Any nuclear reaction in which mass number decreases by four and atomic number decreases by two is an alpha decay.
When nuclei undergo fission, they decay via #alpha#-decay, #beta#-decay or a #gamma#-decay.
In an #alpha#-decay, the atomic number (i.e., the number of protons or electrons in a neutral atom) of the parent nucleus decreases by two units while the mass number (the number of protons and neutrons put together) of the nucleus decreases by four units and an #alpha#-particle is emitted, which basically is a Helium-4 nucleus.
Consider the following examples:
#""_92^238 U -> ""_90^234 Th + ""_2^4 He#
#""_8^16 O + gamma -> ""_6^12 C + ""_2^4 He#
Here, in both the reactions, the parent nuclei breakup to emit an alpha particle (a #""^4 He# nucleus) along with the daughter nucleus. Whereas the first reaction occurs as the # ""_92^238 U# is a comparatively unstable nucleus, in the second reaction, energy is to be given from outside to force the highly stable #""_8^16 O# to breakup in to #""_6^12 C# and an #alpha#. The second example may also be written in short as #""_8^16 O(gamma,alpha)""_6^12 C# reaction.
