The basic structure of an atom is a small central nucleus, made up of protons and neutrons, surrounded by electrons.  The nuclei of radioactive substances are unstable.  They become stable by radioactive decay.  In radioactive decay, the atoms emit radiation and turn into other elements.  The three types of radiation emitted are alpha, beta and gamma.

We cannot predict when an unstable nucleus will decay. It is a random process and is not affected by external conditions.  Background radiation is around us all the time.  Background radiation comes from radioactive substances in the environment, from space, and from special devices such as X-ray tubes.


At one time, scientists thought that atoms consisted of spheres of positive charge with electrons stuck to them.  This became known as the 'plum pudding' model of the atom.  Then Rutherford, Geiger and Marsden devised an alpha particle scattering experiment, in which they fired alpha particles at thin gold foil.  Most of the alpha particles passed straight through the foil, which would suggest that most of an atom is just empty space.  Some of the alpha particles were deflected through small angles, which suggests that the nucleus has a positive charge.  Other alpha particles were deflected through very large angles, which suggests that the nucleus has a large mass and a large positive charge.


Alpha decay:

  • The nucleus loses 2 protons and 2 neutrons
  • 2 protons and 2 neutrons are emitted as an alpha particle
  • Alpha particles have a relative mass of 4 and a relative charge of +2

Beta decay:

  • A neutron in the nucleus changes into a proton and an electron
  • The electron created in the nucleus is instantly emitted
  • A beta particle is a high-speed electron with a relative mass of 0 and a relative charge of -1
  • The proton stays in the nucleus, so the atomic number goes up by one, but the mass number is unchanged

Gamma decay:

  • When a nucleus emits gamma radiation, there is no change in the atomic number or the mass number.  This is because a…


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