Alpha Decay
Alpha decay is one process that unstable atoms can use to become more stable. During alpha decay, an atom's nucleus sheds two protons and two neutrons in a packet that scientists call an alpha particle.
Since an atom loses two protons during alpha decay, it changes from one element to another. For example, after undergoing alpha decay, an atom of uranium (with 92 protons) becomes an atom of thorium (with 90 protons).
Alpha decay is a form of radioactive decay in which an atomic nucleus ejects an alpha particle through the electromagnetic force and transforms into a nucleus with mass number 4 less and atomic number 2 less. For example:
this is usually written as;
Note that an alpha particle IS a helium nucleus, and that both mass number and atomic number are conserved. Alpha decay can essentially be thought of as nuclear fission where the parent nucleus splits into two daughter nuclei. Alpha decay is fundamentally a quantum tunneling process. Unlike beta decay, alpha decay is governed by the strong nuclear force.
Alpha particles with their typical kinetic energy of 5 MeV have a speed of 15,000 km/s.
Because of alpha decay, virtually all of the helium produced in the United States and elsewhere comes from trapped underground deposits associated with minerals containing uranium or thorium, and brought to the surface as a by-product of natural gas production.
Alpha particles emitted by radioactive nuclei are among the most hazardous forms of radiation, if these nuclei are incorporated within a human body. As any heavy charged particle, alpha particles lose their energy within a very short distance in dense media, causing significant damage to surrounding biomolecules. On the other hand, external alpha irradiation is not harmful because alpha particles are completely absorbed by a very thin (micrometers) dead layer of skin as well as by few centimeters of air. However, if a substance radiating alpha particles is injected, ingested or inhaled by an organism it may become a risk, potentially inflicting very serious damage to the organisms' genetic makeup.