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 × 10⁻³¹ 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 (1856–1940) 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 particles—electrons—had actually been

suggested some years earlier by Irish physicist George Johnstone Stoney (1826–1911).

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 (1868–1953) 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 × 10⁻¹⁰ coulomb, is

very close to the value accepted today of 1.602177 × 10⁻¹⁹ 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, ℓ, m_ℓ, 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 ℓ represents the shape of the electron's orbit, that is, how flattened out its orbit is. Quantum number m_ℓ 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 (1902–1984) 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 answer—the opposite of some property—could 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 (1905–1991) 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 ]

Electron

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.

History

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 oftime 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

KEY TERMS

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.

Quarks

—Believed to be the most fundamental units of protons and neutrons.

Terminal velocity

—Since the air resistance force increases with velocity, all objects falling in air reach a fixed or terminal velocity when this force directed upward equals the force of gravity directed down.

Electron

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; that means 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. 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 mass of the electron is about onethousandth that of the smallest atom.

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 (contributed by protons) 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; their exact location at any moment in time is expressed in terms of probability. Thus, for a given electron, it can be more likely that it will be found in some locations than in others. The total probabilities can be drawn as an electron cloud, which is densest nearer to the nucleus and progressively less dense at an increasing distance from the nucleus. 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, which allows them to 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.

Robert L. Stearns

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

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.