An alpha particle (α-particle) is a positively charged particle with absolute value of charge 2e, where e is the elementary charge. An α-particle is in fact the nucleus of the helium-4 isotope, consisting of two protons and two neutrons, thus having a mass close to 4 u (u stands for unified atomic mass unit). More precisely: mα = 4.001 506 179 127 u.
Discovered and named in 1899 by Ernest Rutherford, alpha radiation (α-radiation) was used by him and co-workers in experiments that probed the structure of atoms in thin metallic foils, work that resulted in the first conception of the atom as a heavy nucleus with light electrons orbiting the nucleus (1909–1911). Later Rutherford and collaborators bombarded nitrogen with α-particles, changing it to oxygen, producing in 1919 the first artificial nuclear transmutation.
In 1899 Rutherford determined several properties of the "uranium rays" (thus named because the most common uranium isotope, 238U, is an α emitter and uranium salts were used as a source of α-radiation), but at that time the cause and origin of the radiation emitted by uranium was an enigma. He discovered that there were two kinds of radiation involved, which he called α and β radiation. In retrospect, the nuclear reactions in the uranium salt likely to have been involved were:
Ten years later Rutherford knew the nature of the α-particle: Hans Geiger and he wrote a paper on its nature and charge, in which it was explained that α-particles are helium atoms that have lost their negative charge. In 1908 it was not yet known that an atom consists of a nucleus plus orbiting electrons, though the existence of atoms had already been firmly established by then.
Copy of Rutherford's 1899 paper Retrieved June 12, 2009.