istoria chmiei

8/12/2019 istoria chmiei

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A Antichitatea Babilonienii si egiptenii au observat miscarile planetelor si au au reusit sa prezica eclipsele. Civilizatia greaca acontribuit putin datorita neacceptarii ideilor marilor filosofi, Plato si Aristocle. Totusi au fost inregistrate progrese in Alexandria, centru stiintific al civilizatiei greci. Acolo, matematicianul siinventatorul Arhimede a conceput instrumente macanice variate precum precum parghii si scripeti si a masurat densitatilecorpurilor solide introducandu-le in lichid. Alti important om de stiinta a fost Aristarchus din Smos care a masuratdistantele de la pamant la soare si la luna! matematicianul, astronomul si geograful "ratostene, care a determinatcircumferinta pamantului si a facu o harta a cerului si Ptolemeu si a propus sistemul de miscare a planetelor in care

Pamantul era in mi#loc si Soarele, $una si stelele se misca in #urul lui in orbite circulareB "vul %ediu

&n evul %ediu s-a avansat putin in domeniul fizicii, sau altor stiinte, in afara de pastrarea clasicelor tratate grecesti.'ondarea marilor universitati medievale a esuat sa avanseze in fizica sau alte activitati experimentale .'ilosoful si teologul italian Thomas A(uinas, de exemplu a incercat sa demonstreze calucrarile lui Platon si Aristotel asuntin armonie cu sfintele scripturi

C Secolul )* si )+Avansul stiintei moderne urmata de renastere a fost a#utata de incercarile incununate de succes a oamenilor de stiintaexraordinari care au interpretat comportamentul corpurilor ceresti. 'ilosoful polone, icolaus Copernicus a propussistemul heliocentric in care planetele se misca in #urul soarelui. "l era convins ca orbitele planetare erau circulare si, prinurmare sistemul sau avea nevoie de elaborari complicare, precum sistemul Ptolemeic, pe care intentiona sa-l inlocuiasca.Astronomul danez Tcho Brahe, crezand in sistemul Ptolemeic, a incercat sa-l confirme, intr-o serie de masuratoriremarcabil de precise. Aceasta l-a a#utat pe asistentul sau, astronomul ohannes /epler cu datele acesta rasturnand sistemul Ptolemeic si au

condus la elaborarea a trei legi care s-au conformat cu un sistem heliocentric modificat. 0alileo, auzind de inventiatelescopului, si-a construit unul si, incepand din )1*2, a fost capabil sa confirme sistemul heliocentric observand variatiile

pozitie planetei 3enus. Tot el a observat suprafetele neregulate de pe $una si pe patru dintre cei mai luminosi sateliti ai luiupiter, petele solare si multe stele din Calea $actee. &nteresele lui 0alileo nu se limitau la astronomie! folosind planuriinclinate si un ceas cu apa imbunatatit a demonstrat ca corpurile cu greutati diferite cad la fel de repede si viteza lor cresteuniform cu timpul. 4escoperirile astronomice ale lui 0alileo au prefigurat munca matematicianului englez cel mai maredin secolul al )+-lea , Sir &saac e5ton.

&3 %ecanica e5toniana&ncepand din )**6 , la varsta de 78 de ani ne5ton a enuntat principiile mecanicii, a formulat legera gravitatiei

universale, a propus teoria propagarii luminii si a inventat calculul diferential si integral. Contributiile lui e5ton auacoperit o raza enorma de fenomene naturale9a prezis aparitia cometelor si a explicat formarea echinoctiului.

A 4ezvotarea %ecanicii

4ezvoltarea fizicii datoreava mult legilor miscarii ale lui e5ton, in special celei de-a doua care spune ca forta de careeste nevoie pt a accelera un obiect este proportionala cu produsul dintre masa si acceleratie. 4aca forta , pozitia initiala siviteza sunt cunoscute pot fi calculate, pozitiile consecutive chiar daca forta variaza cu timpul si variatiile pozitiilor.Aceasta lege importanta continea un alt aspect important9 fiecare corp are proprietatiile sale specifice, masa inertialaspecifica, ce ii influenteaza miscarea. Chiar si azi legea este utila atat timp cat corpul nu este prea masiv sau prea mic sinu se misca prea repede. $egea a treia a lui e5ton este exprimata foarte simpla9:pt fiecare actiune se formeaza o reactieegala si opusa: ne spuna in termeni sofisticati ca toate fortele dintre particula vin in perechi directionate opus, dar nuneaparat pe linia determinata de cele doua particule.

B 0ravitatia Contributia cea mai importanta a lui e5ton descrierea fortelor naturii a fost elucidarea fortei gravitatiei. ;amenii destiinta de astazi stiu ca pe langa gravitatie mai sunt doar trei forte fundamentale 9cea a electromagnetismului, cea asa-numitele interactiuni nucleare-tari care tin impreunati neutronii si protonii din atomii nucleici, si interactiunile slabedintre unele particule care formeaza fenomenul numit radioactivitate. &ntelegerea conceptului de forta dateaza de la legeauniversala a gravitatieicare ne spune ca toate particulele materiale si corpurile care le compun au o proprietate numita masa gravitationala.Aceasta proprietate face ca oricare doua particule sa exerciteze forte de atractie una asuprei celeilalte care sunt direct

proportionala cu produsul masei si invers proportionale cu patratul distantei. 'orta gravitational guverneaza miscareplanetelor in #urul soarelui si campul gravitational al Pamantului si este responabila de colapsul gravitational, ultimulstagiu al ciclului vietii unei stele.. Cu toate acestea demonstratiile lui 0alileo, care le antecedeazape cele ale lui e5ton, conforma carora corpurile cad spre Pamant cu aceeasi acceleratie pot fi explicate prin faptul camasa gravitationala a unui corp care determina fortele exercitate pe el si masa inertiala care determina raspunsul la aceaforta se reduc una pe cealalta.


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Semnificatia completa a acestei echivalente dintre masele gravitationale si inertiale nu a fost apreciata pana la fizicianulAlbert "instein, teoretician care a enuntat teoria relativitatii. 'orta gravitationala este cea mai slaba din cele patru forte dinnatura cand sunt considerate particule elementare. 'orta gravitationala dintre doi protoni, de exemplu, care sunt printrecele mai grele particule elementare este la orice distanta data de )?-8* ori cat magnitudinea fortelor electrostatice dintreele si pentru doi dintre acesti protoni din nucleu unui atom este de multe ori mai mica decat interactiunea nucleara tare.&ncarcatura electrica a particulelor elementare care formeaza fortele electrostatice si magnetice sunt sau pozitive sau

negative , sau impreuna pot avea valoarea ?. umai particulele cu incarcaturi de sens contrar se atrag si corpurile maritind sa fie neutre din punct de vedere electric.

inactive. Pe de alta parte fortele nucleare, cea slaba si cea tare, au o raza foarte mica si devin foarte greu de observat ladistante mai mari de a milioana parte din a milioana parte dintr-un centimetru.&n ciuda importantei macroscopice , forte gravitationala ramane asa de slaba incat corpul trebuie sa fie foarte masiv inaiteca influenta sa sa fie oservata. Cu toate acestea legea universala a gravitatiei a fost dedusa din observatiile asupramiscarilor planetelor cu mult inainte de a fi verificata experimental. &n )++) fizicianul si chimistul englez @enrCavedish a confirmat folosid sfere mari atragan mase mici atasate de un pendul si cu a#utorul acestor masuratori a dedusdensitatea Pamantului.&n cele doua secole de dupa e5ton, cu toate ca mecanica a fost analizata, reformulata si aplicata pe sisteme complexe, nus-au introdus idei noi in fizica. %atematicianul suedez, $eonhard "uler a formulat pentru prima data ecuatia miscariicorpurilor rigide. &n aceeasi perioada omul de stiinta danez, 4aniel Bernouli, si alti doi au extins mecanica ne5toniana siau pus bazele mecanicii fluidelor.

C "lectricitate si magnetismCu toate ca grecii antici stiau de proprietatile electricitatii si chinezii din 7+?? i.e.n. confectionau magneti, experimentareasi intelegerea electricitatii si fenomenelor magnetice nu s-a intamplat pana la sfarsitul secolului al )1-lea. &n )+16,

fizicianul francez Charles Auguste de Coulomb a confirmat pentru prima data experimental ca sarcinile electrice se atragsau se resping unul pe altul. ; puternica teorie pt calcularea efectelor unui numar de sarcini electrice a fost elaborata dematematicianul francez Simon 4enis Poisson si de matematicianul german Carl 'riedrich 0auss.; particula incarcata pozitiv atrage o particula incarcata negativ avand tendinta sa accelereze unul spre altul. 4aca

mediul prin care particulele se misca impune rezistenta miscarii, aceasta poate fi redusa la o miscare cu viteza constanta simediul se incalzeste.Teoria clasica a unui circuit electric simplu ce presupune ca, capetele unei baterii sunt mentinuteincarcate pozitiv si negativ. Cand capetele sunt conectate prin intermediul unui fir, incarcatura negativa va fi indepartatade capatul negativ si atrasa de capatul pozitiv. Procesul incalzeste firul care ofera rezistenta miscarii electronilor.'izicianul german 0eorg Simeon ;hm a descoperit pentru prima data existenta unei simple proportionalitati constantaintre curent si forta electrica formata de baterie, cunoscuta sub numele de rezistenta electrica. $egea lui ;hm in carestabileste ca rezistenta ,egala cu raportul dintre volta# si curent, nu este o lege a fizicii aplicabila fundamentala siuniversala, ci mai degraba descrie comportamentul unei clase limitate de materiale solide.The historical concepts of magnetism, based on the existence of pairs of oppositel charged poles, had started in the )+thcentur and o5e much to the 5or of Coulomb. The first connection bet5een magnetism and electricit, ho5ever, 5as

made through the pioneering experiments of the 4anish phsicist and chemist @ans Christian ;ersted, 5ho in )1)2discovered that a magnetic needle could be deflected b a 5ire nearb carring an electric current. ithin one 5ee afterlearning of ;erstedDs discover, the 'rench scientist Andr %arie AmpEre sho5ed experimentall that t5o current-carring 5ires 5ould affect each other lie poles of magnets. &n )18) the British phsicist and chemist %ichael 'aradadiscovered that an electric current could be induced =made to flo5> in a 5ire 5ithout connection to a batter, either bmoving a magnet or b placing another current-carring 5ire 5ith an unstead-that is, rising and falling-current nearb.The intimate connection bet5een electricit and magnetism, no5 established, can best be stated in terms of electric ormagnetic fields, or forces that 5ill act at a particular point on a unit charge or unit current, respectivel, placed at that

point. Stationar electric charges produce electric fields! currents-that is, moving electric charges-produce magnetic fields."lectric fields are also produced b changing magnetic fields, and vice versa. "lectric fields exert forces on charged

particles as a function of their charge alone! magnetic fields 5ill exert an additional force onl if the charges are inmotion.These (ualitative findings 5ere finall put into a precise mathematical form b the British phsicist ames Cler %ax5ell5ho, in developing the partial differential e(uations that bear his name, related the space and time changes of electric andmagnetic fields at a point 5ith the charge and current densities at that point. &n principle, the permit the calculation ofthe fields ever5here and an time from a no5ledge of the charges and currents. An unexpected result arising from thesolution of these e(uations 5as the prediction of a ne5 ind of electromagnetic field, one that 5as produced baccelerating charges, that 5as propagated through space 5ith the speed of light in the form of an electromagnetic 5ave,and that decreased 5ith the inverse s(uare of the distance from the source. &n )11+ the 0erman phsicist @einrich Fudolf@ertz succeeded in actuall generating such 5aves b electrical means, thereb laing the foundations for radio, radar,television, and other forms of telecommunications. See "lectromagnetic Fadiation.The behavior of electric and magnetic fields in these 5aves is (uite similar to that of a ver long taut string, one end of5hich is rapidl moved up and do5n in a periodic fashion. An point along the string 5ill be observed to move up anddo5n, or oscillate, 5ith the same period or 5ith the same fre(uenc as the source. Points along the string at differentdistances from the source 5ill reach the maximum vertical displacements at different times, or at a different phase. "ach


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point along the string 5ill do 5hat its neighbor did, but a little later, if it is further removed from the vibrating source =see;scillation>. The speed 5ith 5hich the disturbance, or the message to oscillate, is transmitted along the string is called the5ave velocit =see ave %otion>. This is a function of the medium, its mass, and the tension in the case of a string. Aninstantaneous snapshot of the string =after it has been in motion for a 5hile> 5ould sho5 e(uispaced points having thesame displacement and motion, separated b a distance no5n as the 5avelength, 5hich is e(ual to the 5ave velocitdivided b the fre(uenc. &n the case of the electromagnetic field one can thin of the electric-field strength as taing the

place of the up-and-do5n motion of each piece of the string, 5ith the magnetic field acting similarl at a direction at rightangles to that of the electric field. The electromagnetic-5ave velocit a5a from the source is the speed of light.

4 $ightThe apparent linear propagation of light 5as no5n since anti(uit, and the ancient 0rees believed that light consisted ofa stream of corpuscles. The 5ere, ho5ever, (uite confused as to 5hether these corpuscles originated in the ee or in theob#ect vie5ed. An satisfactor theor of light must explain its origin and disappearance and its changes in speed anddirection 5hile it passes through various media. Partial ans5ers to these (uestions 5ere proposed in the )+th centur b

e5ton, 5ho based them on the assumptions of a corpuscular theor, and b the "nglish scientist Fobert @ooe and the4utch astronomer, mathematician, and phsicist Christiaan @ugens, 5ho proposed a 5ave theor. o experiment could

be performed that distinguished bet5een the t5o theories until the demonstration of interference in the earl )2th centurb the British phsicist and phsician Thomas Goung. The 'rench phsicist Augustin ean 'resnel decisivel favored the5ave theor.&nterference can be demonstrated b placing a thin slit in front of a light source, stationing a double slit farther a5a, andlooing at a screen spaced some distance behind the double slit. &nstead of sho5ing a uniforml illuminated image of theslits, the screen 5ill sho5 e(uispaced light and dar bands. Particles coming from the same source and arriving at thescreen via the t5o slits could not produce different light intensities at different points and could certainl not cancel each

other to ield dar spots. $ight 5aves, ho5ever, can produce such an effect. Assuming, as did @ugens, that each of thedouble slits acts as a ne5 source, emitting light in all directions, the t5o 5ave trains arriving at the screen at the same

point 5ill not generall arrive in phase, though the 5ill have left the t5o slits in phase. 4epending on the difference intheir paths, :positive: displacements arriving at the same time as :negative: displacements of the other 5ill tend to cancelout and produce darness, 5hile the simultaneous arrival of either positive or negative displacements from both sources5ill lead to reinforcement or brightness. "ach apparent bright spot undergoes a time5ise variation as successive in-phase5aves go from maximum positive through zero to maximum negative displacement and bac. either the ee nor anclassical instrument, ho5ever, can determine this rapid :flicer,: 5hich in the visible-light range has a fre(uenc from H I)?)H to +.6 I )?)H @z, or ccles per second. Although it cannot be measured directl, the fre(uenc can be inferred from5avelength and velocit measurements. The 5avelength can be determined from a simple measurement of the distance

bet5een the t5o slits, and the distance bet5een ad#acent bright bands on the screen! it ranges from H I )?-6 cm =).* I )?-6 in> for violet light to +.6 I )?-6 cm =8 I )?-6 in> for red light 5ith intermediate 5avelengths for the other colors.The first measurement of the velocit of light 5as carried out b the 4anish astronomer ;laus Foemer in )*+*. @e notedan apparent time variation bet5een successive eclipses of upiterDs moons, 5hich he ascribed to the intervening change in

the distance bet5een "arth and upiter, and to the corresponding difference in the time re(uired for the light to reach theearth. @is measurement 5as in fair agreement 5ith the improved )2th-centur observations of the 'rench phsicistArmand @ippolte $ouis 'izeau, and 5ith the 5or of the American phsicist Albert Abraham %ichelson and hisco5orers, 5hich extended into the 7?th centur. Toda the velocit of light is no5n ver accuratel as 722,727.* m=)16,2+).1 mi sec> in vacuum. &n matter, the velocit is less and varies 5ith fre(uenc, giving rise to a phenomenonno5n as dispersion. See also ;ptics! Spectrum! 3acuum.%ax5ellDs 5or contributed several important results to the understanding of light b sho5ing that it 5as electromagneticin origin and that electric and magnetic fields oscillated in a light 5ave. @is 5or predicted the existence of nonvisiblelight, and toda electromagnetic 5aves or radiations are no5n to cover the spectrum from gamma ras =seeFadioactivit>, 5ith 5avelengths of )?-)7 cm =H I )?-)) in>, through J ras, visible light, micro5aves, and radio 5aves,to long 5aves of hundreds of ilometers in length =see J Fa>. &t also related the velocit of light in vacuum and throughmedia to other observed properties of space and matter on 5hich electrical and magnetic effects depend. %ax5ellDsdiscoveries, ho5ever, did not provide an insight into the msterious medium, corresponding to the string, through 5hichlight and electromagnetic 5aves had to travel =see the "lectricit and %agnetism section above>. Based on the experience5ith 5ater, sound, and elastic 5aves, scientists assumed a similar medium to exist, a :luminiferous ether: 5ithout mass,5hich 5as all-pervasive =because light could obviousl travel through a massless vacuum>, and had to act lie a solid=because electromagnetic 5aves 5ere no5n to be transverse and the oscillations too place in a plane perpendicular to thedirection of propagation, and gases and li(uids could onl sustain longitudinal 5aves, such as sound 5aves>. The searchfor this msterious ether occupied phsicistsD attention for much of the last part of the )2th centur.The problem 5as further compounded b an extension of a simple problem. A person 5aling for5ard 5ith a speed of 8.7mKh =7 mph> in a train traveling at *H.H mKh =H? mph> appears to move at *+.* mKh =H7 mph>, to an observer on theground. &n terms of the velocit of light the (uestion that no5 arose 5as9 &f light travels at about 8??,??? mKsec =about)1*,??? miKsec> through the ether, at 5hat velocit should it travel relative to an observer on earth 5hile the earth alsomoves through the etherL ;r, alternatel, 5hat is the earthDs velocit through the etherL The famous %ichelson-%orleexperiment, first performed in )11+ b %ichelson and the American chemist "d5ard illiams %orle using an


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interferometer, 5as an attempt to measure this velocit! if the earth 5ere traveling through a stationar ether, a differenceshould be apparent in the time taen b light to traverse a given distance, depending on 5hether it travels in the directionof or perpendicular to the earthDs motion. The experiment 5as sensitive enough to detect even a ver slight difference binterference! the results 5ere negative. Phsics 5as no5 in a profound (uandar from 5hich it 5as not rescued until"instein formulated his theor of relativit in )2?6.

" Thermodnamics A branch of phsics that assumed ma#or stature during the )2th centur 5asthermodnamics. &t began b disentangling the previousl confused concepts of heat and temperature, b arriving atmeaningful definitions, and b sho5ing ho5 the could be related to the heretofore purel mechanical concepts of 5or

and energ. See also @eat Transfer.") @eat and Temperature A different sensation is experienced 5hen a hot or a cold bod is touched,

leading to the (ualitative and sub#ective concept of temperature. The addition of heat to a bod leads to an increase intemperature =as long as no melting or boiling occurs>, and in the case of t5o bodies at different temperatures brought intocontact, heat flo5s from one to the other until their temperatures become the same and thermal e(uilibrium is reached. Toarrive at a scientific measure of temperature, scientists used the observation that the addition or subtraction of heat

produced a change in at least one 5ell-defined propert of a bod. The addition of heat, for example, to a column of li(uidmaintained at constant pressure increased the length of the column, 5hile the heating of a gas confined in a containerraised its pressure. Temperature, therefore, can invariabl be measured b one other phsical propert, as in the length ofthe mercur column in an ordinar thermometer, provided the other relevant properties remain unchanged. Themathematical relationship bet5een the relevant phsical properties of a bod or sstem and its temperature is no5n as thee(uation of state. Thus, for an ideal gas, a simple relationship exists bet5een the pressure, p, volume 3, number of molesn, and the absolute temperature T, given b p3 M nFT, 5here F is the same constant for all ideal gases. BoleDs la5,named after the British phsicist and chemist Fobert Bole, and 0a-$ussacDs la5 or CharlesDs la5, named after the

'rench phsicists and chemists oseph $ouis 0a-$ussac and ac(ues Alexandre Csar Charles, are both contained in thise(uation of state =see 0ases>.


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initial separate or ordered state is turned into a mixed or disordered state. These ideas can be expressed b athermodnamic propert, called the entrop =first formulated b Clausius>, 5hich serves as a measure of ho5 close asstem is to e(uilibrium-that is, to perfect internal disorder. The entrop of an isolated sstem, and of the universe as a5hole, can onl increase, and 5hen e(uilibrium is eventuall reached, no more internal change of an form is possible.Applied to the universe as a 5hole, this principle suggests that eventuall all temperature in space becomes uniform,resulting in the so-called heat death of the universe.$ocall, the entrop can be lo5ered b external action. This applies to machines, such as a refrigerator, 5here the entropin the cold chamber is being reduced, and to living organisms. This local increase in order is, ho5ever, onl possible at the

expense of an entrop increase in the surroundings! here more disorder must be created.This continued increase in entrop is related to the observed nonreversibilit of macroscopic processes. &f a process 5erespontaneousl reversible-that is, if, after undergoing a process, both it and all the surroundings could be brought bac totheir initial state-the entrop 5ould remain constant in violation of the second la5. hile this is true for macroscopic

processes, and therefore corresponds to dail experience, it does not appl to microscopic processes, 5hich are believed tobe reversible. Thus, chemical reactions bet5een individual molecules are not governed b the second la5, 5hich appliesonl to macroscopic ensembles.'rom the promulgation of the second la5, thermodnamics 5ent on to other advances and applications in phsics,chemistr, and engineering. %ost chemical engineering, all po5er-plant engineering, and air-conditioning and lo5-temperature phsics are #ust a fe5 of the fields that o5e their theoretical basis to thermodnamics and to the subse(uentachievements of such scientists as %ax5ell, the American phsicist illard 0ibbs, the 0erman phsical chemist alther@ermann ernst, and the or5egian-born American chemist $ars ;nsager.

' /inetic Theor and Statistical %echanics The modern concept of the atom 5as first proposed b theBritish chemist and phsicist ohn 4alton in )1?1 and 5as based on his studies that sho5ed that chemical elements enter

into combinations based on fixed ratios of their 5eights. The existence of molecules as the smallest particles of a substancethat can exist in the free-that is, gaseous-state and have the properties of an larger amount of the substance, 5as firsthpothesized b the &talian phsicist and chemist Amedeo Avogadro in )1)), but did not find general acceptance untilabout 6? ears later, 5hen it also formed the basis of the inetic theor of gases =see AvogadroDs $a5>. 4eveloped b%ax5ell, the Austrian phsicist $ud5ig Boltzmann, and other phsicists, it applied the la5s of mechanics and probabilitto the behavior of individual molecules, and dre5 statistical inferences about the properties of the gas as a 5hole.A tpical but important problem solved in this manner 5as the determination of the range of speeds of molecules in thegas, and from this the average inetic energ of the molecules. The inetic energ of a bod, as a simple conse(uence of

e5tonDs second la5, is Lmv7, 5here m is the mass of the bod and v its velocit. ;ne of the achievements of inetictheor 5as to sho5 that temperature, the macroscopic thermodnamic propert describing the sstem as a 5hole, 5asdirectl related to the average inetic energ of the molecules. Another 5as the identification of the entrop of a sstem5ith the logarithm of the statistical probabilit of the energ distribution. This led to the demonstration that the state ofthermodnamic e(uilibrium corresponding to that of highest probabilit is also the state of maximum entrop. 'ollo5ingthe success in the case of gases, inetic theor and statistical mechanics 5ere subse(uentl applied to other sstems, a

process that is still continuing.0 "arl Atomic and %olecular Theories The development of 4altonDs atomic theor and AvogadroDs

molecular la5 had overriding influence on the development of chemistr, in addition to their importance in phsics.0) AvogadroDs $a5 AvogadroDs la5, 5hich 5as easil proved b inetic theor, indicated that a specified

volume of a gas at a given temperature and pressure al5as contained the same number of molecules, irrespective of thegas selected. This number, ho5ever, could not be accuratel determined, and the )2th-centur phsicists therefore had nosound no5ledge of molecular or atomic mass and size until the turn of the 7?th centur, 5hen subse(uent to thediscover of the electron, the American phsicist Fobert Andre5s %illian carefull determined its charge. This finall

permitted accurate determination of the so-called AvogadroDs number, 5hich is the number of molecules in that amount ofmaterial exactl e(ual to its molecular 5eight.Besides the mass, another (uantit of interest 5as the size of an atom. 3arious and onl partl successful attempts atfinding the size of an atom 5ere made during the latter part of the )2th centur! the most successful applied the results ofinetic theor to nonideal gases-that is, gases the behavior of 5hich depended on the fact that molecules 5ere not points

but had finite volumes. ;nl later experiments involving the scattering of J ras, alpha particles, and other atomic andsubatomic particles b atoms led to more precise measurements of their size as being bet5een )?-1 and )?-+ cm =H I )?-+and H I )?-* in> in diameter. A precise statement about the size of an atom, ho5ever, re(uires some explicit definition of5hat is meant b size, since most atoms are not exactl spherical and can exist in various states that change the distance

bet5een the nucleus and the electrons 5ithin the atom.07 Spectroscop ;ne of the most important developments leading to the exploration of the interior of the

atom, and to the eventual overthro5 of the classical theories of phsics, 5as spectroscop! the other 5as the discover ofthe subatomic particles themselves.&n )178 the British astronomer and chemist Sir ohn 'rederic illiam @erschel suggested that a chemical substancemight be identified b examining its spectrum-that is, the discrete 5avelength pattern in 5hich light from a gaseoussubstance is emitted. &n the ears that follo5ed, the spectra of a great man substances 5ere cataloged b t5o 0ermans,the chemist Fobert ilhelm Bunsen and the phsicist 0ustav Fobert /irchhoff. @elium 5as first discovered as a ne5


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element follo5ing the discover of an unexplained spectral line in the sunDs spectrum b the British astronomer Sir osephorman $ocer in )1*1. 'rom the standpoint of atomic theor, ho5ever, the most important contributions 5ere made bthe stud of the spectra of simple atoms, such as hdrogen, 5hich sho5ed fe5 spectral lines. See Chemical Analsis.4iscrete line spectra originate from gaseous substances 5here, in terms of modern no5ledge, the electrons have beenexcited b heat or b bombardment 5ith subatomic particles. &n contrast, a heated solid has a continuous spectrum over thefull visible range and into the infrared and ultraviolet regions. The total amount of energ emitted depends strongl on thetemperature, as does the relative intensit of the different 5avelength components. As a piece of iron is heated, forexample, its radiation is first in the infrared spectrum and cannot be seen! it then extends into the visible spectrum 5here

the glo5 shifts from red to 5hite as the pea of its radiant spectrum shifts to5ard the middle of the visible range. Attemptsto explain the radiation characteristics of solids, using the tools of theoretical phsics available at the end of the )2thcentur, led to the prediction that at an given temperature the amount of radiation increased 5ith fre(uenc and 5ithoutlimit. This calculation, in 5hich no error 5as found, 5as in disagreement 5ith experiment and also led to an absurdconclusion9 A bod at a finite temperature could radiate an infinite amount of energ. This re(uired a ne5 5a of thiningabout radiation and, indirectl, about the atom. See &nfrared Fadiation!


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the amplitude of the vibrations directl related to the temperature. All vibration fre(uencies should be possible and thethermal energ of the solid should be continuousl convertible into electromagnetic radiation as long as energ is supplied.Planc made a radical assumption b postulating that the molecular oscillator could emit electromagnetic 5aves onl indiscrete bundles, no5 called (uanta, or photons. See Photon! Ouantum Theor. "ach photon has a characteristic5avelength in the spectrum and an energ " given b " M hf, 5here f is the fre(uenc of the 5ave. The 5avelength related to the fre(uenc b f M c, 5here c is the speed of light. ith the fre(uenc specified in hertz =@z>, or ccles persecond, h, no5 no5n as PlancDs constant, is extremel small =*.*7* I )?-7+ erg-sec>. ith his theor, Planc againintroduced a partial dualit into the theor of light, 5hich for nearl a centur had been considered to be 5avelie onl.

C Photoelectricit &f electromagnetic radiation of appropriate 5avelength falls upon suitable metals,negative electric charges, later identified as electrons, are e#ected from the metal surface. The important aspects of this

phenomenon are the follo5ing9 =)> the energ of each photoelectron depends onl on the fre(uenc of the illumination andnot on its intensit! =7> the rate of electron emission depends onl on the illuminating intensit and not on the fre(uenc=provided that the minimum fre(uenc to cause emission is exceeded>! and =8> the photoelectrons emerge as soon as theillumination hits the surface. These observations, 5hich could not be explained b %ax5ellDs electromagnetic theor oflight, led "instein to assume in )2?6 that light can be absorbed onl in (uanta or photons, and that the photon completelvanishes in the absorption process, 5ith all of its energ " =Mhf> going to one electron in the metal. ith this simpleassumption "instein extended PlancDs (uantum theor to the absorption of electromagnetic radiation, giving additionalimportance to the 5ave-particle dualit of light. &t 5as for this 5or that "instein 5as a5arded the )27) obel Prize in

phsics.4 J Fas These ver penetrating ras, first discovered b Foentgen, 5ere sho5n to be electromagnetic

radiation of ver short 5avelength in )2)7 b the 0erman phsicist %ax Theodor 'elix von $aue and his co5orers. Theprecise mechanism of J-ra production 5as sho5n to be a (uantum effect, and in )2)H the British phsicist @enr 05n-

effres %osele used his J-ra spectrograms to prove that the atomic number of an element, and hence the number ofpositive charges in an atom, is the same as its position in the periodic table =see Periodic $a5>. The photon theor ofelectromagnetic radiation 5as further strengthened and developed b the prediction and observation of the so-calledCompton effect b the American phsicist Arthur @oll Compton in )278.

" "lectron Phsics That electric charges 5ere carried b extremel small particles had alread beensuspected in the )2th centur and, as indicated b electrochemical experiments, the charge of these elementar particles5as a definite, invariant (uantit. "xperiments on the conduction of electricit through lo5-pressure gases led to thediscover of t5o inds of ras9 cathode ras, coming from the negative electrode in a gas discharge tube, and positive orcanal ras from the positive electrode. Sir oseph ohn ThomsonDs )126 experiment measured the ratio of the charge ( tothe mass m of the cathode-ra particles. $enard in )122 confirmed that the ratio of ( to m for photoelectric particles 5asidentical to that of cathode ras. The American inventor Thomas Alva "dison had noted in )118 that ver hot 5ires emitelectricit, called thermionic emission =no5 called the "dison effect>, and in )122 Thomson sho5ed that this form ofelectricit also consisted of particles 5ith the same ( to m ratio as the others. About )2)) %illian finall determined thatelectric charge al5as arises in multiples of a basic unit e, and measured the value of e, no5 no5n to be ).*?7 I )?-)2

coulombs. 'rom the measured value of ( to m ratio, 5ith ( set e(ual to e, the mass of the carrier, called electron, couldno5 be determined as 2.))? I )?-8) g.'inall, Thomson and others sho5ed that the positive ras also consisted of particles, each carring a charge e, but of the

positive variet. These particles, ho5ever, no5 recognized as positive ions resulting from the removal of an electron froma neutral atom, are much more massive than the electron. The smallest, the hdrogen ion, is a single proton 5ith a mass of).*+8 I )?-7+ g, about )18+ times more massive than the electron =see &on! &onization>. The :(uantized: nature ofelectric charge 5as no5 firml established and, at the same time, t5o of the fundamental subatomic particles identified.

' Atomic %odels&n )2)8 the e5 Qealand-born British phsicist "rnest Futherford, maing use of the ne5l discovered radiations fromradioactive nuclei, found ThomsonDs earlier model of an atom 5ith uniforml distributed positive and negative charged

particles to be untenable. The ver fast, massive, positivel charged alpha particles he emploed 5ere found to deflectsharpl in their passage through matter. This effect re(uired an atomic model 5ith a heav positive scattering center.Futherford then suggested that the positive charge of an atom 5as concentrated in a massive stationar nucleus, 5ith thenegative electron moving in orbits about it, and positioned b the electric attraction bet5een opposite charges. This solar-sstem-lie atomic model, ho5ever, could not persist according to %ax5ellDs theor, 5here the revolving electrons shouldemit electromagnetic radiation and force a total collapse of the sstem in a ver short time.Another sharp brea 5ith classical phsics 5as re(uired at this point. &t 5as provided b the 4anish phsicist iels @enri4avid Bohr, 5ho postulated the existence 5ithin atoms of certain specified orbits in 5hich electrons could revolve 5ithoutelectromagnetic radiation emission. These allo5ed orbits, or so-called stationar states, are determined b the conditionthat the angular momentum of the orbiting electron must be a positive multiple integral of PlancDs constant, divided b7 R, that is, M nhK7p, 5here the (uantum number n ma have an positive integer value. This extended :(uantization: todnamics, fixed the possible orbits, and allo5ed Bohr to calculate their radii and the corresponding energ levels. Also in)2)8 the model 5as confirmed experimentall b the 0erman-born American phsicist ames 'ranc and the 0erman

phsicist 0ustav @ertz.Bohr developed his model much further. @e explained ho5 atoms radiate light and other electromagnetic 5aves, and also


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proposed that an electron :lifted: b a sufficient disturbance of the atom from the orbit of smallest radius and least energ=the ground state> into another orbit, 5ould soon :fall: bac to the ground state. This falling bac is accompanied b theemission of a single photon of energ " M hf, 5here " is the difference in energ bet5een the higher and lo5er orbits."ach orbit shift emits a characteristic photon of sharpl defined fre(uenc and 5avelength! thus one photon 5ould beemitted in a direct shift from the n M 8 to the n M ) orbit, 5hich 5ill be (uite different from the t5o photons emitted in ase(uential shift from the n M 8 to n M 7 orbit, and then from there to the n M ) orbit. This model no5 allo5ed Bohr toaccount 5ith great accurac for the simplest atomic spectrum, that of hdrogen, 5hich had defied classical phsics.Although BohrDs model 5as extended and refined, it could not explain observations for atoms 5ith more than one electron.

&t could not even account for the intensit of the spectral colors of the simple hdrogen atom. Because it had no more thana limited abilit to predict experimental results, it remained unsatisfactor for theoretical phsicists.

0 Ouantum %echanics ithin a fe5 ears, roughl bet5een )27H and )28?, an entirel ne5 theoreticalapproach to dnamics 5as developed to account for subatomic behavior. amed (uantum mechanics or 5ave mechanics, itstarted 5ith the suggestion in )27H b the 'rench phsicist $ouis 3ictor, Prince de Broglie, that not onl electromagneticradiation but matter could also have 5ave as 5ell as particle aspects. The 5avelength of the so-called matter 5avesassociated 5ith a particle is given b the e(uation M hKmv, 5here m is the particle mass and v its velocit. %atter 5aves5ere conceived of as pilot 5aves guiding the particle motion, a propert that should result in diffraction under suitableconditions. This 5as confirmed in )27+ b the experiments on electron-crstal interactions b the American phsicistsClinton oseph 4avisson and $ester @albert 0ermer and the British phsicist 0eorge Paget Thomson. Subse(uentl,erner @eisenberg, %ax Born, and "rnst Pascual ordan of 0erman and the Austrian phsicist "r5in Schrdingerdeveloped BroglieDs idea into a mathematical form capable of dealing 5ith a number of phsical phenomena and 5ith

problems that could not be handled b classical phsics. &n addition to confirming BohrDs postulate regarding the(uantization of energ levels in atoms, (uantum mechanics no5 provides an understanding of the most complex atoms,

and has also been a guiding spirit in nuclear phsics. Although (uantum mechanics is usuall needed onl on themicroscopic level =5ith e5tonian mechanics still satisfactor for macroscopic sstems>, certain macroscopic effects, suchas the properties of crstalline solids, also exist that can onl be satisfactoril explained b principles of (uantummechanics.0oing beond BroglieDs notion of the 5ave-particle dualit of matter, additional important concepts have since beenincorporated into the (uantum-mechanical picture. These include the discover that electrons must have some permanentmagnetism and, 5ith it, an intrinsic angular momentum, or spin, as a fundamental propert. Spin 5as subse(uentl foundin almost all other elementar particles. &n )276 the Austrian phsicist olfgang Pauli expounded the exclusion principle,5hich states that in an atom no t5o electrons can have precisel the same set of (uantum numbers. 'our (uantum numbersare needed to specif completel the state of an electron in an atom. The exclusion principle is vital for an understandingof the structure of the elements and of the periodic table. @eisenberg in )27+ put forth the uncertaint principle, 5hichasserted the existence of a natural limit to the precision 5ith 5hich certain pairs of phsical (uantities can be no5nsimultaneousl.'inall, a snthesis of (uantum mechanics and relativit 5as made in )271 b the British mathematical phsicist Paul

Adrien %aurice 4irac, leading to the prediction of the existence of the positron and bringing the development of (uantummechanics to a culmination.$argel as a result of BohrDs ideas, a different and statistical approach developed in modern phsics. The fulldeterministic cause-effect relations produced b e5tonian mechanics 5ere supplanted b predictions of future events interms of statistical probabilit onl. Thus, the 5ave propert of matter also implies that, in accordance 5ith the uncertaint

principle, the motion of the particles can never be predicted 5ith absolute certaint even if the forces are no5ncompletel. Although this statistical aspect plas no detectable role in macroscopic motions, it is dominant on themolecular, atomic, and subatomic scale.

@ uclear PhsicsThe understanding of atomic structure 5as also facilitated b Bec(uerelDs discover in )12* of radioactivit in uranium ore=see . ithin a fe5 ears radioactive radiation 5as found to consist of three tpes of emissions9 alpha ras, laterfound b Futherford to be the nuclei of helium atoms! beta ras, sho5n b Bec(uerel to be ver fast electrons! and gammaras, identified later as ver short 5avelength electromagnetic radiation. &n )121 the 'rench phsicists %arie and PierreCurie separated t5o highl radioactive elements, radium and polonium, from uranium ore, thus sho5ing that radiationscould be identified 5ith particular elements. B )2?8 Futherford and the British phsical chemist 'rederic Sodd hadsho5n that the emission of alpha or beta ras resulted in the transmutation of the emitting element into a different one.Fadioactive processes 5ere shortl thereafter found to be completel statistical! no method exists that could indicate 5hichatom in a radioactive material 5ill deca at an one time. These developments, in addition to leading to FutherfordDs andBohrDs model of the atom, also suggested that alpha, beta, and gamma ras could onl come from the nuclei of ver heavatoms. &n )2)2 Futherford bombarded nitrogen 5ith alpha particles and converted it to hdrogen and oxgen, thus

producing the first artificial transmutation of elements.%ean5hile, a no5ledge of the nature and abundance of isotopes 5as gro5ing, largel through the development of themass spectrograph. A model emerged in 5hich the nucleus contained all the positive charge and almost all the mass of theatom. The nuclear-charge carriers 5ere identified as protons, but except for hdrogen, the nuclear mass could beaccounted for onl if some additional uncharged particles 5ere present. &n )287 the British phsicist Sir ames Chad5ic


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discovered the neutron, an electricall neutral particle of mass ).*+6 I )?-7+ g, slightl more than that of the proton.o5 nuclei could be understood as consisting of protons and neutrons, collectivel called nucleons, and the atomic numberof the element 5as simpl the number of protons in the nucleus. ;n the other hand, the isotope number, also called theatomic mass number, 5as the sum of the neutrons and protons present. Thus, all atoms of oxgen =atomic no. 1> haveeight protons, but the three isotopes of oxgen, ;)*, ; )+, and ;)1, also contain 5ithin their respective nuclei eight, nine,or ten neutrons.Positive electric charges repel each other, and because atomic nuclei =except for hdrogen> have more than one proton,the 5ould fl apart except for a strong attractive force, called the nuclear force, or strong interaction that binds the

nucleons to each other. The energ associated 5ith this strong force is ver great, millions of times greater than theenergies characteristic of electrons in their orbits or chemical binding energies. An escaping alpha particle =consisting oft5o protons and t5o neutrons>, therefore, 5ill have to overcome this strong interaction force to escape from a radioactivenucleus such as uranium. This apparent paradox 5as explained b the American phsicists "d5ard , 5hich is promptl e#ected,and a residual proton. The proton left behind leaves the :daughter: nucleus 5ith one more proton than its :parent: andthus increases the atomic number and the position in the periodic table. Alpha or beta emission usuall leaves the nucleus5ith excess energ, 5hich it unloads b emitting a gamma-ra photon.&n all these nuclear processes a large amount of energ, given b "insteinDs " M mc7 e(uation, is released. After the

process is over, the total mass of the product is less than that of the parent, 5ith the mass difference appearing as energ.See uclear "nerg.

3& 4"3"$;P%"TS & P@GS&CS S&C" )28?The rapid expansion of phsics in the last fe5 decades 5as made possible b the fundamental developments during the

first third of the centur, coupled 5ith recent technological advances, particularl in computer technolog, electronics,nuclear-energ applications, and high-energ particle accelerators.

A AcceleratorsFutherford and other earl investigators of nuclear properties 5ere limited to the use of high-energ emissions fromnaturall radioactive substances to probe the atom. The first artificial high-energ emissions 5ere produced in )287 b theBritish phsicist Sir ohn 4ouglas Coccroft and the &rish phsicist "rnest Thomas Sinton alton, 5ho used high-voltagegenerators to accelerate protons to about +??,??? e3 and to bombard lithium 5ith them, transmuting it into helium. ;neelectron volt is the energ gained b an electron 5hen the accelerating voltage is ) 3! it is e(uivalent to about ).* I )?-)2

#oule =>. %odern accelerators produce energies measured in million electron volts =usuall 5ritten mega-electron volts, or%e3>, billion electron volts =giga-electron volts, or 0e3>, or trillion electron volts =tera-electron volts, or Te3>. @igher-voltage sources 5ere first made possible b the invention, also in )287, of the 3an de 0raaff generator b the American

phsicist Fobert . 3an de 0raaff.

This 5as follo5ed almost immediatel b the invention of the cclotron b the American phsicists "rnest ;rlando$a5rence and %ilton Stanle $ivingston. The cclotron uses a magnetic field to bend the tra#ectories of charged particlesinto circles, and during each half-revolution the particles are given a small electric :ic: until the accumulate the highenerg level desired. Protons could be accelerated to about )? %e3 b a cclotron, but higher energies had to a5ait thedevelopment of the snchrotron after the end of orld ar && =)282-)2H6>, based on the ideas of the American phsicist"d5in %attison %c%illan and the Soviet phsicist 3ladimir &. 3esler. After orld ar &&, accelerator design maderapid progress, and accelerators of man tpes 5ere built, producing high-energ beams of electrons, protons, deuterons,heavier ions, and J ras. 'or example, the accelerator at the Stanford $inear Accelerator Center =S$AC> in Stanford,California, accelerates electrons do5n a straight :run5a,: 8.7 m =7 mi> long, at the end of 5hich the attain an energof more than 7? 0e3.hile lo5er-energ accelerators are used in various applications in industr and laboratories, the most po5erful ones areused in studing the structure of elementar particles, the fundamental building blocs of nature. &n such studieselementar particles are broen up b hitting them 5ith beams of pro#ectiles that are usuall protons or electrons. Thedistribution of the fragments ields information on the structure of the elementar particles.To obtain more detailed information in this manner, the use of more energetic pro#ectiles is necessar. Since theacceleration of a pro#ectile is achieved b :pushing: it from behind, to obtain more energetic pro#ectiles it is necessar toeep pushing for a longer time. Thus, high-energ accelerators are generall larger in size. The highest beam energreached at the end of orld ar && 5as less than )?? %e3. A bigger accelerator, reaching 8 0e3, 5as built in the earl)26?s at the Broohaven ational $aborator at


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photographic emulsions and to energize fluorescent materials. The actual paths of ionized particles 5ere first observed bthe British phsicist Charles Thomson Fees ilson in a cloud chamber, 5here 5ater droplets condensed on the ions

produced b the particles during their passage. "lectric or magnetic fields can be used to bend the particle paths, ieldinginformation about their momentum and electric charges. A significant advance on the cloud chamber 5as the constructionof the bubble chamber b the American phsicist 4onald Arthur 0laser in )267. &t uses a li(uid, usuall hdrogen, insteadof air, and the ions produced b a fast particle become centers of boiling, leaving an observable bubble trac. Because thedensit of the li(uid is much higher than that of air, more interactions tae place in a bubble chamber than in a cloudchamber. 'urthermore, the bubbles clear out faster than 5ater droplets, allo5ing more fre(uent ccling of the bubble

chamber. A third development, the spar chamber, evolved in the )26?s. &n this device, man parallel plates are ept at ahigh voltage in a suitable gas atmosphere. An ionizing particle passing bet5een the plates breas do5n the gas, formingspars that delineate its path.

A different tpe of detector, the discharge counter, 5as developed earl during the 7?th centur, largel b the 0ermanphsicist @ans 0eiger, and later improved b the 0erman-American phsicist alther %ller. &t is no5 commonl no5nas the 0eiger-%ller counter, and although small and convenient, it has been largel replaced b faster and moreconvenient solid-state counting devices, such as the scintillation counter, developed about )2H+ b the 0erman-American

phsicist @artmut Paul /allmann and others. &t uses the abilit of ionized particles to produce a flash of light as the passthrough certain organic crstals and li(uids. See Particle 4etectors.

C Cosmic FasAbout )2)) the Austrian-American phsicist 3ictor 'ranz @ess discovered that cosmic radiation, consisting of rasoriginating outside the earthDs atmosphere, arrived in a pattern determined b the earthDs magnetic field =see Cosmic Fas>.The ras 5ere found to be positivel charged and to consist mostl of protons 5ith energies ranging from about ) 0e3 to

)?)) 0e3 =compared to about 8? 0e3 for the fastest particles produced b artificial accelerators>. Cosmic ras trappedinto orbits around the earth account for the 3an Allen radiation belts discovered during an artificial-satellite flight in )262=see Fadiation Belts>.hen a ver energetic primar proton smashes into the atmosphere and collides 5ith the nitrogen and oxgen nuclei

present, it produces large numbers of different secondar particles that spread to5ard the earth as a cosmic-ra sho5er.The origin of the cosmic-ra protons is not et full understood! some undoubtedl come from the sun and the other stars."xcept for the slo5est ras, ho5ever, no mechanism can be found to account for their high energies and the lielihood isthat 5ea galactic fields operate over ver long periods to accelerate interstellar protons =see 0alax! %il a>.

4 "lementar Particles To the electron, proton, neutron, and photon have been added a number offundamental particles. &n )287 the American phsicist Carl 4avid Anderson discovered the antielectron, or positron,

predicted in )271 b 4irac. Anderson found that the stopping of an energetic cosmic gamma ra near a heav nucleusielded an electron-positron pair out of pure energ. hen a positron subse(uentl meets an electron, the annihilate eachother 5ith a burst of photons of energ.

4) 4iscover of the %uon

&n )286 the apanese phsicist Gua5a @idei developed a theor explaining ho5 a nucleus is held together, despite themutual repulsion of its protons, b postulating the existence of a particle intermediate in mass bet5een the electron and the

proton. &n )28* Anderson and his co5orers discovered a ne5 particle of 7?+ electron masses in secondar cosmicradiation! no5 called the mu-meson or muon, it 5as first thought to be Gua5aDs nuclear :glue.: Subse(uent experiments

b the British phsicist Cecil 'ran Po5ell and others led to the discover of a some5hat heavier particle of 7+? electronmasses, the pi-meson or pion =also obtained from secondar cosmic radiation>, 5hich 5as eventuall identified as themissing lin in Gua5aDs theor.%an additional particles have since been found in secondar cosmic radiation and through the use of large accelerators.The include numerous massive particles, classed as hadrons =particles that tae part in the :strong: interaction, 5hich

binds atomic nuclei together>, including hperons and various heav mesons 5ith masses ranging from about one to threeproton masses! and intermediate vector bosons such as the and Q? particles, the carriers of the :5ea: nuclear force.The ma be electricall neutral, positive, or negative, but never have more than one elementar electric charge e."nduring from )?-1 to )?-)H sec, the deca into a variet of lighter particles. "ach particle has its antiparticle and carriessome angular momentum. The all obe certain conservation la5s involving (uantum numbers, such as baron number,strangeness, and isotopic spin.&n )28) Pauli, in order to explain the apparent failure of some conservation la5s in certain radioactive processes,

postulated the existence of electricall neutral particles of zero-rest mass that nevertheless could carr energ andmomentum. This idea 5as further developed b the &talian-born American phsicist "nrico 'ermi, 5ho named the missing

particle the neutrino.


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photon, and all the particles discovered since )287 are collectivel called elementar particles. But the term is actuall amisnomer, for most of the particles, such as the proton, have been found to have ver complicated internal structure."lementar particle phsics is concerned 5ith =)> the internal structure of these building blocs and =7> ho5 the interact5ith one another to form nuclei. The phsical principles that explain ho5 atoms and molecules are built from nuclei andelectrons are alread no5n. At present, vigorous research is being conducted on both fronts in order to learn the phsical

principles upon 5hich all matter is built.;ne popular theor about the internal structure of elementar particles is that the are made of so-called (uars =seeOuar>, 5hich are subparticles of fractional charge! a proton, for example, is made up of three (uars. This theor 5as

first proposed in )2*H b the American phsicists %urra 0ell-%ann and 0eorge Q5eig. 4espite the theorDs abilit toexplain a number of phenomena, no (uars have et been found, and current theor suggests that (uars ma never bereleased as separate entities except under such extreme conditions as those found during the ver creation of the universe.The theor postulated three inds of (uars, but later experiments, especiall the discover of the Kpsi particle in )2+H bthe American phsicists Samuel C. C. Ting and Burton Fichter, called for the introduction of three additional inds.

47


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produce a controlled fusion reaction that output 6.* million 5atts of po5er. @o5ever, the toama consumed more po5erthan it produced during its operation.

' Solid-State Phsics &n solids, the atoms are closel paced, leading to strong interactive forces andnumerous interrelated effects that are not observed in gases, 5here the molecules largel act independentl. &nteractioneffects lead to the mechanical, thermal, electrical, magnetic, and optical properties of solids, 5hich is an area that remainsdifficult to handle theoreticall, although much progress has been made.A principal characteristic of most solids is their crstalline structure, 5ith the atoms arranged in regular and geometricallrepeating arras =see Crstal>. The specific arrangement of the atoms ma arise from a variet of forces! thus, some solids,

such as sodium chloride, or common salt, are held together b ionic bonds originating in the electric attraction bet5een theions of 5hich the materials are composed. &n others, such as diamond, atoms share electrons, giving rise to covalent

bonding. &nert substances, such as neon, exhibit neither of these bonds. Their existence is a result of the so-called van deraals forces, named after the 4utch phsicist ohannes 4ideri van der aals. These forces exist bet5een neutralmolecules or atoms as a result of electric polarization. %etals, on the other hand, are bonded b a so-called electron gas, orelectrons that are freed from the outer atomic shell and shared b all atoms, and that define most properties of the metal=see %etallograph! %etals>.The sharp, discrete energ levels permitted to the electrons in individual atoms become broadened into energ bands 5henthe atoms become closel paced in a solid. The 5idth and separation of these bands define man properties, and thus theseparation b a so-called forbidden band, 5here no electrons ma exist, restricts their motion and results in a good electricand thermal insulator. ;verlapping energ bands and their associated ease of electron motion results in their being goodconductors of electricit and heat. &f the forbidden band is narro5, a fe5 fast electrons ma be able to #ump across, ieldinga semiconductor. &n this case the energ-band spacing ma be greatl affected b minute amounts of impurities, such asarsenic in silicon. The lo5ering of a high-energ band b the impurit results in a so-called donor of electrons, or an n-

tpe semiconductor. The raising of a lo5-energ band b an impurit lie gallium results in an acceptor, 5here thevacancies or :holes: in the electron structure act lie movable positive charges and are characteristic of p-tpesemiconductors. A number of modern electronic devices, notabl the transistor, developed b the American phsicists ohnBardeen, alter @ouser Brattain, and illiam Bradford Shocle, are based on these semiconductor properties.%agnetic properties in a solid arise from the electronsD acting lie tin magnetic dipoles. "lectron spin plas a big role inmagnetism, leading to spin 5aves that have been observed in some solids. Almost all solid properties depend ontemperature. Thus, ferromagnetic materials, including iron and nicel, lose their normal strong residual magnetism at acharacteristic high temperature, called the Curie temperature. "lectrical resistance usuall decreases 5ith decreasingtemperature, and for certain materials, called superconductors, it becomes extremel lo5, near absolute zero. These andman other phenomena observed in solids depend on energ (uantization and can best be described in terms of effective:particles: such as phonons, polarons, and magnons.

0 CrogenicsAt ver lo5 temperatures =near absolute zero>, man materials exhibit striingl different characteristics =see Crogenics>.At the beginning of the 7?th centur the 4utch phsicist @eie /amerlingh ;nnes developed techni(ues for producing

these lo5 temperatures and discovered the superconductivit of mercur9 &t loses all electrical resistance at about H /.%an other elements, allos, and compounds do the same at their characteristic near-zero temperature, 5ith originallmagnetic materials becoming magnetic insulators. The theor of superconductivit, developed largel b the American

phsicists ohn Bardeen, $eon . Cooper, and ohn Fobert Schrieffer, is extremel complicated, involving the pairing ofelectrons in the crstal lattice.Another fascinating discover 5as that helium does not freeze but changes at about 7 / from an ordinar li(uid, @e &, tothe superfluid @e &&, 5hich has no viscosit and has a thermal conductivit about )??? times greater than silver. 'ilms of@e && can creep up the 5alls of their containing vessels and @e && can readil permeate some materials lie platinum. ofull satisfactor theor is et available for this behavior.

@ Plasma Phsics A plasma is an substance =usuall a gas> 5hose atoms have one or more electronsdetached and therefore become ionized. The detached electrons remain, ho5ever, in the gas volume that in an overallsense remains electricall neutral. The ionization can be effected b the introduction of large concentrations of energ,such as bombardment 5ith fast external electrons, irradiation 5ith laser light, or b heating the gas to ver hightemperatures =see $aser>. The individuall charged plasma particles respond to electric and magnetic fields and cantherefore be manipulated and contained.Plasmas are found in gas-filled light sources, such as a neon lamp, in interstellar space 5here residual hdrogen is ionized

b radiation, and in stars 5hose great interior temperatures produce a high degree of ionization, a process closelconnected 5ith the nuclear fusion that supplies the energ of stars. 'or the hdrogen nuclei to fuse into heavier nuclei,the must be fast enough to overcome their mutual electric repulsion. This implies high temperature =millions of degrees>5hen the hdrogen ionizes into a plasma. &n order to produce a controlled fusion, or thermonuclear reaction, it isnecessar to generate and contain plasmas magneticall! this is an important but difficult problem that falls in the field ofmagnetohdrodnamics.

& $asersAn important recent development is that of the laser, an acronm for light amplification b stimulated emission ofradiation. &n lasers, 5hich ma have gases, li(uids, or solids as the 5oring substance, a large number of atoms are raised


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to a high energ level and caused to release this energ simultaneousl, producing coherent light 5here all 5aves are inphase. Similar techni(ues are used for producing micro5ave emissions b the use of masers. The coherence of the lightallo5s for ver high intensit, sharp 5avelength light beams that remain narro5 over tremendous distances! the are farmore intense than light from an other source. Continuous lasers can deliver hundreds of 5atts of po5er, and pulsed laserscan produce millions of 5atts of po5er for ver short periods. 4eveloped during the )26?s and )2*?s, largel b theAmerican engineer and inventor 0ordon 0ould and the American phsicists Charles @ard To5nes, T. @. %aiman, Arthur$eonard Scha5lo5, and Ali avan, the laser toda has become an extremel po5erful tool in research and technolog,5ith applications in communications, medicine, navigation, metallurg, fusion, and material cutting.

Astrophsics The construction of large and speciall designed optical telescopes has led to the discoverof ne5 stellar ob#ects, including (uasars, 5hich are billions of light-ears a5a, and has led to a better understanding ofthe structure of the universe. Fadio astronom has ielded other important discoveries, such as pulsars and the interstellar

bacground radiation, 5hich probabl dates from the origin of the universe. The evolutionar histor of the stars is no55ell understood in terms of nuclear reactions. As a result of recent observations and theoretical calculations, the belief isno5 5idel held that all matter 5as originall in one dense location and that bet5een )? and 7? billion ears ago itexploded in one titanic event often called the big bang. The aftereffects of the explosion have led to a universe that appearsto be still expanding. A puzzling aspect of this universe, recentl revealed, is that the galaxies are not uniformldistributed. &nstead, vast voids are bordered b galactic clusters shaped lie filaments. The pattern of these voids andfilaments lends itself to nonlinear mathematical analsis of the sort used in chaos theor. See also &nflationar Theor.

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