what is an alpha particle

An alpha particle is a type of radiation consisting of two protons and two neutrons bound together, identical to a helium-4 nucleus. It is generally produced in the process of alpha decay, but may also be produced in other ways. Alpha particles are relatively slow and heavy compared to other forms of nuclear radiation, traveling at 5 to 7% of the speed of light and having a mass approximately equivalent to 4 protons. They have low penetrating power and a short range, being stopped by a few centimeters of air or less than a tenth of a millimeter of biological tissue. Due to their double positive charge, large mass, and relatively slow speed, alpha particles are highly ionizing and can cause multiple ionizations within a very small distance. They are emitted from the nuclei of some radioactive elements and are hazardous when an alpha-emitting isotope is inside the body. Alpha particles were crucial in Ernest Rutherfords discovery of the atomic nucleus, which was the foundation of his atomic model.