Chlorine has two naturally occurring isotopes: chlorine-35 and chlorine-37. Explain what is meant by the term isotope and describe how these two isotopes of chlorine are similar to, and different from, each other in terms of their subatomic particles.
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 a different number of neutrons.
Both chlorine‑35 and chlorine‑37 have 17 protons.
Both isotopes also have 17 electrons.
Chlorine‑35 contains 18 neutrons, whereas chlorine‑37 contains 20 neutrons, so the two isotopes have different numbers of neutrons.
Because the neutron numbers differ, the two isotopes have different mass numbers and therefore different masses.
Both chlorine‑35 and chlorine‑37 have 17 protons.
Both isotopes also have 17 electrons.
Chlorine‑35 contains 18 neutrons, whereas chlorine‑37 contains 20 neutrons, so the two isotopes have different numbers of neutrons.
Because the neutron numbers differ, the two isotopes have different mass numbers and therefore different masses.
Examiner tips
- Use the definition of isotope first, then list the common proton and electron numbers, followed by the differing neutron numbers and the resulting mass difference.
- Show the neutron count for each isotope to justify the mass difference.
Common mistakes
- Confusing neutrons with protons or electrons; students may say the isotopes have different numbers of protons.
- Failing to mention that both isotopes have the same number of electrons.
Mark scheme (5 marks)
- Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
- Both isotopes have the same number of protons (17 protons).
- Both isotopes have the same number of electrons (17 electrons).
- Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons, so the two isotopes have different numbers of neutrons.
- The two isotopes therefore have different mass numbers / different masses.
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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