Alpha decay is a type of radioactive decay where an unstable nucleus changes to another element by emitting an alpha particle, which is composed of two protons and two neutrons. The ejected particle is known as an alpha particle and is simply a helium nucleus. During alpha decay, the atomic number of the nucleus decreases by two and the mass number decreases by four. This means that the parent atom changes elements, since the number of protons changes. Alpha decay occurs most often in massive nuclei that have too large a proton to neutron ratio. Alpha particles have a relatively large mass and a positive charge, which means they cant go very far through the air or get very deep into a solid. Alpha decay is rarely used in external medical radiation therapy because alpha particles only affect surfaces. Although not very penetrating, the ingestion of a substance that undergoes alpha decay is harmful as the ejected alpha particles can damage internal tissues very easily despite its short range.