alpha (a )  particles    Two  neutrons  and  two  protons held together  by  nuclear forces. An a particle is equivalent to  helium  nucleus.  These  are  emitted  when radioactive decay  of  certain heavy nuclei takes place (process known as a - decay). e.g.

       ₉₂U²³⁸ ® ₉₀Th²³⁴ + _2a ⁴

The  difference  of  mass  in  the  two  sides of the reaction  appears  as kinetic energy of the a - particle and the nucleus.

While passing through a medium, it quickly loses its kinetic energy, the rate of decrease of kinetic energy of a particle  in  a medium being proportional to the square of the   charge . Therefore it has short ranges in air (~cm).