Electron is an elementary subatomic particle with negative electric charge, fundamental constituent of the atom, in which one or more electrons, in different number according to the chemical element, form the cloud of negatively charged particles surrounding the positive nucleus and determining all chemical properties of the element. It is the constituent of electric current in solid conducting materials.
The term electron derives from the Greek word ήλεκτρον (pronounced électron), whose meaning is amber. This name is historically due to the fact that amber played a key role in the discovery of electrical phenomena: in particular since the seventh century BC the ancient Greeks were aware of the fact that rubbing an object of amber or ebonite with a woollen cloth, the object in question acquired the ability to attract light particles, such as grains of dust. This experimental evidence was taken up in the 16th century by William Gilbert, who identified several substances, including diamond and sulfur, that exhibited the same behavior as amber. He gave the name “electrical force” to the force that attracted the corpuscles, and called “electrified” materials that exhibited this property.
Together with protons and neutrons, it is a component of the atom and, although it contributes to its total mass for less than 0,06%, it sensibly characterizes its nature and determines its chemical properties: the covalent chemical bond is formed as a consequence of the redistribution of the electronic density between two or more atoms. The electron motion generates a magnetic field, while the variation of its energy and acceleration cause the emission of photons; it is also responsible for the conduction of electric current and heat.
Most of electrons present in the universe was produced by Big Bang, but they can be generated also by beta decay of radioactive isotopes and in high energy collisions, while they can be annihilated by collision with positrons or absorbed in a stellar nucleosynthesis process.
The advent of electronics and the related development of information technology have made the electron a major player in the technological development of the twentieth century. Its properties are exploited in a variety of applications, such as cathode ray tubes, electron microscopes, radiotherapy and lasers.
Fundamental properties of the electron
The electron has a rest mass of 9.1093837015(28)×10-31 kg, which, according to the mass-energy equivalence principle, corresponds to a rest energy of 0.511 MeV, with a ratio with respect to the proton mass of about 1 to 1836. It is the lightest known stable subatomic particle among those with electric charge.
Astronomical measurements have shown that the ratio between proton and electron masses has remained constant for at least half of the age of the universe, as it is predicted in the standard model.
The electron has an electrical charge of -1.602176634×10-19 C, defined as “elementary charge” and used as the standard unit for the charge of subatomic particles. Within the limits of experimental error, the value of the electron charge is equal to that of the proton, but with the opposite sign. The value of the elementary charge is indicated with the symbol e, while the electron is commonly indicated with the symbol e–, where the minus sign indicates the fact that this particle has negative charge; similarly, for its antiparticle, the positron, which has the same mass and charge of opposite sign, is used as symbol e+.
The electron has no known substructure and is described as a material point, since experiments made with Penning trap have shown that the upper limit for the particle radius is 10-22 meters.
There is also a physical constant, the classical radius of the electron, which corresponds to a value of 2.8179×10-15 m; however, this constant is derived from a calculation that neglects the quantum effects present. The electron is believed to be stable because, since the particle possesses unitary charge, its decay would violate the law of conservation of electric charge. The experimental lower limit for the mean lifetime of the electron is 4.6×1026 years, with a 90% confidence interval.
The idea of a discontinuous structure of electricity originated in the years between 1831 and 1834, with the research of M. Faraday on the phenomenon of electrolysis. Faraday himself in his writings talked about “discrete quantities of electricity associated to atoms” or “atoms of electricity”. G.J. Stoney, in 1874, recognized more explicitly that “in electrolysis phenomena nature presents a single definite quantity of electricity, independent of the particular body that is considered” and indirectly determined the numerical value. While in the electrolytic phenomena the elementary charge did not show its individuality as a particle isolable from the atom, with the progress of the vacuum technique was possible to identify it as an elementary particle.
The studies started from the experiments on cathode rays performed by J. Plücker (1857), W. Hittorf (1869), J. J. Thomson (1894-97) and J. B. Perrin (1895). The characteristics of such rays are explained by admitting that they consist of identical particles in motion, negatively charged. Even before the identification of these particles was certain and complete G. J. Stoney proposed, in 1891, to call them electrons.
Experiments based on the phenomenon of condensation of vapors on ions and the deflection of their traces by electromagnetic fields were performed by Thomson, J. Townsend and others between 1897 and 1910, but the best result was obtained by R. A. Millikan in a famous experience of 1909. The main characteristics of electron were identified when, in 1924, L. de Broglie advanced the hypothesis that we should associate to electron a wave of length λ=h/p with Planck constant “h” and “p” momentum of particle. This hypothesis emerged from the critical re-examination of the Bohr-Sommerfield atomic model and brought with it the crisis of the purely corpuscular model of matter; indeed it frustrated the research of a geometric and mechanical model at atomic or subatomic level.
In 1927 C. J. Davisson and L. H. Germer confirmed this hypothesis with electron diffraction experiments. To explain the Zeeman effect, observed since 1896, in 1925 G. E. Uhlenbeck and S. A. Goudsmit introduced the hypothesis of the rotating electron, which justified the assignment of an intrinsic magnetic moment to the electron: this explained the phenomena related to the splitting of the atomic spectral lines in the presence of magnetic fields. Finally, in 1928, P. Dirac, in establishing a relativistic theory of electron, was faced with the need to assume the existence of an antiparticle for electron, that is a positive electron. C. D. Anderson, in 1932, identified the presence of these positive electrons (positrons) in cosmic rays.