chemical principle/ revifited
Edited by DAN KALLUS
Midland Senior High School
906 W. lllinoi~
Midland, TX 79705
Updating the Atomic Theory in General Chemistry Mark Whitman William Henry Harrison H.S., 5700 N. Road 50 West, West Lafayette, IN 47906 Each year in our high school chemistry courses, we teachers take great pains to descrihe the events and investigations which culminate in what we have come to call the "modern" atomic theory. With the aid of our textbooks, we impress upon our students the importance of the contributions of Dalton, Thomson, Rutherford, Bohr, Schroedinger, and others, and then. havine reviewed their work. we move on. However. in the area of zomic theory, theory has grown much more rapidlv than standard hirh s c h d text revisions can handle. This information lag can and often does result in the presentation of incomnlete or erroneous data. Fiese&hen in the field of atomic structure are in the midst of an information explosion of a magnitude unprecedented in the history of chemical investigation. The prelates of the new order include the theorists such as Gell-Mann. Weinberg, Salam, Glashow, and Feynman and the experimentalists such as Ricter, Ting, Lederman, and Ruhhia. Although these names will certainly never supplant those previously mentioned, they will douhtless fit comfortably alongside them, and, of even ereater imnortance to the educator attemntine to motivate " students, these people are our contemporaries. I t is the intent of this article to present a descriptive overview of the recent achievements that have done so much to further our knowledee of atomic stucture and thus Drovide instructors with the hackground necessary to enhance their classrwm presentations.
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What are Quarks Anyway?
Through the early 1930's, atomic theory included protons, neutrons, and electrons as the fundamental sub-atomic particles. However, with the advent of high-energy particle accelerators. the orevionslv simple picture of the atom became more andmore cluttered. ~ & e n of i nuclear particles were discovered as researchers were able to view the products of countless collisions of particles. While accepting the proliferation of nuclear particles, many theorists were disturhed by the possibility of so many "building blocks," and the atomic model a t this point was in no way simple, which seemed a contradiction t o the ways of nature. ~ n r i c oFermi was distressed to the point of commenting that so many particles existed that it would take a botanist to remember the names (1). This featureis imended as a review of basic demicai plncipies and as a reappaisa1ofme state a t h a r t . Camnents, suggestions fw twin. and contributions should be sent to the feature editor.
952
Journal of Chemical Education
HADRONS
Figure 1. The subdlvislons of hadrons.
In 1963, two theorists from the California Institute of Technology, Murray Gell-Mann and George Zweig, indenendentlv arrived a t a conceot that hroueht order to this eomplex bicture. Both concepts used thejdea of a fundamental narticle and its anticinated counter~art.Gell-Mann named his particle the " q u a r k (Zweig's w& named "aces.") The present-day quark theory is quite different from that originally proposed in 1963 (Z), hut the essence of the concept remains the same: suh-nuclear oarticles Le.. protons and neutrons) are composed of findamenti1 &icles that cannot he divided into smaller entities. The fundamental particles are identified as quarks and their antiquark counterparts. GeU-Mann originallypostulated the existence of three types of quarks: the up (u) quark, the down (d) quark, and the strange (s) quark (I).' The theory specified that each suhnuclear particle (termed hadron) was composed of a set of quarks, e.g., 1proton = 2u I d and 1neutron = 1u 2d. (No longer are protons and neutrons to he thought of as elementary particles.) Thus, depending primarily on their quark composition, hadrons have been subdivided into three groups: baryons, which are particles composed of three quarks (protons and neutrons belong to this group), antiharyons, which are composed of three antiquarks, and mesons, which are composed of a quark-antiquark pair (3-5).(See Fig. 1.) Thus in the present particle theory quarks remain the fundamental particles, and as experimentation has continued the number of ouarks has nroliferated. The theorv now encompasses six and six antiquark's, hringing ;he total to twelve fundamental oarticles which are differentiated by the quantum numbers that descrihe them. (See Table 1.) A quantum number is assigned for each inherent property possessed by a quark, so that no two quarks within a hadron
+
+
A word of caution to the reader: To be comfortablewith the language of high-energy physics, one must be ready to accept new meanings for common words, which have been adapted for descriptive uses only remotely connected to their everyday definitions.
Table 1. Fundamental Parllcles (6) EleCtllc Barvon
S p . Spin
~~avcw bo~
Charge ~un;ber Sbange
(J)
Q
% % % % % %
+% -%
% % %
-% +% -%
b
+%
ness (sl
(6)
Cham Trulh Beauty (c) (t) (b)
Quarks
u d
UP Dom Wlarmed
c
S'Janse
s
Trvm Beauty
t
Amlup Amldnvn Ami-
c-
b
d
a
+'I3
+% +% +% +%
+Y3 -'la
+% ' 1
+'Is
Amiquarks -% -# -%
ChamF
ed Am&
6
Anti-
-1
0
-%
0
0
0
beauty
Table 2. A Partlal Li.1 of Hadrons and melr Quark Composltlom (3.6,7 )
Amibewons
Barvons
Ran Neutrm Lambda Charmed
wd +1
Amlpmton
udd
0 Antlneubon
uds udc
0
Lambda Omega sss
uud
-1
Pion(?r+)
ud
0 Kaon(Kf)
Amilambda
0 Charmed
Mesans
udc
Antilambda
+1 +1
0 Upsilon (Y)
bb
0
0 Phi Psi
$5
0
CZ
0
-1 D Meson (D+)
Neutral D
cd ci
+1
0
Meson 10'1 A h s h n hidexist foreach pwrlble qua*
cmblnatm.
can have the same set of quantum numbers. This rule of ex-
clusivity nicely parallels Pauli's exclusion principle as it applies to electrons in the same orbital. To simplify further, except for spin (intrinsic angular momentum), the quantum values for the auarks are additive. and the sums eaual the quantum valuesof the hadrons thehuarks compose (4). (See Table 2.) For examnle. a oroton is comnosed of two un. auarks . and onedown qua;k. m here fore,
+
Electric Charge = +V3 YQ- = +1 Baryon Quantum Number = +% + % + 35 = +I.
+
For a pion, a 1meson, composed of an up quark and an anti-down quark, the sums are Electric Charge = +V3 + % = -1 Baryon Quantum Numher = +% - 'h = 0 The additive property of these values runs true for all hadrons, thus the quantum properties of all hadrons are explained by combinations of quarks, and all combinations of quarks should create all hadrons. This is a useful rule, as discovery of an unexplained badron may prophesy an undiscovered quark, which, once discovered, will in turn prophesy yet to be discovered hadrons. Heavy hadrons normally decay very rapidly (on the order of 10-2"econd). Gell-Mann noted, however, that nome heaw hadrons decay at a strangely slow rate (10-11 second). To eiplain this observation, he postulated that some possessed a property he originally called strangeness (1, 6). Empirical evidence for the original conjecture of three quarks was verified a t Brookhaven in 1964, but the verification of the strange
quark upnet the symmetry of the theory. As more data were analyzed, it became evident that the elementary particles are paired into what are termed "doublets." If up and down were a doublet, why should not strangeness also have a partner? In 1974, a particle (the J or psi meson) was discovered that pmesxed quantum properties which could not be accounted for by the use of the three known quarks. It too was a heavy hadron, and it had a decay rate 1000 times slower than predicted. The nsi meson was said to Dossess charm and to be composed o i a charmed quark, wLose quantum properties would fit those needed for the osi meson. Truth and beauty (referred tb by some as top and bottom) are similar to strangeness and charm, except that quarks possessing truth and beauty are even heavier than those possessing strangeness and charm. Quarks possessing truth and beauty have been predicted since circa 1975. However, due to their inordinate masses, a great deal of energy is required to produce the evidence needed to confirm their existence. As of this writing, no such empirical data for a truth~ossessineauark have been fortbcomine. The oresence of a beauty-po&ssing quark was recently conhnedby a research group from Cornell University (7). Present theory provides us 12 fundamental nuclear particles. auarks and antiauarks. each with a uniaue set of auant u g numbers. ~ o w e i e rse;eral , nuclear parthes havebeen postulated to contain two of the same type of quarks. A proton, for example, contains two up quarks. Such multiplicity of quarks within the same baryon is, however, counter to the exciusion rule stated earlier (no two quarks in the same hadron will have the same set of quantum numbers). To overcome this apparent inconsistency, a new quantum property termed "color" was invented. The quark color represents the force needed to hold quarks together. There are three colors available to quarks. These are the primary colors of red, green, and blue. The complementary anti-colors (anti-red, anti-green, and anti-blue) are available to the antiquarks. Given three quarks in a baryon, the theory states that each of the three primary colors will be represented, collectively producing what is considered to be a colorless barvon. The assienment of which color is associated with which &ark in th& set cannot he specified and may in fact undergo constant change. A colorless meson is produced by two quarks bounded together by a color and its anti-color. Thus.. the oresence of the auantum nrooertv . of color insures that no two quarks shall possess thesame set of quantum properties in the same hadron. If they did, the hadron would not be colorless, and according to theory, this is imoossible ( 1 . 4 ) .Though the reader may a t this ooint be t e m h d to judge the theory as seemingly Lbitrariand capricious, all aspecta including color withstand even the severest of tests. Perhaps the theory will crumble with time or he modified to explain new observations, but a t this point the theory is not contradictory to experimental results. Leapin' Leptons1 The atomictheory, as taught a t the high schoollevel, usually identifies the electron as a uniaue narticle outside of the nucleus. Experimental evidence, however, now suggests that the electron is not uniaue. In fact. it is onlv " one member of the family of particles Ealled "lep&ns." Lentons. like auarks. are considered elementan oarticles that :annot be broken down into smaller entities. hey differ from quarks in three distinct ways: (1) leptons are not fractionallycharged, hot possess unit charges of -1,0, and +1; (2) leptons do not combine to form more complex particles, such as quarks form hadrons; and (3) leptons are not influenced by the strong forceWat binds quarks together.2 The sbong face is one d fou faces presently accepted as existing in natue and will be discussed later.
Volume 61 Number 11 November 1984
953
Table 9.
Summary ol Leptons and AntlteptaM ( 5 ) LeDhms
Mble elemon e i e m n neutrino mwn mwn neutrino tau tau nevbino
Symbol 8-
Charge -1
POBRIY.electron Wltron) electron antlneutrlno pwitive muon mwn amlneublno positive tau tau antinetn1no
The lepton-antilepton family (again as with quarks) has 12 members, consisting of electrons, muons (heavy electrons), taus (superheavy electrons), neutrinos (neutral electron-type particles) and their antiparticle counterparts (5, 7). Electrons. muons. and taus amear . . to be similar exceDt in mass (a muon is approximately 206 times more massive than the electron, while the tau particle is almost 3500 times more massive th& the electron). The neutrino, however, is an oddball. Like quarks and the other three leptons, it may he a dimensionless point-like particle, but unlike the others, neutrinos are neutral and may be massless. Thus, it is difficult to substantiate directly the existence of neutrinos. Verification has resulted only by viewing events in which they interact with other particles. However, such observations are plentiful, and there is a strong indication that there are apparently three types of neutrinos (one for each lepton) and three types of antineutrinos-one for each type of antilepton (Table 3). Considering both ouarks and leotons. as well as their antiparticles, present theory postulates 2 4 subatomic building blocks 16). Chinese hieh-enerev have. in fact.. hv-" nhvsicists .. pothes&d a never-elding chain of minute pakicles, each beine constructed of vet another smaller unit. At the vew leasLevidence may s;meday be found for a precursor of thk quark, which will comprise quarks themnelven ( 1 ) . ?
May the Forces be wnh You
Quarks and leptons are only part of the picture presented bv todav's theorv of atomic structure. The other part belongs 6 the forces thai are currently considered to affeit subatomic narticles. The present theory goes heyond definitions and delves into the-means by which forces are carried hetween particles participating in an interaction. Particle interactions can be thought of as events in which two particles influence one another by their presence, or in which force is exerted between the particles. Research in high-energy physics concerns itself with sucb interactions. Four forces are thought to exist, including the familiar gravity and the electric force, which are easily observed at the macroscopic levels. Of these forces, gravity is by far the weakest, requiring interactions between relatively hugh bodies before its influence becomes sienificant. In snite of the meater strength of the electric force, its almost infiite range accounts for its detectability at the macroscopic level. However, the other two forces (the strong force and the weak force) are not as easily detected and are apparently detectable only at the subatomic level. The strong force is the strongest of the four forces (much stronger than the runner-up electric force). Its influence is restricted to the distance between quarks. The property of color, discussed earlier as the property responsible 954
Journal of Chemical Education
for binding quarks together, may relate to the strong force as charge does to the electric force. The weak force is just as its name suggests. It is the second weakest of the forces, although its strength is much greater than that of gravity. In addition, its range is very short, even shorter than-the quark-to-quark rangeof the strong force. Despite its weakness and range limitations, the weak force may be the most interesting of the group, for it promotes transmutations between particles, most commonly in the form of article decavs 11.4). .. , The effect of gravity, as stated, is considered to be significant only on the macroscopic level; therefore, gravity is of little interest at the subatomic level and will be considered no further. The other forces. however. are imwrtant at this level and are significant when examining interactions between elementary particles. Present theorv ~ostulatesthat when the weak fo%, the electric force, or thk strong force is transmitted between the particles, a short-lived entitv is exchan~ed.This intermediate force-carrier particle is called a "bos&n." For each force, there is theorized to he a unique boson, or set of bosons; i.e., the carrier of the electric force is thought to be the photon, the carrier of the strong force between quarks is thought to be the gluon, while the weak force is thought to utilize a set of massive carriers, the W+, and W-, and Zo bosons 13). ~o'dsualizeparticle interactions, investigators utilize a tool called the Fevnman diaeram. named for its creator. the eminent mer rick theorisc~ichardFeynman. Figure 2 is an illustration of a Fevnman diamam as a ~ o l i e dto electron-electron repulsions. when two el&trons interact, their like charges cause them to re~el.Accordina to resent theorv. the electrons repel one anothkr by transmitting the electric force through a photon. Electric interactions also occur in the collision of a particle and an anti~article.An example of sucb an interaction occurs when an el&ron and a posiGon collide. They are both annihilated as a photon, which carries the electric force, is produced (Fig. 3). This photon then transforms into a muonantimuon pair (7). The in~eiactionisquite different when the strong color force (an attractive force) is involved. Gluons, so named because thev serve as the elue between ouarks. are thoueht to transmit the"strong color f&e from one {uark & anothe;, thus bonding the oarticles toeether. Fieure 4 illustrates a nossible interact i ~ ~ b e t w eG e nu p quarcand a down quark: Gluons are said to be color sensitive, but they are insensitive to flavor, meaning that they do not possess the ability to transform quarks from one type (flavor) to another. Thus when a gluon transmittance
~igure2. Electron~lectronrepulsion,
photon e+ Figure 3. Elemon-positronannihilation.
"
(red) u- -
(blue) gluon
(blue) Figure 4. Stmng color force between an up and down quark.
Table 4. Tv~e Strong Face Figure 5. Up quark decay.
Ranoe
Forces of Nature
Sbenoth
cm 100 times (diameter greater of a man the hadron) electric force
Eleclric Force
Infinite
Weak Forre
lo-'$ cm
Carrier a. Giurns
a. Binds q&s logwther
b. M s m
b. Binds potons and neuhons together Attraction and repulsion between charged particles Acts on q&s and te~tons
Photons
Figure 6. Tau lepton decay.
occurs. the flavor of the resulting-auarks does not change, . - . but
their color may. Such color changes can occur only in pairs, such that one auark interchanges colors with a second auark, thus maintainkg the integritiof the parent hadron in Ghich the auarks are hound (4.7). he weak force is unique in its ability to promote particle transformations, such as altering the identity of a quark or a lepton. The presence of the weak force is most commonly noticed in particle decays. An up quark may, for example, decay into a down quark and a W+ boson, with the boson materializing (in perhaps second) into an electron antineutrino and a positron (Fig. 5). A tau lepton may analogously decay into a tau neutrino and a W- boson with the hoson materializing into a muon and a muon antineutrino. (See Figure 6.) Lest the reader search for a deeoer meaning than is intended, it is important to point out that the above discussion of bosons and the accompanying figures are presented to inform the reader of the mechanics required to investigate and support present theories. For one must realize that to develop a complete theory of elementary particles, it is now necessary to have more than a superficial understanding of the forces that affect the attractions, repulsions, collisions, and decays of articles ( 4 ) . The c o n c e ~of t bosons is a creative solution to perplexi'n~prohlem,&d it may he empirically validated within the near future as research facilities are being upgraded t o test for their existence. The information in Table 4 summarizes the properties of the four forces (I).
a
Unification The final focal point of this discussion is centered around the unified fields theory. If one acceptr the "Rig Bang" theory of the orieinr of the universe. then it follows that all oariicles and forces have a common starting point, the implication being that these particles and forces must have been originally unified or singular. Early in this century, Einstein showed us that mass and enerev are convertible. In 1967. Steven Weinberg and ~ b d u c ~ a l a m working , independently, advanced the theory of unification still further. They hypothesized the unification of the electric force and the weak force into a single entitv called the "electroweak" force. This hvpothesis &ried with it a corollary that relates the massle& chargeless photon to the massive, singularly charged W bosons. Though this may appear to be an unlikely relationship, subsequent proofs demonstrate that given sufficient energy such would be the case (I). Energy, in fact, is the crux of the nnification theory. If there were a Big Bang, then it originated by definition with a tremendous outpouring of energy that dissipated from that initial moment. As the universe cooled, elementary particles and the forces "froze out" as products of the process. To reconstruct the event, one simply needs to investigate what would happen to the elementary particles if the system were placed in reverse and if mass were transformed into energy. Such investigations in this area seem to indicate that a gradual increase in energy u
lo-"
mat of the electric face Weakest of all forces
Effect
WC.W-.Z0 bosons
Qavilon
Signlflcam only between bodies
would fmt result in the unification of the strong force with the electroweak force. Then gravity would be unified with the resulting strong-electroweak force, and it is theorized that a t some point all forces in nature would unify into one (8). A similar theory is postulated for the particles. Addition of enerw would first seoarate the hadrons from each other and finali; the quarks from one another. At some point, the quarks and lentons would become interchangeable. and finallv the particfes and the forces of nature wouk he iidistinguishahle. If these events were to be viewed from the oers~ectiveof the moment of the Big Bang, the creation of the particles and the forces would have been completed within the first three minutes of the original explosion, and the four forces would have frozen out within the first 10-10 second (8). Therefore, according to the theorists, almost anything is exohinable given enough enerev. When events substantiatinr! thk quark model wererecorded a t Brookhaven in the earl; 1960's, the acceleration energy was t he approximate equivalent of 33 GeV (33 hillion electron volts) (I). In 1985, when remodeling is finished, Fermi National Accelerator (Fermilab) in ~ a t a v i a ,Illinois, will utilize a superconducting 6.4-km underground magnetic ring to accelerate protons and antiprotons with an energy of 2 TeV (2 trillion electron volts). I t is hoped that experimentation will provide further empirical evidence of the weak intermediate W+, W-, and Zo hosons which were recently found a t the CERN particle accelerator. Acknowledgment The author would like to thank Laslow Gutay, Purdue University, who has offered a number of helpful suggestions in the preparation of this paper.
I21 Clint. Dasld B..Rubhra. Carlo. \an der Mar, Sirnm. "The Semh for lnvrmediav V m r h o r n . " So A w r .246,319R2, 1.3 Dak#n.Jamca'r.."ThrQwk-A Decade Lala,." 77wPhya Teorh..la. Ill9111 I O Mamhak. ti E . " n s Fnvnh € o m in Ksurc." mI+pTrarh..9,9~191l,.
Glossary of Terms Antibaryon: A hadron that is composed of three antiquarks. Antiquark: An elementary particle. Antiquarks are the mirror images of quarks. For each quark, there exists an antiquark. Volume 61 Number 11 November 1984
955
Baryon: A hadron that is composed of three quarks. Beauty: See strangeness. Boson: A short-lived force carrier. Bosons transmit forces from one particle to another. Charm: See strangeness. Color: A quantum property of quarks, responsiblefor binding quarks. No two quarks in the same hadron may pmsess the same color. The colors present in a baryon are the primary colors of red, blue, and green. The colors present in an anti-baryon are the primary complements anti-red, anti-blue, and anti-green. The colors present in a meson are a primary color and its complement. Colors combine in hadrons to leave the hadron colorless. Electric Farce: A force between two charged objects. Infinite range, and effective on hoth the macroscopic and subatomic particle Levels. Electron: A member of the lepton family, -1 charge. Flavor: A term to differentiate between types of quarks. Gell-Mann, Murray: First hypothesized the existence uf quarks at the same time as George Zweig in 1963. For this. and subsequent work, considered hy most aq the father of the quark theory. Gluon: A massless buson which transmits the strong force between quarks. 1 s mnge is limited to thc diameter uf a hadron, lo-'" Hadmn: A particle suhject tothe strong force. Hadmns are corn& of quarks. Hadrons can he subdivided into baryons, antibaryons, and mesons. Lepton: An elementary particle that is not affected by the strong force, and leptons do not combine to form mare complex particles. Leptons possess unit charges of -1,0, +l.Present theory accepts six leptons: electron, muon, tau, and three different neutrinos. Meson: A hadronthat is composed of a quark and an antiquark. Thought to transmit the strong force between haryons, antibaryons, and mesons.
958
Journal of Chemical Education
Muon (heavy electron): A lepton, charge -1. Muons possess a mass 206 times greater than the mass of an electron. Neutrino: A virtually masslean neutral lepton. Three different neutrinos have heen identified, one paired with each of the negative leptons (electron neutrino, muon neutrino, and tau neutrino). Neutron: A neutral baryon composed of two down quarks and one up quark. Photon: A maasleas h o n which transmits the electric force. Its range is infinite. Proton: A baryon that is composed of two up quarks and one down quark. +1charge. Quark: An elementary particle which is subject to the strong force, &g afractianal charge of -% or +%. The quark is considered to he a point-like, possihly dimensionless particle. Present theory accepts sir quarks, up (u), down (d), strange (s), charmed (4,truth (t), and beauty (h). Strangeneas: Along with beauty, &um, and truth, a unique property possessed by individual quarks. The presence of these quarks in a heavy hadron slows down the decay of the particle by as much as several orders of 10. Strong force: The strongest known force. Responsible for bonding protons to neutrons, and quarks to quarks. Tau (superheavy electron): A lepton. Tau particles possess a charge of -1 and are almost 3500 times heavier than electrons. Truth: See strangeness. Unified Field Theory A theory which attempts to unify all forces and elementary particles into one singular unit. W+, W-, Z? Heavy, short-lived hosons which transmit the weak force. Range of 10-16 em. Weak Force: Next to gravity, the weakest of all known forces. The shonest range of all forres, 1W'Scm. The weak force is responsible for particle transmutations
