Periodicity refers to trends that recur at regular intervals across the Periodic Table as atomic number increases (e.g., repeating peaks in first ionisation energy or repeating trends in atomic radii across Periods 2, 3, and 4).

❌ Common Errors

• Omitting the word repeating (simply stating "a pattern of properties" gets 0 marks).
• Referring to trends down a group instead of across periods (penalised by examiners).

🧠 Exam Technique

Memorise the exact phrasing: "A repeating pattern or trend of physical or chemical properties." You may mention a named property (e.g., repeating pattern of ionisation energies), which is also accepted.

✅ Correct Answer

Bromine (or Br )

Note: Krypton / Kr is also accepted on the mark scheme.

💡 Key Knowledge

Electronegativity increases across a period because the nuclear charge increases while electron shielding remains approximately constant, creating a stronger attraction for a bonding pair of electrons.

❌ Common Errors

• Writing the molecular formula Br₂ instead of the element symbol Br or name Bromine (explicitly rejected in the mark scheme).
• Naming an element from Period 2 or 3 (such as Fluorine or Chlorine) by failing to read "Period 4".

🧠 Exam Technique

When asked to identify an element, give the single symbol (e.g., Br ) or write the full elemental name. Diatomic formulas ( Br₂ , Cl₂ , O₂ ) describe molecules, not the elemental identity, and will lose you the mark.

✅ Correct Answer & Marks

• Element: Potassium (or K ) [1 mark]
• Explanation:
  • Has the smallest number of protons / smallest nuclear charge (in Period 4) [1 mark]
  • Has similar shielding / same number of electron shells (as other Period 4 elements) [1 mark]

💡 Key Knowledge

Atomic radius decreases across any period:

• Electrons are added to the same shell, so shielding remains similar.
• Nuclear charge increases (more protons), pulling the electron cloud closer to the nucleus.
• Therefore, Group 1 elements (leftmost) always possess the largest atomic radius in their period.

❌ Common Errors

• Naming an element in the wrong period: Selecting Na (Period 3) or Rb (Period 5) loses M1, though you can still get M2 and M3. Choosing any other wrong Period 4 element results in 0/3.
• Stating "potassium has no shielding" or "less shielding" — all Period 4 elements have three inner shielding shells!
• Failing to mention shielding altogether.

🧠 Exam Technique: The Three-Part Shielding Structure

Whenever explaining periodic size trends, always cover these three pillars:

1. Nuclear charge: Count of protons.
2. Shielding: Same or different number of inner shells.
3. Net electrostatic attraction: Pull on outermost electrons.

✅ Correct Answer

Amphoteric

💡 Key Knowledge

Look at how As₂O₃ behaves in each reaction:

• As₂O₃ + 6HCl → 2AsCl₃ + 3H₂O : Reacts with an acid → acting as a base.
• As₂O₃ + 6NaOH → 2Na₃AsO₃ + 3H₂O : Reacts with a base → acting as an acid.

A substance that reacts with both acids and bases is defined as amphoteric (compare with Al₂O₃ in Period 3).

❌ Common Errors

• Confusing amphoteric with amphiprotic (amphiprotic specifically means donating/accepting H⁺ ions).
• Writing "neutralisation" (this describes the reaction type, not the property of the oxide).

✅ Final Balanced Equation

As₂O₃ + 6Zn + 12HNO₃ → 2AsH₃ + 6Zn(NO₃)₂ + 3H₂O

❌ Common Errors

• Forgetting that Hydrogen atoms appear in two products ( AsH₃ and H₂O ), leading to an incorrect coefficient for HNO₃ .
• Balancing oxygen by counting all nitrate oxygens individually, increasing the risk of arithmetic slips. Keep polyatomic ions like NO₃⁻ as single units!