# 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.

> Cambridge International IGCSE Chemistry (0620) — 2.3 Isotopes · Explain · 4 marks

> Uranium is a radioactive element used as a fuel in nuclear reactors. Two isotopes of uranium exist in nature: uranium-235 and uranium-238.

## Mark scheme (4 marks)

1. Both isotopes have the same number of protons / same atomic number (92)
2. Both isotopes have the same number of electrons, so they have the same electronic structure / arrangement of electrons
3. Chemical properties depend on the number / arrangement of electrons, so chemical properties are identical
4. The isotopes have different numbers of neutrons (uranium-235 has 143 neutrons; uranium-238 has 146 neutrons), giving them different masses

## Key terms

- [isotopes](https://www.gradenine.co.uk/glossary/isotopes)

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