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Electron

Electron

The electron is a subatomic (smaller than an atom) particle that carries a single unit of negative electricity. All matter consists of atoms that, in turn, contain three very small particles: protons, neutrons, and electrons. Of these three, only electrons are thought to be fundamental particles, that is, incapable of being broken down into simpler particles.

The presence or absence of an excess of electrons is responsible for all electrical phenomena. Suppose a metal wire is connected to two ends of a battery. Electrical pressure from electrons within the battery force electrons in atoms of the metal to flow. That flow of electrons is an electric current.

Electron energy levels

The protons and neutrons in an atom are packed together in a central core known as the nucleus of the atom. The size of the nucleus is many thousands of times smaller than the size of the atom itself. Electrons are distributed in specific regions outside the nucleus. At one time, scientists thought that electrons traveled in very specific pathways around the nucleus, similar to the orbits traveled by planets in the solar system.

Words to Know

Electric current: A flow of electrons.

Energy level: A region of the atom in which there is a high probability of finding electrons.

Nucleus (atomic): The central core of an atom, consisting of protons and (usually) neutrons.

Positron: The antiparticle of the electron. It has the same mass and spin as the electron, but its charge, though equal in magnitude, is opposite in sign to that of the electron.

But it is known that the orbit concept is not appropriate for electrons. The uncertainty principle, a fundamental law of physics (the science of matter and energy), says that the pathway traveled by very small particles like an electron can never be defined perfectly. Instead, scientists now talk about the probability of finding an electron in an atom. In some regions of the atom, that probability is very high (although never 100 percent), and in other regions it is very low (but never 0 percent). The regions in space where the probability of finding an electron is high corresponds roughly to the orbits about which scientists talked earlier. Those regions are now called energy levels.

Electron properties

Electrons have three fundamental properties: charge, mass, and spin. By definition, the electric charge on an electron is 1. The mass of an electron has been measured and found to be 9.109389 × 1031 kilograms. Electrons also spin on their axes in much the same way that planets do. Spinning electrons, like any other moving electric charge, create a magnetic field around themselves. That magnetic field affects the way electrons arrange themselves in atoms and how they react with each other. The field is also responsible for the magnetic properties of materials.

History

During the nineteenth century, scientists made a number of important basic discoveries about electrical phenomena. However, no one could explain the fundamental nature of electricity itself. Then, in 1897, English physicist J. J. Thomson (18561940) discovered the electron. He was able to show that a flow of electric current consisted of individual particles, all of which had exactly the same ratio of electric charge to mass (e/m). He obtained the same result using a number of different materials and concluded that these particles are present in all forms of matter. The name given to these particleselectronshad actually been

suggested some years earlier by Irish physicist George Johnstone Stoney (18261911).

Although Thomson was able to measure the ratio of electric charge of mass (e/m) for an electron, he did not know how to determine either of these two quantities individually. That problem puzzled physicists for more than a decade. Finally, the riddle was solved by American physicist Robert Andrew Millikan (18681953) in a series of experiments conducted between 1907 and 1913. The accompanying figure outlines the main features of Millikan's famous oil drop experiment.

The oil drops needed for the experiment are produced by a common squeeze-bulb atomizer. The tiny droplets formed by this method fall downward and through the hole in the upper plate under the influence of gravity. As they fall, the droplets are given a negative electric charge.

Once droplets enter the space between the two plates, the highvoltage source is turned on. The negatively charged oil droplets are then attracted upward by the positive charge on the upper metal plate. At this point, the droplets are being tugged by two opposite forces: gravity, pulling them downward, and an electrical force, pulling them upward.

By carefully adjusting the voltage used, Millikan was able to keep oil droplets suspended in space between the two plates. Since the droplets moved neither upward or downward, he knew that the gravitational force on the droplets was exactly matched by the electric force. From this information, he was able to calculate the value of the electric charge on a droplet. The result he obtained, a charge of 1.591 × 1010 coulomb, is

very close to the value accepted today of 1.602177 × 1019 coulomb. (The coulomb is the standard metric unit of electrical charge.)

Quantum Number

How would you send a letter to an electron? As strange as that question seems, electrons have "addresses," just as people do.

Think of an oxygen atom, for example. Every oxygen atom has eight electrons. But those eight electrons are all different from each other. The differences among the eight electrons are represented by quantum numbers. A quantum number is a number that describes some physical property of an object (in this case, of an electron).

We know that any electron can be completely described by stating four of its properties. Those properties are represented by four different quantum numbers represented by the letters n, , m, and s. Quantum number n, for example, represents the distance of an electron from the nucleus. Any electron for which n = 1 is in the first orbit around the nucleus of the atom. Quantum number represents the shape of the electron's orbit, that is, how flattened out its orbit is. Quantum number m represents the magnetic properties of the electron. And quantum number s represents the spin of the electron, whether it's spinning in a clockwise or counter-clockwise direction.

So if you decide to send a letter to electron X, whose quantum numbers are 3, 2, 0, + ½, you know it will go to an electron in the third orbit, with a flattened orbital path, certain magnetic properties, and a clockwise spin.

The positron

One of the interesting detective stories in science involves the discovery of an electron-type particle called the positron. During the 1920s, English physicist Paul Dirac (19021984) was using the new tools of quantum mechanics to analyze the nature of matter. Some of the equations he solved had negative answers. Those answers troubled him since he was not sure what a negative answerthe opposite of some propertycould mean. One way he went about explaining these answers was to hypothesize the existence of a twin of the electron. The twin would have every property of the electron itself, Dirac said, except for one: it would carry a single unit of positive electricity rather than a single unit of negative electricity.

Dirac's prediction was confirmed only two years after he announced his hypothesis. American physicist Carl David Anderson (19051991) found positively charged electrons in a cosmic ray shower that he was studying. Anderson called these particles positrons, for posi tive electrons. Today, scientists understand that positrons are only one form of antimatter, particles similar to fundamental particles such as the proton, neutron, and electron, but with one property opposite to that of the fundamental particle.

[See also Antiparticle; Quantum mechanics; Subatomic particles ]

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electron

electron,elementary particle carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricity). The electron is one of the basic constituents of matter. An atom consists of a small, dense, positively charged nucleus surrounded by electrons that whirl about it in orbits, forming a cloud of charge. Ordinarily there are just enough negative electrons to balance the positive charge of the nucleus, and the atom is neutral. If electrons are added or removed, a net charge results, and the atom is said to be ionized (see ion). Atomic electrons are responsible for the chemical properties of matter (see valence). The name electron was first used for a unit of negative electricity by the English physicist G. J. Stoney in the late 19th cent. The actual discovery of the particle, however, was made in 1897 by J. J. Thomson, who showed that cathode rays are composed of electrons and who measured the ratio of charge to mass for the electron. In 1909, R. A. Millikan measured the charge of the electron. Combining these two results gives the mass of the electron (about 1/1,840 of the mass of the proton). Ernest Rutherford, in 1903, showed that beta rays (see radioactivity) are high-energy electrons. In 1927, Davisson and Germer, working with high-speed electron beams, discovered that electrons sometimes exhibit the wave property of diffraction. This confirmed L. V. de Broglie's hypothesis that electrons, which had previously been thought of as particles, also possess certain wave properties (see quantum theory). The wavelike properties of electrons are utilized in the electron microscope and other devices. The electron is the lightest particle having a non-zero rest mass. It belongs to the lepton class of particles and, together with its antiparticle, the positron, and its associated neutrino and antineutrino, constitutes a subfamily of the leptons. In any particle reaction involving any of the four members of the electron family, the total electron family number (+1 for ordinary particles, -1 for antiparticles) must be conserved (see conservation laws, in physics). As a consequence, an electron and a positron (total electron family number equals zero) can annihilate each other to yield two or more photons or a neutrino-antineutrino pair, but not two neutrinos (total electron family number equals two).

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electron

electron (symbol e) Stable elementary particle with a negative charge (−1.602 × 10−19C), a rest mass of 9.1 × 10−31 kg and a spin of Aw fermion. J. J. Thomson first identified electrons in 1897. They are not made up of smaller particles and are one of the three primary constituents of atoms. They form orbitals that surround the positively charged nucleus. In a free atom, the electrons' total negative charge balances the positive charge of the protons in the nucleus. Removal or addition of an atomic electron produces a charged ion. Free electrons (not bound to an atom) produce electrical conduction. Electronic devices, such as cathode-ray tubes, oscilloscopes and electron microscopes, use beams of electrons. An electron is a lepton (light particle). Its anti-particle is the positron (positive particle). See also Broglie, Prince Louis Victor de; chemical bond; matter; neutrons; particle physics; photoelectric effect; valence

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electron

electron Elementary particle of mass 9.11 × 10−31kg and negative electrical charge of 1.602×10−19C (coulombs). Electrons can exist independently, or in groups around the nucleus of an atom. Experiments show that electrons in an atom may occur at a range of distances from the nucleus but are most likely to exist in certain low-energy orbits or shells, and within these shells there are further subshells, the configuration being such that no two electrons in any one atom have identical properties. When an electron moves from one subshell to another of lower energy, electromagnetic radiation is given off; if an electron moves to a subshell of higher energy, electromagnetic radiation is absorbed. An electron moves about the nucleus in a circular or elliptical orbit and also spins on its axis.

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electron

e·lec·tron / iˈlekˌträn/ • n. Physics a stable subatomic particle with a charge of negative electricity, found in all atoms and acting as the primary carrier of electricity in solids.

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electron

electron (phys.) elementary particle with a negative charge of electricity. XIX (applied to the unit of electric charge). f. ELECTRIC + -on of ION.
Hence electronic, electronics XX.

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electron

electron An elementary particle present in all atoms in groupings called shells around the nucleus. When electrons are detached from the atom they are called free electrons.

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electron

electron (i-lek-tron) n. a negatively charged particle in an atom, one or more of which orbit around the positively charged nucleus of the atom.

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electron

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