Explain the difference between the mass number and the atomic number of an atom, and state what is meant by isotopes of the same element.
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.
The mass number is the total number of protons and neutrons in the nucleus.
Isotopes are atoms of the same element that have the same number of protons.
Isotopes have different numbers of neutrons (so different mass numbers).
The mass number is the total number of protons and neutrons in the nucleus.
Isotopes are atoms of the same element that have the same number of protons.
Isotopes have different numbers of neutrons (so different mass numbers).
Examiner tips
- Use the exact terms ‘atomic number’ and ‘mass number’.
- Show that isotopes share the same element but differ in neutron count.
Mark scheme (4 marks)
- The atomic number is the number of protons in the nucleus
- The mass number is the total number of protons and neutrons in the nucleus
- Isotopes are atoms of the same element that have the same number of protons
- Isotopes have different numbers of neutrons (so different mass numbers)
Key terms in this question
mass number · atomic number · isotopes
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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