The Alpha Particle, a positively charged nuclear particle, consists of two
protons bound to two neutrons. Alpha particles are emitted spontaneously in some types of
radioactive decay. They are also produced when Helium-4 atoms are completely ionised.
Alpha particles are also known as Helium nuclei because of its chemical properties.
Because Alpha particles consist of neutrons and protons they can only be emitted from
the nucleus of an atom (the nucleus consists only of protons and neutrons). Loss of an
Alpha particle by a nucleus results in the formation of a new nucleus, lighter than the
original by four mass units (the masses of the neutron and of the proton are about one
unit each). An atom of the uranium isotope of mass 238, upon emitting an Alpha particle,
becomes an atom of another element of mass 234.
Each of the two protons that form part of the Alpha particle emitted from an atom of
uranium-238 possesses a unit of positive electric charge. Because the charge on the
uranium-238 nucleus decreases by two units as a result of Alpha emission, the atomic
number of the newly formed atom is 2 fewer than that of the original, which was 92. The
new atom has an atomic number of 90 and hence is an isotope of the element Thorium.
92238U => 90234Th + 24He
Usually very large (and heavy) nuclei emit Alpha particles spontaneously.