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Isotopes (CIE AS Chemistry)

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Defining Isotopes

  • Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
  • The name of an isotope is the chemical symbol (or word) followed by a dash and then the mass number
    • Eg. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively

  • The chemical symbol of an isotope will still be shown in the usual form, as shown:

A general chemical symbol

Atomic structure notation, IGCSE & GCSE AS & A Level Chemistry revision notes

Isotopes of the same element have the same chemical symbol with a different mass number

  • For chemical symbols, isotopes have:
    • The same chemical symbol
      • e.g. isotopes of chlorine all have the symbol Cl
    • The same atomic number
      • e.g. isotopes of chlorine all have an atomic number of 17
    • A different mass / nucleon number
      • e.g. the chlorine-35 isotope has a mass number of 35, while the chlorine-37 isotope has a mass number of 37

The atomic structure and symbols of three isotopes of hydrogen

Atomic Structure Hydrogen Isotopes, downloadable AS & A Level Chemistry revision notes

All three hydrogen isotopes have the chemical symbol H, the same number of protons but a different number of neutrons

Chemical & Physical Properties of Isotopes

  • Isotopes have similar chemical properties but different physical properties

Chemical properties

  • Isotopes of the same element display the same chemical characteristics
  • This is because they have the same number of electrons in their outer shells
  • Electrons take part in chemical reactions and therefore determine the chemistry of an atom

Physical properties

  • The only difference between isotopes is the number of neutrons
  • Since these are neutral subatomic particles, they only add mass to the atom
  • As a result of this, isotopes have different physical properties such as small differences in their mass and density

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.