In essence, Alpha decay occurs when an atomic nucleus emits an alpha particle. This alpha particle consists of two protons and two neutrons bound together to form a particle. The new atom's atomic number is lowered by two and its atomic mass lowered by four.
In this example, radium decays into radon and an alpha particle. The equation for this example of alpha decay is shown below:
22688 Ra → 22286 Rn + 2 2 He
Alpha particles are the largest of the three types of radioactive emission and are often referred to as helium nuclei. Just like a helium nuclei, the mass number and atomic number of an alpha particle are the same.
Because they are so large they tend to bump into other particles in the air and so lose their energy quickly, slowing down as they collide.
Alpha radiation is not very penetrative and is easily stopped by a sheet of paper however it is the most intensely ionising radiation which is why Alpha radiation works very well with the spark counter.