Uranium has two naturally occurring isotopes: uranium-235 and uranium-238. Explain why these two isotopes have identical chemical properties but different physical properties such as mass.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Uranium is a radioactive element used as a fuel in nuclear reactors. Two isotopes of uranium exist in nature: uranium-235 and uranium-238.
Model answer (4 marks)
Both isotopes have the same number of protons (atomic number 92) and therefore the same number of electrons, giving them identical electronic structures. Chemical properties depend on the number and arrangement of electrons, so the two isotopes behave the same chemically. The difference lies in the number of neutrons: uranium‑235 has 143 neutrons, uranium‑238 has 146 neutrons, giving them different masses and physical properties.
Examiner tips
- State the common atomic number and electron count first; link this to identical chemical behaviour. Then mention the different neutron numbers to explain mass differences. Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing mass number with atomic number; writing that the isotopes have different numbers of protons. Including unnecessary details about nuclear reactions or radioactivity.
Mark scheme (4 marks)
- Both isotopes have the same number of protons / same atomic number (92)
- Both isotopes have the same number of electrons, so they have the same electronic structure / arrangement of electrons
- Chemical properties depend on the number / arrangement of electrons, so chemical properties are identical
- The isotopes have different numbers of neutrons (uranium-235 has 143 neutrons; uranium-238 has 146 neutrons), giving them different masses
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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