Isotopes (Cambridge International A-Level Chemistry)

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

  • E.g. 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

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

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

• The chemical behaviour of an atom depends on its electron configuration, especially the electrons in the outer (valence) shell

• This is because all isotopes of the same element have the same number of electrons and the same electron arrangement, they:

  • Form the same types of bonds
  • React with the same elements
  • Show identical chemical reactivity

Physical properties

• Although isotopes have identical chemical properties, they differ in physical properties because they have different numbers of neutrons

• Neutrons add mass to the nucleus but carry no charge and don’t influence chemical bonding

• As a result, isotopes have:

  • Different relative atomic masses
  • Slight differences in density

Exam Hint

• These small differences in physical properties become important in processes like:

  • Mass spectrometry (detecting different isotopes by mass)
  • Isotope separation (e.g. in nuclear chemistry)
  • Medical imaging (radioisotopes rely on differences in physical properties)