Elementary charge is . Robert A. Millikan's oil drop experiment first measured the magnitude of the elementary charge in 1909. In some natural unit systems, such as the system of atomic units, "e" functions as the unit of electric charge, that is "e" is equal to 1 e in those unit systems. The use of elementary charge as a unit was promoted by George Johnstone Stoney in 1874 for the first system of natural units, called Stoney units. Later, he proposed the name "electron" for this unit. At the time, the particle we now call the electron was not yet discovered and
Alpha particle less likely to be referred to as "alpha particles". The best-known source of alpha particles is alpha decay of heavier (> 106 u atomic weight) atoms. When an atom emits an alpha particle in alpha decay, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom goes down by exactly two, as a result of the loss of two protons – the atom becomes a new element. Examples of this sort of nuclear transmutation are when uranium becomes thorium, or radium becomes radon gas, due