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الکترون - Electron

Electron

I

INTRODUCTION

Electron, negatively charged particle found in an atom. Electrons, along with neutrons and protons, comprise the basic building blocks of all atoms. The electrons form the outer layer or layers of an atom, while the neutrons and protons make up the nucleus, or core, of the atom. Electrons, neutrons, and protons are elementary particles—that is, they are among the smallest parts of matter that scientists can isolate. The electron carries a negative electric charge of –1.602 x 10-19 coulomb and has a mass of 9.109 x 10-31 kg. See also Atom.

Electrons are responsible for many important physical phenomena, such as electricity and light, and for physical and chemical properties of matter. Electrons form electric currents by flowing in a stream and carrying their negative charge with them. All electrical devices, from flashlights to computers, depend on the movement of electrons. Electrons also are involved in creating light. The electrons in the outer layers of the atom sometimes lose energy, emitting the energy in the form of light. Because electrons form the outer layers of atoms, they are also responsible for many of the physical and chemical properties of the chemical elements. Electrons help determine how atoms of an element behave with respect to each other and how they react with atoms of other elements. See also Chemistry.

II

ELECTRONS AS ELEMENTARY PARTICLES

The electron is one of the most fundamental and most important of elementary particles. The electron is also one of the few elementary particles that is stable, meaning it can exist by itself for a long period of time. Most other elementary particles can exist independently for only a fraction of a second.

Electrons are among the smallest of all elementary particles and have no detectable shape or structure. At the same time, they do have a property that scientists can measure called spin, or intrinsic angular momentum. An electron’s spin makes it act as a tiny magnet. Electrons can spin clockwise or counterclockwise.

The electron is affected by three of the four fundamental forces that define the nature and interaction of everything in the universe: gravitation, the electromagnetic force, and the weak nuclear force. Gravitation is the attractive force between every object in the universe that has mass. Gravitation affects the electron because the electron has mass. The electromagnetic force affects objects with an electric charge, so the electron’s negative electric charge subjects it to the forces of electromagnetism. The electron attracts positively charged particles, such as protons, and repels negatively charged particles, such as other electrons. The electron is also sensitive to the weak nuclear force, a very feeble force that affects certain types of elementary particles and is only important over very short distances. The one fundamental force that does not affect the electron is the strong nuclear force, which is the force that binds protons and neutrons in the atom’s nucleus

III

ELECTRONS IN ATOMS

An atom consists of neutrons and protons packed into a dense nucleus with electrons orbiting around the nucleus. The neutrons have no electrical charge, while each proton carries a positive charge that is equal and opposite to the negative charge of the electron. Each chemical element is defined by the number of protons in the nucleus of its atoms; this number is the element’s atomic number. The electrons are equal in number to the protons in the atom, balancing the electrical charge of the nucleus. In other words, the atom’s net charge is zero, and the atom is said to be neutral.

A

Electron Orbitals

Scientists cannot simultaneously measure both the exact location of an electron and its precise speed and direction, so they cannot measure the path a specific electron takes as it orbits the nucleus. The law of physics governing this phenomenon is called the uncertainty principle. Scientists can, however, determine the area an electron will probably occupy, and the probability of finding the electron at some place inside this area. A map of this area and its probabilities forms a cloudlike pattern known as an orbital. Each orbital can contain two electrons, but these electrons can not have identical properties, so they must spin in opposite directions. Orbitals are grouped into shells, like the layers of an onion, around the nucleus. Each shell can contain a limited number of orbitals, which means that each shell can contain a limited number of electrons. Each shell corresponds to a certain level of energy, and all the electrons in the shell have this same level of energy. As the shells get farther from the nucleus, they can contain more electrons, and the electrons in the shells have higher energy. See also Chemistry: Electron Cloud.

B

Electrons and Light Emission

When an atom’s energy is at its minimum, it is said to be in a ground state. In this ground state, the atom’s electrons occupy the innermost available shells, those closest to the nucleus. When atoms are excited by heat, by an electric current, or by light or some other form of radiation, the atoms’ electrons can acquire energy and jump from an inner to an outer shell, leaving a vacancy in the inner shell. The atom seeks to shed this surplus energy, leading the electron in the outer orbit to fall back down to an inner vacancy. As it falls, the electron releases energy in the form of a photon, a tiny flash of light. The color of the light depends on the amount of energy emitted.

When an electron moves to a different shell, it does not gradually go from one shell to another, but instead jumps directly to the other shell. These jumps are like steps on a staircase (and are different from a smooth incline, or hill). The electron also absorbs or emits the energy to make jumps in steps. It cannot gradually build up or lose energy, but must instantly absorb the exact amount of energy needed to make a certain jump, or instantly emit the exact amount needed to fall to a lower shell. Each element has a different pattern of allowed jumps within its electronic structure, so the element’s atoms can only absorb or emit a distinct set of energies, or spectrum of colors. In this way, a scientist can tell which elements are present in a sample by looking at the colors absorbed or emitted when the sample is excited by heat, electricity, or light. See also Spectroscopy

IV

ELECTRONS AND CHEMICAL BONDING

The electrons in the valence, or outermost, shell of atoms determine the chemical behavior of most elements. The atoms of noble gases (helium, neon, argon, krypton, xenon, and radon) have complete, or full, valence shells. The configuration of a complete outer shell is very stable, so the noble gases usually exist as single atoms and rarely react with other elements. Atoms of the other elements attempt to imitate the stable configuration of the noble gases. They do this by donating, accepting, or sharing electrons in chemical reactions with atoms of the same element or atoms of other elements.

When atoms donate, accept, or share electrons with other atoms to complete their valence shells, they form chemical bonds. The resulting substance is called a compound. The type of bond depends on whether the electrons are transferred or shared.

An atom with few electrons in its valence shell will tend to donate these electrons to fill an almost complete shell in another atom. For example, an atom of lithium has two electrons filling its inner shell and a lone electron in an outer shell that could accommodate eight electrons. An atom of fluorine, on the other hand, has seven electrons in the outer shell (as well as two in the inner shell). The lithium atom transfers its outer electron to the fluorine atom. Both atoms now have filled outer shells. Fluorine has ten electrons, with eight electrons completing its outer shell. Lithium no longer has a second shell, but has two electrons completing the first shell. Because the lithium atom lost an electron, it now has a positive charge, while the fluorine atom gains a negative charge. Atoms that have an electrical charge are called ions. These oppositely charged ions attract each other, and an ionic bond forms between them. The compound created by lithium and fluorine is called lithium fluoride.

covalent bond forms between atoms when the valence electrons of one atom are shared with another atom with no discrete transfer of electrons. For example, two atoms of hydrogen, each with a single electron (and just one shell), can share their electrons. Each hydrogen atom’s shell is now complete with two electrons. This covalent bond yields a molecule of hydrogen. In molecules, each valence electron belongs to the molecule, not to the individual atoms.

When metal atoms combine with each other, the outermost electrons lose contact with their parent atoms. The remaining positively charged atomic centers form an ordered structure while the outer electrons move freely around the whole sample. These freely moving electrons, called conduction electrons, can carry heat energy and electric charge easily throughout the metal, making metals good conductors of heat (see Heat Transfer) and electricity.

Elements with atoms that have similar valence shell structures react in the same way to complete their outer shells. This predictable behavior led scientists to form the periodic law, which states that the physical and chemical properties of the elements tend to repeat at certain intervals as the atomic number (and number of electrons in the atom) increases. Elements that behave similarly are grouped in columns in the periodic table. For example, the valence shells of hydrogen and the alkali metals (lithium, sodium, potassium, rubidium, cesium, and francium) found in column 1 (or Ia) of the periodic table all contain a single electron, which makes them all highly reactive.

V

ELECTRONS AND ELECTRICITY

Electricity refers to the group of effects caused by charged particles, such as electrons and protons. Each charged particle creates an electric field around it that attracts or repels other charged particles. A difference in the amount of attraction or repulsion between any two points in an electrical field is known as a potential difference and is usually measured in volts. The two terminals of a working battery hold different charges: the positively charged terminal attracts electrons, the negative terminal repels them. Because of this difference in attraction, there is a voltage between the terminals. When a piece of metal is connected to the positive and negative terminals of a battery, freely moving conduction electrons will be attracted to and move toward the positive terminal. Such a movement of electric charge is an electric current.

Insulators are substances that do not normally conduct electricity. Scientists can make these substances conduct, however, by applying a very high electric field to the substance, a field strong enough to overcome the outer electron’s attraction to its nucleus and pull the electrons from the atoms. The electrons will move toward the positive terminal and, in a gas, the positive ions (the atoms stripped of their outer electrons) will move toward the negative terminal. Such currents are seen as electrical discharges of light—for example, in neon lamps.

VI

APPLICATIONS

In addition to using electrons for electrical devices, manufacturers use beams of pure electrons to produce television pictures and X rays, and to illuminate objects in electron microscopes. The electron beam for each of these devices is created by heating a cathode, a negatively charged metal that emits electrons. The electrons accelerate as they are attracted to the anode, a positively-charged piece of metal.

Electron beams are used in the cathode-ray tube (or picture tube) of traditional television screens. In the cathode-ray tube, the electrons race toward a hollow anode so that a narrow, fast beam of electrons shoots out through the hole in the anode. The higher the positive charge on the anode, the greater the speed—and thus the energy—of the beam. The tube must be emptied of air to prevent the electrons from being slowed or scattered by collisions with air molecules. The beam of electrons is focused so that it hits a specific spot on the television screen, which is covered with luminescent material. When the electrons hit this material, they excite its atoms. The excited atoms then lose this extra energy by releasing flashes of light. A changing electromagnetic field inside the picture tube affects the negatively charged electrons and makes the electron beam rapidly scan across the screen, moving horizontally and vertically. The flashes caused by the beam build up a continually changing picture. See also Television: Picture Tube.

When a high-powered electron beam hits a metal anode, it can create X rays for medical or industrial purposes. A fast-moving electron can knock an inner-shell electron out of an atom. As an outer-shell electron jumps inward to fill the inner-shell vacancy, the atom emits an X ray, a high-energy photon invisible to the eye. X rays are absorbed by heavier atoms, such as those in bones, but pass through lighter atoms, such as those in flesh. X rays can also react with chemicals in specialized film to create a picture (see Photography). If a patient’s arm is placed in front of a photographic film, exposing the arm to an X-ray beam will create an image of the bone on the film.

Scientists use powerful X rays created by electrons to probe the structure of atoms and molecules. They produce these X rays by accelerating a beam of electrons, confined by magnets in a circular tube, to a very high energy. Higher and higher energy electrons release radiation with shorter and shorter wavelengths, in this case, X rays. The shorter the wavelength, the finer the detail the X rays reveal.

While scientists usually describe the electron as a particle, the electron can also behave like a wave. Scientists use this aspect of electron behavior to illuminate extremely small objects. Ordinary light can only resolve objects that are larger than the wavelength of the light waves illuminating them. For smaller objects, the light waves scatter randomly off the object and do not reveal its shape. The wavelength of visible light is about a millionth of a meter. Electrons can have smaller wavelengths than visible light and thereby reveal objects many times smaller. Electron microscopes, using beams of electrons instead of light, can create images of objects, such as viruses, too small to be visible by ordinary microscopes. Electron energies are usually measured in electron volts (eV), where 1 eV is the energy acquired by an electron when it is accelerated in a vacuum by 1 volt. Physicists can use electrons they’ve accelerated to very high energies (giga-electron volts, or 109 eV, which is 1 billion electron volts) to reveal elementary particles such as protons, neutrons, and even quarks.

VII

HISTORY

In the early 19th century, British scientist Michael Faraday explored the phenomenon of electrolysis. Electrolysis involves passing an electric current through a substance, such as an ionic compound dissolved in a solution of water. The current separates the constituent elements of the compound—the positively charged ions collect at the (negative) cathode and the negatively charged ions collect at the (positive) anode. Faraday discovered that the amount of an element formed increased in proportion to the amount of electricity passed through the substance (see Electrochemistry). This suggested that atoms, although themselves electrically neutral, are made up of smaller particles that carry electric charge.

Toward the end of the 19th century, physicists realized that if they applied a high voltage between two electrodes (a cathode and an anode) in a vacuum tube, the cathode would release a discharge. This discharge was called a cathode ray. In 1897 the British physicist Sir Joseph J. Thomson revealed that these rays were made up of tiny particles almost 2,000 times lighter than an atom of hydrogen. Thomson also showed that electric and magnetic fields could move around the particles, thus proving they were electrically charged. These tiny, light, and electrically charged particles were named electrons, and because of his work Thomson is regarded as the discoverer of the electron.

In the 1900s, physicists began to realize that light waves could act like particles, so they wondered whether electrons could act like waves. In 1905 German-born American physicist Albert Einstein showed that light—a form of radiation—sometimes behaves as though it is made of particles of fixed energy. In 1923 French physicist Louis de Broglie suggested that electrons—particles of fixed energy—should also be able to behave like radiation. In 1927 American physicists Clinton Davisson and Lester Germer showed that a beam of electrons passing through a crystal diffracts, or bends, in the same way that light does. This dual particle-radiation behavior is the basis of the electron microscope.

Also in 1927, British physicist Paul A. M. Dirac theorized that electrons must have the property now known as spin. The electron was the first elementary particle to be attributed with spin, now considered to be a general attribute of all elementary particles. Dirac also predicted that electrons should have antiparticles, elementary particles with exactly the same properties as electrons but carrying a positive electric charge. In 1932 American physicist Carl David Anderson discovered these electron antiparticles, called positrons.

In modern physics experiments, scientists carefully prepare and collide speeding beams of electrons and positrons. When the beams meet, electrons and positrons destroy each other, producing bursts of energy. The energy released in these collisions can make many new kinds of elementary particles. Such electron-positron colliders are among the main tools of today's particle physics research. See also Particle Accelerators.


Contributed By:
Gordon Fraser

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

 

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