Isotopes (CIE A Level Chemistry)

• 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

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

• 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