Electron
From Encyclopædia
The electron is a negatively charged subatomic particle and a constituent of all ordinary matter. Electrons are responsible for many physical and chemical properties of matter, such as the formation of chemical compounds. They are FUNDAMENTAL PARTICLES in that they are not now thought to be composed of simpler particles in the way that, for example, protons are composed of quarks.The electron was the first subatomic particle to be discovered. The discovery was made by the British physicist Sir
Joseph John THOMSON in 1897, though analysis of CATHODE RAYS formed in the discharge of
electricity through gases. By deflecting the rays with electric and magnetic forces, Thomson showed that they are a stream of negatively charged particles, all of which have the same ratio of electric charge to
Mass. He also showed that the particles, for which he adopted the name electrons, had
Mass thousands of times smaller than those of the
atomS from which they emerged. Recent precise measurements show the charge of an electron to be 1.60218 X 10 (to the -19th power) COULOMBS, and the
Mass of an electron to be 9.10939 X 10 (to the -31st power) kilograms.Electrons in AtomsEach neutral
atom contains as many electrons as it has protons in its
nucleus. The electrons are bound to the
nucleus by their mutual electrostatic forces (see
electrostatics). Electrons occupy most of the volume of
atoms, although they contribute only a small part of the atomic
Mass. The spatial distribution of the electrons in an
atom follows rules discovered in the 1920s by Niels BOHR, Wolfgang PAULI, and others (see ELECTRON CONFIGURATION). The
average distance of the outer electrons from the
nucleus is a few tens of thousands of nanometers (one nanometer is a billionth of a meter) in all
atoms, while the inner electrons in heavy
atoms are much closer to the
nucleus. Explaining this size of
atoms is one of the important accomplishments of QUANTUM
mechanics. The tendency of
atoms to resist interpenetration, which is responsible for the solidity of matter, is a consequence of Pauli's EXCLUSION PRINCIPLE, which holds that two electrons cannot occupy the same point in space.When two or more
atoms are near each other, their electrons can bind them into stable configurations known as
moleculeS (see CHEMICAL BOND). This binding can take place by having different
atoms share some electrons (covalent bonds) or by the transfer of electrons from one
atom to another (ionic bonds).Unbound Electrons in
metals and PlasmasIn crystalline solids, such as
metals (see
crystal), some of the electrons are detached from
atoms and move almost freely through the whole solid. These unattached electrons are responsible for such characteristic properties of
metals as electrical conductivity (see CONDUCTION, ELECTRICAL). Unbound electrons are also found in PLASMAS, a form of matter that exists at high temperatures inside
stars.Quantum Theory of ElectronsIn addition to charge and
Mass, electrons have properties that can be understood only with the use of quantum theory. One such
property is spin, which can be thought of as rotation about an axis passing through the electron. Electron spin, however, does not behave entirely in that simply manner. Its magnitude is fixed to be h/4pi, where h is PLANCK'S
constant. Furthermore, the projection of electron spin along any direction-for example, can only be plus or minus h/4pi. Spinning electrons act as small magnets, a fact that plays an important role in such phenomena as ferromagnetism (see MAGNETISM).According to quantum theory, electrons and all other subatomic particles have wave properties. This was first suggested by the French physicist Louis DE BROGLIE in 1923, and later verified in experiments by Clinton DAVISSON, Lester Germer, and G.P. Thomson (the son of J.J. Thomson), who showed that electron beams could be made to undergo DIFFRACTION just as
light beams do. The de Broglie wave associated with any particle has a wavelength equal to h/p, where p is the particle's momentum. For an electron with an energy of an ELECTRON VOLT, this wavelength is a few nanometers, the size of an
atom?itself. Thus the wave properties of electrons should have important effects for electrons in
atoms. This idea was realized in 1926 by Erwin SCHRODINGER, when he invented wave
mechanics and used it to give detail and accurate description of atomic properties. An interpretation of de Broglie waves as defining the
probability of finding the electron at various positions in space was given in 1927 by Max BORN.Another remarkable consequence of quantum
mechanics was the prediction of the existence of the POSITRON, the antiparticle of the electron (see ANTIMATTER). Positrons have the same
Mass and spin as electrons but opposite charge. They were predicted by Paul DIRAC in 1930 and discovered by Carl David ANDERSON in 1932. Electrons and positrons can be created in pairs--for example, when GAMMA RAYS pass through matter. These positrons do not survive for long in ordinary matter, since a positron can be annihilated in a very short time along with any electron that it encounters. The total energy of the electron and positron is converted into that of gamma
radiation. For the short time that they coexist before annihilation, positrons and electrons sometimes combine to form a simple type of
atom known as positronium.Technological Applications of ElectronsBecause of their electric charge and small
Mass, electrons are easy to manipulate with electric and magnetic forces. When electrons are accelerated, as in a radio antenna, they emit
light (see
electromagnetic radiation). These facts have led to an immense
number of technological applications of electrons (see
ELECTRONICS). One example is the
television tube, a development of the CATHODE-RAY TUBE in which electrons were first discovered. Another is the ELECTRON MICROSCOPE, which uses the fact that the wavelengths of the de Brolie waves associated with electrons can be much smaller than those of visible
light, and therefore can resolve otherwise invisible objects such as viruses.Gerald FeinbergBibliography: Anderson, D. L., and Cohen, I. B., eds., The Discovery of the Electron (1981); Hughes, I. S., Elementary Particles, 2d ed. (1985); Kitaigorodsky, A. K.,
physics for Everyone: Electrons (1981); Misell, D. L., and Brown, E. B., Electron Diffraction (1988); Pauling, Linus, The Nature of the Chemical Bond and the Structure of
molecules and
crystals, 3d ed. (1960); Tuck, B., and Christopoulos, C., Physical
electronics (1986); Wolkenstein, T., Electrons and
crystals (1985).
