Explain what is meant by the term 'isotopes' and describe how two isotopes of the same element are similar and how they differ in terms of their atomic structure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
The two isotopes:
1. have the same number of protons (same atomic number).
2. have different numbers of neutrons (different mass numbers).
3. have the same number of electrons, so the same electronic structure.
Because the electron arrangement is identical, isotopes of the same element exhibit the same chemical properties.
The two isotopes:
1. have the same number of protons (same atomic number).
2. have different numbers of neutrons (different mass numbers).
3. have the same number of electrons, so the same electronic structure.
Because the electron arrangement is identical, isotopes of the same element exhibit the same chemical properties.
Examiner tips
- Use the definition first, then list the three similarities/differences in order.
- Show that the same atomic number gives identical chemistry.
- Mention the different mass numbers to show the difference.
Common mistakes
- Confusing isotopes with isobars (same mass number, different elements).
- Saying isotopes have different chemical properties – they do not.
- Omitting the point that the number of electrons is the same.
Mark scheme (5 marks)
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons
- The two isotopes have the same number of protons (accept: same atomic/proton number)
- The two isotopes have different numbers of neutrons (accept: different mass numbers)
- The two isotopes have the same number of electrons (accept: same electron arrangement / same electronic structure)
- Because the number of electrons is the same, isotopes of the same element have the same chemical properties (accept: behave the same chemically / react in the same way)
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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