A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Mass Number, Atomic Number, Isotopes, and Mass Spectrometry — mark scheme explained
The short answer
In the realm of chemistry, understanding atomic structure is fundamental to grasping more complex concepts. This section focuses on mass number (A), atomic (proton) number (Z), and isotopes.
The question
An atom has a mass number of 23 and an atomic number of 11. Determine the number of protons, neutrons, and electrons in this atom.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.
What the command words demand
- Calculate
- Perform a numerical calculation using given data and appropriate formulas.
- Determine
- Find or derive a specific value or quantity based on given information.
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Interpret
- Analyze and explain the meaning of data or graphs.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.
- Identify the given values.0 marksMass number (A) = 23Atomic number (Z) = 11
- Calculate the number of protons.1 markNumber of protons = Atomic number (Z) = 11
- Calculate the number of neutrons.1 markNumber of neutrons = Mass number (A) - Atomic number (Z) = 23 - 11 = 12
- Calculate the number of electrons in a neutral atom.1 markNumber of electrons = Atomic number (Z) = 11
Final answer: Protons: 11, Neutrons: 12, Electrons: 11
Work through every step correctly and you earn all 3 marks.
Another worked example
An ion has a mass number of 35 and an atomic number of 17. It carries a charge of +1. Determine the number of protons, neutrons, and electrons in this ion.
- Identify the given values.0 marksMass number (A) = 35Atomic number (Z) = 17Charge = +1
- Calculate the number of protons.1 markNumber of protons = Atomic number (Z) = 17
- Calculate the number of neutrons.1 markNumber of neutrons = Mass number (A) - Atomic number (Z) = 35 - 17 = 18
- Calculate the number of electrons in the ion.1 markNumber of electrons = Atomic number (Z) - charge = 17 - 1 = 16
Final answer: Protons: 17, Neutrons: 18, Electrons: 16
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing mass number (A) with atomic number (Z).
Why it happens: Students often mix up the definitions of mass number and atomic number, leading to incorrect calculations.
Fix: Always remember that the mass number is the total number of protons and neutrons, while the atomic number is the number of protons only.
Forgetting to account for charge in ions when determining the number of electrons.
Why it happens: Students may overlook the effect of charge on the number of electrons, leading to incorrect answers.
Fix: Always adjust the number of electrons by adding or subtracting the charge from the atomic number (Z).
Misinterpreting the peaks in a mass spectrum as molecular masses instead of isotopic masses.
Why it happens: Students may not understand that the m/z values represent isotopic masses, not necessarily molecular masses.
Fix: Practice identifying and interpreting the peaks in a mass spectrum to distinguish between isotopic and molecular masses.
Using incorrect units for mass number and atomic number.
Why it happens: Students may use decimal values instead of whole numbers for mass number and atomic number, leading to errors in calculations.
Fix: Always ensure that the mass number and atomic number are whole numbers when substituting into formulas or interpreting data.
Failing to convert percentages to decimal form when calculating relative atomic mass.
Why it happens: Students may forget to convert percentages to decimals, leading to incorrect calculations of the weighted average.
Fix: Always convert percentages to decimal form before using them in the formula for relative atomic mass (A r ).
Misunderstanding how to read a time-of-flight (TOF) mass spectrum.
Why it happens: Students may not fully grasp the relationship between the time-of-flight and the mass-to-charge ratio of ions, leading to incorrect interpretations.
Fix: Practice analyzing TOF mass spectra and relate the time-of-flight to the m/z values. Lighter ions travel faster than heavier ones, so their peaks appear earlier in the spectrum.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Atomic Structure Composition | Work out the numbers of protons, neutrons and electrons from mass and atomic numbers. | 3 |
| Atomic Structure Calculation | Work out the protons, neutrons, and electrons in a charged ion from its symbol. | 3 |
| Calculate Relative Atomic Mass | Use isotopic masses and their percentage abundances to find the weighted average atomic mass. | 3 |
| Relative Atomic Mass | Calculate an element's relative atomic mass from its isotopic masses and their percentage abundances. | 3 |
| Total across these question types | 12 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.