AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2021: Question 2

✅ Correct Answer (Mark Scheme)

• (Atoms with the) same number of protons [1 mark]
• (but with) different numbers of neutrons [1 mark]

💡 Key Knowledge

Every atom of a specific element must have the same atomic number (number of protons). If the proton number changes, it becomes a completely different element.

However, the nucleus can contain differing numbers of neutrons, resulting in different mass numbers. These variants are called isotopes.

🧠 Exam Technique

Notice the prompt explicitly instructs: "You should answer in terms of subatomic particles."

Always name the exact particles (protons and neutrons). Avoid answering purely in terms of "mass number" or "atomic number", as the examiner is directly testing your knowledge of subatomic particles.

❌ Common Errors

• Writing "different relative atomic mass" – this is an explicit reject in the mark scheme.
• Stating "different numbers of electrons" (neutral isotopes of the same element always have equal numbers of electrons).
• Vague statements like "elements with different masses" without mentioning atoms or subatomic particles.

📐 Step-by-Step Calculation

1. State formula and substitute values:
   Aᵣ = [(mass₁ × %₁) + (mass₂ × %₂)] / 100
   Aᵣ = [(69 × 60) + (71 × 40)] / 100 [1 mark]
2. Multiply each bracket:
   69 × 60 = 4140
   71 × 40 = 2840
3. Add and divide by 100:
   (4140 + 2840) / 100 = 6980 / 100 = 69.8
4. Check decimal places:
   69.8 (already 1 decimal place) [1 mark]

✅ Final Answer

69.8

🧠 Exam Technique & Reality Check

Always perform a sense-check on your answer:

• The isotopes have masses 69 and 71.
• Your answer must lie between 69 and 71.
• Because ⁶⁹Ga is more abundant (60%), the average must be closer to 69 than to 71. 69.8 makes physical sense!

❌ Common Calculation Traps

• Simply averaging the mass numbers: (69 + 71) / 2 = 70.0 (ignores percentages; scores 0).
• Rounding to the nearest whole number (70) and losing the second mark because the question asked for 1 decimal place.
• Entering the sum incorrectly into a calculator without brackets (e.g. 69 × 60 + 71 × 40 / 100 = 4140 + 28.4 = 4168.4).

✅ Correct Answers

• Number of electrons: 31 [1 mark]
• Number of neutrons: 38 [1 mark]

💡 Key Knowledge

• Bottom number (31) = Atomic number = number of protons. In a neutral atom, number of protons = number of electrons (31).
• Top number (69) = Mass number = protons + neutrons.
• Neutrons = Mass number − Atomic number = 69 − 31 = 38.

❌ Common Errors

• Mixing up the top and bottom numbers: giving 69 electrons or subtracting incorrectly.
• Using the relative atomic mass from table 1 (or part 02.2) instead of the mass number given in the specific symbol ⁶⁹₃₁Ga.

✅ Correct Answer

Tick box 3: Ga³⁺ [1 mark]

💡 Key Knowledge

• Gallium is in Group 3 of the periodic table.
• Elements in Group 3 have 3 electrons in their outer shell.
• As a metal, gallium loses these 3 outer electrons to achieve a full outer shell (stable noble gas configuration).
• Losing 3 negatively charged electrons leaves behind a 3+ charge (Ga³⁺).

❌ Common Errors

• Choosing Ga³⁻: confusing electron loss (forming positive cations) with electron gain (forming negative anions). Metals always form positive ions.
• Ticking multiple boxes: if more than one box is ticked without clearly crossing out mistakes, 0 marks are awarded.

✅ Correct Answers (Give two reasons)

• Gallium fitted into a gap that Mendeleev had left [1 mark]
• Gallium's properties matched/agreed with his predictions (or matched the rest of the group) [1 mark]

💡 Key Knowledge

Mendeleev arranged known elements mainly in order of atomic mass, but he boldly:

• Left gaps for undiscovered elements (e.g. "eka-aluminium").
• Used the trends in groups to predict their physical and chemical properties (density, melting point, formulas of oxides).

When gallium was discovered in 1875, its actual properties matched Mendeleev's predictions with astonishing accuracy, convincing the scientific community that his periodic table was correct.

🧠 Exam Technique: Two Distinct Points

Make sure your two bullet points make two fundamentally different observations:

1. Point 1: Physical placement – It filled an empty space / gap he deliberately left.
2. Point 2: Chemical verification – Its properties closely matched what he predicted.

❌ Common Errors

• Simply stating "it was discovered" without linking it to the gap Mendeleev left.
• Vaguely writing "it proved he was right" without identifying what was confirmed (his predictions of properties).