Explain why the atomic number of an element uniquely identifies that element, and distinguish between the terms atomic number and mass number.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The atomic number is the number of protons in the nucleus of an atom.
Because each element has a fixed number of protons, the atomic number uniquely identifies that element; changing the proton number would create a different element.
The mass number is the total number of protons and neutrons in the nucleus.
Unlike the atomic number, the mass number can vary for atoms of the same element because the number of neutrons can differ (isotopes), whereas the atomic number remains constant for all atoms of the same element.
Because each element has a fixed number of protons, the atomic number uniquely identifies that element; changing the proton number would create a different element.
The mass number is the total number of protons and neutrons in the nucleus.
Unlike the atomic number, the mass number can vary for atoms of the same element because the number of neutrons can differ (isotopes), whereas the atomic number remains constant for all atoms of the same element.
Examiner tips
- Use the exact definition of atomic number and mass number; mention protons and neutrons.
- Show why the atomic number is unique – link to element identity.
- Explain isotope variation for mass number.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing mass number with atomic mass (the weighted average).
- Saying that atomic number can change – it cannot for a given element.
- Not mentioning neutrons when defining mass number.
Mark scheme (4 marks)
- The atomic number is the number of protons in the nucleus of an atom.
- Each element has a unique/fixed number of protons, so the atomic number uniquely identifies the element; changing the proton number would create a different element.
- The mass number is the total number of protons AND neutrons in the nucleus.
- Unlike atomic number, the mass number can vary for atoms of the same element because the number of neutrons can differ (isotopes), whereas the atomic number remains constant for all atoms of the same element.
Key terms in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain / Distinguish" question →
- Decode the mark scheme abbreviations →
More The nuclear atom questions
- Explain why atoms of the same element can have different masses, and outline how…
- Explain why the mass number of an atom is always a whole number, yet the relativ…
- Explain why the results of the Geiger–Marsden (gold foil) experiment led to the …
- Explain why two atoms of the same element may behave identically in chemical rea…