Nuclear Stability

Distinguish between stable and radioactive isotopes and describe the conditions under which a nucleus is unstable

  • Isotopes: Atoms with the same number of protons, but with different numbers of neutrons.
  • Isotopes have the same atomic number and are the same element, but have different mass numbers.
  • Isotopes are generally named by their mass numbers, such as rubidium-85 and rubidium-87.
  • Isotopes are often represented using the symbol: 

    • M is the chemical symbol for the element.
    • A is the mass number.
    • Z is the atomic number.
  • Stable isotopes: Isotopes that have stable nuclei and do not emit radiation.
  • Radioisotopes: Isotopes that have unstable nuclei and emit radiation.
  • For some elements, all isotopes are radioactive, such as uranium and radium.
  • For some elements, only one or some isotopes are radioactive, such as rubidium and rhenium, which each have two isotopes, with only one being radioactive.
  • The nucleus of an isotope can be unstable for several reasons:
    • If there are too many neutrons for the number of protons, a neutron changes to a proton and an electron, with the emitted electron being known as a negative beta (β-) particle. This occurs in the case of cobalt-60.

    • If there are too many protons for the number of neutrons, a proton changes to a neutron and a positron, with the emitted positron being known as a positive beta (β+) particle. This occurs in the case of sodium-22.

    • If there are too many protons and neutrons (the nucleus is too heavy), two protons and two neutrons are emitted from the nucleus as a helium nucleus or an alpha (α) particle, such as in the case of radon.

Plot of atomic isotopes with Z protons and N neutrons, coloured according to stability (half-life)

Easychem - Nuclear Stability