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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the given values.
- S2
Mass number (A) = 23
- S3
Atomic number (Z) = 11
- S4
Step 2: Calculate the number of protons.
- S5
Number of protons = Atomic number (Z) = 11
- S6
Step 3: Calculate the number of neutrons.
- S7
Number of neutrons = Mass number (A) - Atomic number (Z) = 23 - 11 = 12
- S8
Step 4: Calculate the number of electrons in a neutral atom.
- S9
Number of electrons = Atomic number (Z) = 11
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the given values.
- S2
Mass number (A) = 23
- S3
Atomic number (Z) = 11
- S4
Step 2: Calculate the number of protons.
- S5
Number of protons = Atomic number (Z) = 11
- S6
Step 3: Calculate the number of neutrons.
- S7
Number of neutrons = Mass number (A) - Atomic number (Z) = 23 - 11 = 12
- S8
Step 4: Calculate the number of electrons in a neutral atom.
- S9
Number of electrons = Atomic number (Z) = 11
Final answer: Protons: 11, Neutrons: 12, Electrons: 11
Common mistakes
- Confusing mass number (A) with atomic number (Z). — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)3 marks