There's three types of decay: alpha beta and gamma.
*Alpha decay is the emission of a helium nucleus (2protons and 2 neutrons)
*Beta decay is the emission of an electron or a positron ([tex] e^{-1} [/tex] or [tex] e^{+1} [/tex] ). It does affect the weight of the atom.
*Gamma decay is the emission of photons with a high energy. It does affect the weight of the atom.
To answer the question, any nuclear reaction which decreases the atomic weight by 4 and the protons by 2 is an alpha decay. (release of an alpha or helium nucleus)
