Alpha particles are 2 protons and 2 neutrons, the nucleus of a helium atom. they are created when radioactive substances decay producing the particle, in this process the substance's mass goes down by 4 and its atomic mass by 2, creating a new element. they are the largest form of radiation, they are slow and therefore don't penetrate far into any materials, even air. they are strongly ionising, meaning they create a lot of ions, positively charged atoms, when they penetrate a material.