The electron is a negatively charged subatomic particle which is an important component of the atoms which make up ordinary matter. The electron is fundamental, in that it is not believed to be made up of smaller constituents. The size of the charge on the electron has for many years been considered the fundamental unit of charge found in nature. All electrical charges were believed to be integral multiples of this charge. Recently, however, considerable evidence has been found to indicate that particles classified as mesons and baryons are made up of objects called quarks, which have charges of either 2/3 or 1/3 the charge on the electron. For example, the neutrons and protons, which make up the nuclei of atoms, are baryons. However, scientists have never been able to observe an isolated quark, so for all practical purposes the charge on the electron can still be considered the fundamental unit of charge found in nature. The magnitude of this charge, usually designated by e, has been measured very precisely and is 1.602177 × 10-19 coulombs. The mass of the electron is small even by atomic standards and has the value 9.109389 × 10-31 kg (0.5110 M V/c2 e , being only about 1/1836 the mass of the proton.
All atoms found in nature have a positively charged nucleus about which the negatively charged electrons move. The atom is electrically neutral and thus the positive electrical charge on the nucleus has the same magnitude as the negative charge due to all the electrons. The electrons are held in the atom by the attractive force exerted on them by the positively charged nucleus. They move very rapidly about the nucleus in orbits which have very definite energies, forming a sort of electron cloud around it. Some of the electrons in a typical atom can be quite close to the nucleus, while others can be at distances which are many thousands of times larger than the diameter of the nucleus. Thus, the electron cloud determines the size of the atom. It is the outermost electrons that determine the chemical behavior of the various elements. The size and shape of the electron clouds around atoms can only be explained utilizing a field of physics called quantum mechanics.
In metals, some of the electrons are not tightly bound to atoms and are free to move through the metal under the influence of an electric field. It is this situation that accounts for the fact that most metals are good conductors of electricity and heat.
Quantum theory also explains several other rather strange properties of electrons. Electrons behave as if they were spinning, and the value of the angular momentum associated with this spin is fixed; thus it is not surprising that electrons also behave like little magnets. The way electrons are arranged in some materials, such as iron, causes these materials to be magnetic. The existence of the positron, the antiparticle of the electron, was predicted by French physicist Paul Dirac in 1930. To predict this antiparticle, he used a version of quantum mechanics which included the effects of the theory of relativity. The positron's charge has the same magnitude as the electron's charge but is positive. Dirac's prediction was verified two years later when the positron was observed experimentally by Carl Anderson in a cloud chamber used for research on cosmic rays. The positron does not exist for very long in the presence of ordinary matter because it soon comes in contact with an ordinary electron and the two particles annihilate, producing a gamma ray with an energy equal to the energy equivalent of the two electron masses, according to Einstein's famous equation E = mc2.
As has been the case with many developments in science, the discovery of the electron and the recognition of its important role in the structure of matter evolved over a period of almost 100 years. As early as 1838, English physicist Michael Faraday found that when a charge of several thousand volts was applied between metal electrodes in an evacuated glass tube, an electric current flowed between the electrodes. It was found that this current was made up of negatively charged particles by observing their deflection in an electric field. Credit for the discovery of the electron is usually given to the English physicist J. J. Thomson. He was able to make quantitative measurements of the deflection of these particles in electric and magnetic fields and measure e/m, the ratio of their charge to mass.
Later, similar measurements were made on the negatively charged particles emitted by different cathode materials and the same value of e/m was obtained. When the same value of e/m was also obtained for "electrons" emitted by hot filaments (called thermionic emission) and for photoelectrons emitted when light hits certain surfaces, it became clear that these were all the same type of particle, and the fundamental nature of the electron began to emerge. From these and other measurements it soon became known that the charge on the electron was roughly 1.6 × 10-19 coulombs. But the definitive experiment, which indicated that the charge on the electron was the fundamental unit of charge in nature, was carried out by Robert A. Millikan at the University of Chicago between 1907 and 1913. A schematic diagram of this famous "oil drop" experiment is shown in Figure 1. Charged oil drops, produced by an atomizer, were sprayed into the electric field maintained between two parallel metal plates. By measuring the terminal velocity of individual drops as they fell under gravity and again as they rose under an applied electric field, Millikan was able to measure the charge on the drops. He measured the charge on thousands of drops and was able to follow some drops for long periods of time and to observe changes in the charge on these drops produced by ionizing x rays. He observed many drops with only a single electronic charge and never observed a charge that was not an integral multiple of this fundamental unit. Millikan's original measurements gave a value of 1.591 × 10-19 coulombs. These results do not prove that nonintegral charges do not exist, but because many other different experiments later confirmed Millikan's result, he is generally credited with discovering the fundamental nature of the charge on the electron, a discovery for which he received the Nobel Prize in physics in 1923.
See also Neutron; Subatomic particles.
Robert L. Stearns
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