Alpha particles are helium-4 nuclei, made up of two protons and two neutrons. These particles are ejected by large nuclei, but they can also be used as projectiles to bombard nuclei and trigger nuclear processes.
Alpha particles are positively charged and are deflected by electric and magnetic fields. Their penetrating power that low, being stopped by a sheet of paper.
The following nuclear equation represents the emission of alpha particles by a uranium-238 nucleus with formation of thorium-234.
\begin{equation} ^{238}_{92}U\rightarrow ^{234}_{90}Th+^{2}_{4}He \end{equation} As can be seen, the sum of atomic and mass numbers coincides on both sides of the reaction.