Electron I INTRODUCTION Models of the Atom Once scientists discovered the electron, they set out to explain how electrons behave in atoms. In the model developed by British physicist Ernest Rutherford, electrons moved around a tightly packed, positively charged nucleus. Danish physicist Neils Bohr began with Rutherford's model, but then postulated further that electrons could only move in certain orbits and have certain energies. In the later model developed by Austrian physicist Erwin Schrödinger, electrons are described not by the paths they take but by the regions of space where they are most likely to be found. © Microsoft Corporation. All Rights Reserved. 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 Electron Density and Orbital Shapes Atomic orbitals are mathematical descriptions of where the electrons in an atom (or molecule) are most likely to be found. These descriptions are obtained by solving an equation known as the Schrödinger equation, which expresses our knowledge of the atomic world. As the angular momentum and energy of an electron increases, it tends to reside in differently shaped orbitals. The orbitals corresponding to the three lowest energy states are s, p, and d, respectively. The illustration shows the spatial distribution of electrons within these orbitals. The fundamental nature of electrons prevents more than two from ever being in the same orbital. The overall distribution of electrons in an atom is the sum of many such pictures. This description has been confirmed by many experiments in chemistry and physics, including an actual picture of a p-orbital made by a Scanning Tunneling Microscope. © Microsoft Corporation. All Rights Reserved. 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 Light Absorption and Emission When a photon, or packet of light energy, is absorbed by an atom, the atom gains the energy of the photon, and one of the atom's electrons may jump to a higher energy level. The atom is then said to be excited. When an electron of an excited atom falls to a lower energy level, the atom may emit the electron's excess energy in the form of a photon. The energy levels, or orbitals, of the atoms shown here have been greatly simplified to illustrate these absorption and emission processes. For a more accurate depiction of electron orbitals, see the Atom article. © Microsoft Corporation. All Rights Reserved. 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. Spectral Lines of Atomic Hydrogen When an electron makes a transition from one energy level to another, the electron emits a photon with a particular energy. These photons are then observed as emission lines using a spectroscope. The Lyman series involves transitions to the lowest or ground state energy level. Transitions to the second energy level are called the Balmer series. These transitions involve frequencies in the visible part of the spectrum. In this frequency range each transition is characterized by a different color. © Microsoft Corporation. All Rights Reserved. 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 Ionic Bonding: Salt The bond (left) between the atoms in ordinary table salt (sodium chloride) is a typical ionic bond. In forming the bond, sodium becomes a cation (a positively charged ion) by "giving up" its valence electron to chlorine, which then becomes an anion (a negatively charged ion). This electron exchange is reflected in the size difference between the atoms before and after bonding. Attracted by electrostatic forces (right), the ions arrange themselves in a crystalline structure in which each is strongly attracted to a set of oppositely charged "nearest neighbors" and, to a lesser extent, all the other oppositely charged ions throughout the entire crystal. © Microsoft Corporation. All Rights Reserved. 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. Covalent Bonds In a covalent bond, the two bonded atoms share electrons. When the atoms involved in the covalent bond are from different elements, one of the atoms will tend to attract the shared electrons more strongly, and the electrons will spend more time near that atom; this is a polar covalent bond. When the atoms connected by a covalent bond are the same, neither atom attracts the shared electrons more strongly than the other; this is a non-polar covalent bond. © Microsoft Corporation. All Rights Reserved. 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. A 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. Metallic Bonding Silver, a typical metal, consists of a regular array of silver atoms that have each lost an electron to form a silver ion. The negativly charged electrons distribute themselves throughout the entire piece of metal and form nondirectional bonds between the positive silver ions. This arrangement, known as metallic bonding, accounts for the characteristic properties of metals: they are good electrical conductors because the electrons are free to move from one place to another, and they are malleable (as shown here) because the positive ions are held together by nondirectional forces. A force applied to a malleable substance shifts the positions of the atoms without breaking the bonds that hold them together. © Microsoft Corporation. All Rights Reserved. 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 Television Picture Tube A color television picture tube contains three electron guns, one corresponding to each of the three primary colors of light--red, green, and blue. Electromagnets direct the beams of electrons emerging from these guns to continuously scan the screen. As the electrons strike red, green, and blue phosphor dots on the screen, they make the dots glow. A screen with holes in it, called a shadowmask, ensures that each electron beam only strikes phosphor dots of its corresponding color. The glow of all the dots together forms the television picture. © Microsoft Corporation. All Rights Reserved. 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. Scanning Electron Microscope This scanning electron microscope (SEM) at the University of California, Berkeley is located to the left with the computer images of the specimen on the computer screens to the right. Although a SEM cannot resolve objects as small as a transmission electron microscope, the images produced by the SEM are more useful for seeing the three-dimensional aspect of the surface structure of small objects. Lawrence Migdale/Photo Researchers, Inc. 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. Wave Aspect of Electrons This pattern is produced when a narrow beam of electrons passes through a sample of titanium-nickel alloy. The pattern reveals that the electrons move through the sample more like waves than particles. The electrons diffract (bend) around atoms, breaking into many beams and spreading outward. The diffracted beams then interfere with one another, cancelling each other out in some places and reinforcing each other in other places. The bright spots are places where the beams interfered constructively, or reinforced each other. The dark spots are areas in which the beams interfered destructively, or cancelled each other out. Science Source/Photo Researchers, Inc. 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 Michael Faraday One of the most prominent scientists of the 19th century, Michael Faraday made significant contributions to both physics and chemistry. He discovered the phenomenon known as electromagnetic induction by observing that a current flows in a wire that is moved through a magnetic field. His discovery of electromagnetic induction contributed to the development of Maxwell's equations, and led to the invention of the electric generator. Faraday's earlier work in chemistry included articulating the laws of electrolysis and the discovery of benzene. Culver Pictures 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. Sir Joseph Thomson British physicist Sir Joseph Thomson won the 1906 Nobel Prize in physics. Perhaps best remembered as the discoverer of the electron, he also conducted research into the conduction of electricity by gasses. © The Nobel Foundation 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. Louis Victor de Broglie French physicist Louis Victor de Broglie won the Nobel Prize in physics in 1929. He discovered the wave nature of electrons. © The Nobel Foundation 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 ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
