Explain what is meant by the term 'isotopes' and describe how the structure of two isotopes of the same element differs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Carbon exists in several forms. Carbon-12 and carbon-14 are two examples.
Model answer (5 marks)
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. For carbon, both isotopes have 6 protons and therefore 6 electrons, giving identical chemical properties. Carbon‑12 contains 6 neutrons, while carbon‑14 contains 8 neutrons, so the two isotopes differ in mass and nuclear stability but not in chemistry.
Examiner tips
- State that isotopes share the same atomic number but differ in neutron number; give the neutron counts for the two isotopes; mention identical electron number and chemistry; keep answer concise to fit 5 marks.
Common mistakes
- Confusing isotopes with ions or isotones; giving wrong neutron numbers; omitting that electrons are the same and therefore chemical properties are unchanged.
Mark scheme (5 marks)
- Isotopes are atoms of the same element
- Isotopes have the same number of protons (or same atomic/proton number)
- Isotopes have different numbers of neutrons
- Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons (correct values for both)
- Both isotopes have the same number of electrons (6 electrons) / same chemical properties
Key terms in this question
Related
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