<P> The best - known source of alpha particles is alpha decay of heavier ( > 106 u atomic weight ) atoms . When an atom emits an alpha particle in alpha decay , the atom 's mass number decreases by four due to the loss of the four nucleons in the alpha particle . The atomic number of the atom goes down by exactly two , as a result of the loss of two protons -- the atom becomes a new element . Examples of this sort of nuclear transmutation are when uranium becomes thorium , or radium becomes radon gas , due to alpha decay . </P>
<P> The best - known source of alpha particles is alpha decay of heavier ( > 106 u atomic weight ) atoms . When an atom emits an alpha particle in alpha decay , the atom 's mass number decreases by four due to the loss of the four nucleons in the alpha particle . The atomic number of the atom goes down by exactly two , as a result of the loss of two protons -- the atom becomes a new element . Examples of this sort of nuclear transmutation are when uranium becomes thorium , or radium becomes radon gas , due to alpha decay . </P>
when a radioactive atom emits an alpha particle the original atom 's mass number decreases by
when a radioactive atom emits an alpha particle the original atom 's mass number decreases by
four
four