Edexcel A-Level Chemistry Paper 1, June 2025: Question 1

✅ Model Answer

The mass of an atom of an isotope [1 mark]

compared to (or relative to) 1/12^th the mass of an atom of carbon-12 [1 mark]

💡 Key Knowledge

• Isotopic vs Atomic: Isotopic mass refers to a single specific atom of an isotope, not an average.
• The Standard: Carbon-12 (¹²C) is the universal international standard, defined as having exactly 12 atomic mass units. Hence, 1 unit = 1/12^th of an atom of ¹²C.

❌ Common Errors & Misconceptions

• Mentioning “average” or “weighted mean”: This confuses relative isotopic mass with relative atomic mass (A_r). Examiners penalise any mention of “average/mean mass”.
• Omitting “atom”: Saying “mass of an isotope relative to carbon-12” loses mark 1. You must state mass of an atom.
• Mentioning “moles”: Definitions are based on individual atoms, not molar masses (e.g. do not say “mass of 1 mole of an isotope relative to 1 mole of carbon-12”).

🧠 Exam Technique

• Commit the two distinct halves of the definition to memory: (1) mass of an atom of an isotope, (2) relative to 1/12^th the mass of a carbon-12 atom.
• If you write the fraction form, ensure “atom” is written in both the numerator and denominator.

✅ Model Answer

• All isotopes have 18 protons (and 18 electrons) [1 mark]
• The isotopes have different numbers of neutrons: [1 mark]
  • ³⁶Ar has 18 neutrons (36 − 18)
  • ³⁸Ar has 20 neutrons (38 − 18)
  • ⁴⁰Ar has 22 neutrons (40 − 18)

🧠 Exam Technique & Guidance

• Always link theory to the specific example: The question explicitly says “Illustrate your answer by referring to the numbers of sub-atomic particles in these isotopes of argon”. Giving only a generic textbook definition caps you at a maximum of 1 mark!
• Check your Periodic Table: Argon has atomic number Z = 18. Therefore, every argon atom must contain 18 protons.
• Number of neutrons = Mass number (A) − Atomic number (Z).

❌ Common Errors

• Giving vague terms: “Same atomic number, different mass number” does not satisfy the requirement to refer to sub-atomic particles.
• Incorrect electron counts: If you mention electrons, you must state 18 electrons. An incorrect electron number will lose mark 1.

💡 Key Knowledge

• Because all isotopes of an element have identical numbers and arrangements of electrons, they display identical chemical properties.
• Different neutron counts lead to differing physical properties (e.g. mass, density, rate of diffusion).

❌ Calculation Traps & Common Errors

• Rounding errors: Rounding to 40, 39.9, or 39.94 loses the second mark. Look at the third decimal digit (39.94 5 ...) → rounds up to 39.95.
• Using nominal mass numbers (36, 38, 40): When exact isotopic masses are provided (e.g. 35.97), you must use them. Using integer mass numbers leads to an inaccurate final value.
• Adding incorrect units: Relative atomic mass is a dimensionless ratio. Do not write “g” or “amu” (though the mark scheme ignores “g mol⁻¹”, it penalises other incorrect units).

🧠 Top-Scorer Reality Check

• Sanity check your answer: ⁴⁰Ar makes up 99.600% of argon, so your calculated value must be extremely close to 39.96. If your result is 37 or 38, check your arithmetic!
• Always re-read the final line of the question: “Give your answer to two decimal places.” Highlight this instruction before writing down your final answer.