What is the general equation for the alpha-decay of a nucleus X, with nucleon number A and proton number Z, into nucleon Y??
AZ X --> A - 4z - 2Y + 42α
During alpha-decay, the nuclus emits an alpha particle which consists of two protons and two neutrons (a totoal of 4 nucleons). Therefore the new nuclide formed will have a nucleon number which is decreased by 4, and a proton number decreased by 2.
