L. Salem Theoretical Chemistry Laboratory (ERA 549) University of Paris-Sud 91405 orsay, France
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A Faithful Couple: The Electron Pair
...to present some o f the simplest facts of Theoretical Chemistry in a language which is as close as possible to our everyday language, a n d . . . to introduce one o f the basic principles which governs the mechanism in chemical reactions.
Lest the rather Gallic title of my Discourse mislead you, may I reassure you that my purpose is a serious one and is twofold: first, to present some of the simplest facts of Theoretical Chemistry in a lancuage which is as close as possible t u our ewryilny 1;inguage--and recmd, t u i n r r d ~ c onr e OI the Imsic 11r1n1 iplcs u,hich governs the inecha~lia~n of c~hemir,iI reactibns. I hope that the scientists in the audience will hear with me in spite of what may seem to them an awkward translation of the true language of chemistry. Molecules-you see models of them here-are made out of atoms iust like words are made out of letters. The atoms are symbokzed by small halls, which are held together by sticks renresentina" the chemical bonds. Let me now take this rather big atom and make a section of it. You see that the atom is like a fruit: a t the center a stone-which scientists call the nucleus-and around it the pulp. This pulp is made out of electrons. The electron is very difficult to describe. Sometimes it is useful, and correct, to describe electrons as charged particles which run through the pulp of the atom-fruit. Tonight, however, we will describe electrons as waves-and rather special ones a t that: waves that extend in three dimensions. CREST
TROUGH
CREST
indicate a crest, a white area, a trough. There is one specific property of the electron-wave which must he mentioned: whereas the waves of the sea advance, the waves of electrons stay put. Scientists say that they are "stationary." Finally, you will notice that, as shown in the model which I am holding, there may he waves with adjacent crests and troughs, and sharp breaks between them, where there is no wave motion. We will return to the role of these breaks later. Atoms can have waves with different shapes and sizes, and with different numbers of crests and troughs. The simplest wave of all is that shown here: a single crest with the shape of
TROUGH
On the blackboard I have first drawn a wave which extends in one dimension. This is the wave which you would obtain by taking a rope and wiggling one end. If you take an instant photograph of the rope you will see crests and troughs. This wave extends in one dimension, along the direction of the rope; the second, vertical dimension, simply measures the amplitude 344 / Journal of Chemical Education
of the wave and the size of the crest or trouah. - There is no oscillation or undulation in that direction. Looking now a t the waves of the sea, shown below, they extend in two dimensions: they can move forward, or sideways. Again the third dimension serves to measure the size of the wave. You will note that the dark green color of the crests and light green color of the troughs on the blackboard are those of the North Sea, rather than the colors of more Mediterranean seas. Now the electron is a three-dimensional wave. Such a wave is far more difficult to describe since there is no dimension left to show the size of the wave. To build a model of the electron-wave one constructs a volume which encloses the greater part of the wave within its confines. Of course the shape of the
This paper was originally presented as a Discourse at the Royal Institution in Landan in November 1977. The teat of the Discourse will also appear, with photographs of the models, in the Proceedings of the Royd Institution, Volume 51. The Discourses at the Royal Institutiun date back to Faraday and are intended to present scientific ideas to non-specialists.
Two electrons-but no more than two-can "form"or "enter"or 6, occupy,"asit were, thesame w a v e . . For two electrons to enter the same wave, they must have opposite characteristics.
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formed by the single electron in the simplest atom of all, the hydrogen atom. Next we have the section of a more complicated hall-shaped wave: there are two crests and one trough. This is the wave formed by the eleventh and outer electron in the sodium atom.
We now turn to the wave which we were looking a t earlier, with one crest and one trough. Professor Porter, in his Discourse twelve years ago, called this a double-egg wave. You might also call it a wave in the shape of an eight. There are three such waves, pointing in three perpendicular directions,
two horizontal ones and one vertical. These different directions allow the electrons to avoid each other. Metals-such as chromium, nickel, iron, etc.-commonly use another family of waves, which we might call cloverleaf waves. There are five of these. There are two horizontal clo-
verleaves; in the second one the leaves point at 45' relative to the first one. Then we have two vertical cloverleaves; finally the fifth wave of the family does not look like a cloverleaf at all. It resembles a toy with which children play in France, called "diaholo." There are still other tvnes of waves. Atoms can mix their waves together to form new waves. For instance the carbon atom can mix one hall-shaved wave with three double-eee waves to make four waves which are like petals. These four
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petal-shaped waves point towards the corners of a tetrahedron. Such "hvhrid" waves. as thev are sometimes called. were discovered by Linus Pauling many years ago. They provide the explanation for whv carbon can make four eauivalent bonds in tetrahedral directions. This tetrahedral bonding of
carbon is extremely important, since it is the basis of an entire field of chemistry, stereochemisiry. I t explains, for instance, why there are left-handed and right-handed substances. The "Electron Couple"
I will not pursue further the study of the different waves in atoms. This would lead us eventually to the classification of atoms, as it was established by the Russian scientist Mendeleev. But this topic is not the purpose of our Discourse tonight. I just wish to mention one last important property of electrons. Two electrons-hut no more than two--can "form" or "enter" or "occupy," as it were, the same wave. For this to occur, there must be a special matching. Electrons have an intrinsic, intimate, characteristic property. For two electrons to enter the same wave, they must have opposite characteristics. Hence, without pushing the analogy too far, it is legitimate to speak of an electron couple. T o illustrate the intrinsic property of an electron. chemists use an arrow ~ o i n t i "n eeither uowards or downwards. In the helium atom, for instance, there is a sinele - "electron couple" in a hall-shaped wave. and which is symbolized by a pair of opposite arrows.
Let us now consider the encounter of two atoms-and of their waves-to form a molecule or fragment of molecule. Let us, for instance, bring two hall-shaped waves together. If these were waves in the sea, there would be just one big splash. Here, however, two new waves are formed which extend over both atoms. One wave is a laree " crest which covers the entire molecule. Electron couples are particularly happy to enter such a crest, where they find additional stahility. In a sense the
favorable nature of this wave is due to its larger extension. At the same time asecond wave is formed, in which, as if by magic (hut nothing is magic in science), one crest has become a trough. Such a wave, with a sharp break in the middle, is exrremely unfavorable fur accommodating elertnrn couples. Electron cuuples will do their urmost to awid having to enter such a wave. whose effect is disastrous.
We might say that the first type of wave is an "auspicious" wave, or simply a "good" wave. The second type of wave we could call "ill-fated" or simvlv "had". The creation of these two waves is due to an "interference" effect. I t is somewhat similar to the result of throwing two stones in a pond: the ripples may add, or they may destroy each other.. Volume 55, Number 6, June 1978 1 345
By taking away an ill-fated electron and adding an auspicious one, we have now seriously decreased the opposition to the approach of the two molecules. Now the existence of these two types of waves lies a t the heart of the dual nature of chemistry. Let us first bring together two hydrogen atoms, illustrated by these two ballshaped waves each carrying a single arrow. The arrows are
have the methane molecule, in which a central carhon atom is linked to four hydrogen atoms at the four corners of a tetrahedron. We saw earlier how carhon formed four petals. In methane each petal-wave will overlap with the hall-shaped wave of a hydrogen atom to form an auspicious wave, and each auspicious wave carries an electron couple. Hence, the eight electrons.of methane-there are two additional electrons very close to the carhon nucleus-all occupy auspicious waves.
opposed, so as to allow matching. As the waves come together, they form an auspicious wave and an ill-fated wave. The electron couple can enter the auspicious wave, where each electron is happier than hefore in its atomic wave. A stable hydrogen molecule is formed.
But let us repeat the same experiment with two helium atoms. There are now going to he two electron couples, since each helium atom carries one couple. One couple can again enter the auspicious wave. But the second couple would have to enter the ill-fated wave. Of course, i t does not want to do
this; it refuses obstinately to enter the wave. Hence the two helium atoms cannot come toeether and the helium molecule " is not formed. I would now like to illustrate our first experiment here tonight. In this test-tuhe we have a solution of ruhrene. The ruhrene molecule is much more complicated than the molecules we haveseen till now: it has a large skeleton of carhon atoms, attached to each other, and external hydrogen atoms attached to the carbons. But, for the purpose of this experiment, ruhrene behaves like helium: two rubrene molecules do not like to come together, because an electron couple would have to enter an ill-fated wave. I t is oossihle. however. to overcome the effect of ill-fated waves dy subtle engineer& Electrochemistrv is a method wherebv. " . thanks to electricitv provided by m a 6 electrons can be fed in and out of molecules. In this experiment one ruhrene molecule loses an electron to one of the electrodes in the test-tuhe. This electron is, precisely, one which would have entered the ill-fated wave. At the same time the other electrode gives an electron to the second rubrene molecule. This electron, as it so haupens, enters an auspicious wave. By taking away an ill-fated electron and adding an auspicious one, we have now seriously decreased the opposition to the approach of the two molecules. The molecules come together, but only fleetingly. As soon as they loose the extra energy which has been provided to them, they move apart. The additional energy released appears as light. In the experiment, after electricity goes through the electrodes, a bright luminescence appears, telling us that the molecules are movine- aoart. . Let me now illustrate a few more examples of how electron couples link atoms together to form stable bonds. Here we 346 / Journal of Chemical Education
Turn now to ethylene, which is slightly more complex. Each carbon atom is linked to two hydrogen atoms: we know how these bonds are formed and will not consider them anv further. But the carbon atoms are also bonded together dy two electron couples. First we have an auspicious wave made by overlap of two petal waves, one from each carbon. This wave contains an "internal" electron couple. Second. there is a novel wave which is made from two dou.hle-egg waves one on each carbon.This wave extends from one douhle-egg wave, to the other. The upper part is a crest, the lower part a trough. This wave which is made from two double-egg waves, one on each above, but you could equally have them hoth below, or one electron above and one below.
The external wave of ethylene is particularly important. Benzene and ruhrene belong to a large family of molecules where similar waves exist, which extend over the entire molecule. They are real rihbon-waves. There are many such ribbons: each auspicious ribbon carries an electron couple. The ribbon-waves determine most of the physical and chemical properties of the molecules. For example the electrons can tra;el within the ribbons, from one end of a molecule to the other. Suppose then that you stack up a great number of ruhrene or rubrene-like molecules to form a crystal, and you apply an electric field. The electrons will go from one end of one molecule to the other, and then jump toa second molecule, repeat the same process, etc. . . . . The crystal will conduct an electrical current. We understand then why there has been extensive research into this family of molecules, as potential conductors or even superconductors. We also get some insight into how the properties of waves can affect our everyday life. T o summarize, the electron couple is an excellent cement to hold atoms toeether. The conceot of the electron conule as " a molecular cement dates hack to G. N. Lewis, more than sixty years ago (1916).
Let us now bring the two molecules together: a hydrogen chloride "Apollo spacecraftnand the ammonia "Lem". Breaking Up the "Couple"
Let us now turn from structure to reactivity. What happens to the electron couples when molecules react, bonds are broken, atoms rearrange? The answer is that electron couples do not like to break. I might say that there is a Commandment in Chemistry: Thou shalt not break on electron couple.
This Commandment was implied in the title of our Discourse, and we must now show that it is true. The essential part of the proof is that it costs alot of energy to hreak a couple. As an example, let us return to ethylene. In this molecule the external couple is the "weaker" one, although the more appropriate term would he "less strong." How can we set about to break it? If you simply pull the carbon atoms away from each other, yuu will be fighting both couples a t the same time and-as in a hattle-it is better to pick your enemies one at a time. So, rather than pull, we can twist the molecule.. bv. rotatine" one terminal erouo " . of atoms relative to the other group. As we twist to 90°, the small rihhon-like waves are broken. and the electron c o u ~ l ealso: each electron must return to its'atomic douhle-egg wave which has
been restored. Now, even though the external electron couple is not amongst the strongest ones, the reaction requires a great amount of heating, up to a t least 500°C. Hence. in the ereat maioritv of reactions, electron c o u ~ l e s will be prwervedr~will ill;stra& this with two examples. ~ i r s t we will consider the reaction of hvdrogen chloride with ammonia. We start, as an additional exercke in the construction of auspicious waves, by building the bond in hydrogen chloride. The chlorine atom has 17 electrons and 9 waves, but here we show only the wave which has a single electron (arrow). We bring up a hydrogen atom, with its electron: the eight-shaped wave on chlorine and the hall-shaped wave on hydrogen form an auspicious wave in which the electron couple enters hap-
oilv. We now turn to the ammonia molecule. This molecule. kith a slightly pyramidal shape as you see, has a nitrogen atom bonded to three hydrogen atoms. But these three honds are not our concern here: we know now they are built. However there is an additional oetal wave, oointina awav from the three bonds, which also has an electron coupre. chemists call this
couple a "lone" couple-we might even call it a lonely couple: it is quite eager to capture a nucleus or an appropriate atom. Let us now bring the two molecules together: a hydrogen chloride "Apollo spacecraft" and the ammonia "Lem." The hydrogen atom is transferred from the chlorine atom to the nitrogen atom. But in so doing, it leaves its electron behind on the chlorine, so that the electron couple is preserved. As it were, only the hydrogen nucleus is transferred together with an emotv . .wave. The ammonium molecule which is formed is actually electron-poor; it hears a positive charge. The remaining chlorine atom has an extra electron and is electronrich; it gears a negative charge. These two charged systems are called "ions." Thev attract each other. And, because of the attraction between a great number of negatively charged chlorines and ~ositivelvcharged ammoninms. the molecules all stay together and form solid. In our experiment, M. Coates is going to blow through concentrated solutions of hydrogen chloride and ammonia to create the gases, which then mix: the result, as you see, is a white smoke (the solid). In our second example, we return again to ethylene. Suppose we take two ethylene molecules and try and hring them together to form a cyclobutane molecule. The latter is characterized by a ring of four carbon atoms, two new carboncarbon bonds having been formed. We can understand that the outcome of this reaction will deoend on whether or not the electron couples in the external waves of the two ethylenes can transform smoothly into the electron couples in the auspicious
a
waves of the new bonds. Professors Woodward and Hoffmann have shown that i t is imoossible to carrv out this transformation without breaking an electron couple. Hence the reaction is "forbidden": it will not occur, at least in this manner. Woodward and Hoffmann have extended their study to a vast class of reactions. which thev have divided into "allowed" reactions, in which all electron couples are preserved, and "forbidden" reactions, in which an electron couple is broken.' When a reaction is forbidden, either it does not occur, or it occurs via an alternative pathway. Hence molecules do go out of their way to preserve electron couples. Well, is it then impossible to hreak an electron couple? One should not he too pessimistic. I t is possible to hreak a couple by using external means. First, with adequate heat one can eventually hreak any couple. The trouble is that with strong
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Salem. L.,.Yum
J ('hrm suhm~trrrlfur publication.
Volume 55.Number 6.June 1978 1 347
heating molecules are broken up indiscriminatelv. Generallv chemists wish to treat their mb~eculesgently. ~ e me t next mention briefly the use of metal surfaces. If we bring a hydrogen molecule onto a surface of nickel atoms, the molecule instantlv dissociates: the electron couple appears to he easily b ~ o k e n . ~ metal ~ h e surface acts as a sdrt ofbrain or computer which organizes the dissociation. This process is not a t all understood: it is one of the mysteries of chemistry. The metal is called a "catalyst."3The operation does not require a surface; sometimes it is performed by a single metal atom if this atom is appropriately surrounded. I now would like to consider three methods which can be used to hreak an electron couple. I will illustrate these methods on the acetone molecule. which vou see here. We want to hreak a couple in the auspicious wave between two carhon atoms. which holds these two carbon atoms together in a bond. w h a t w e would like to do is to hreak this couple and form an acyl molecule, with one electron of this couple and a methyl
molecule (the latter is actually planar rather than pyramidal as shown here) with the other electron. This couole is extremely solid: heating the molecule to hreak it wouid he even more n,stly than f w rhe twisting oiethylrn~.. The first technique which w ? will cmsider is elertrurhemislry. I mentionrd that t l i i ~melhod allows electnrns to 11r fed in and out of molecules. In the present case we will extract an electron frbm acetone and give it to the appropriate electrode.
With only one electron of the original couple left in the ausoicious wave. the carbon atoms are not stronelv .. . held toeether m v more.Thr mtrlecule l~reuksinto two. Actually this reaction has nor h e n carried out for acetone itcell. hut for mi, of 11s higher "homologs." But there seems ever; reason to believe that the reaction could work with acetone. By the way, we
Although the bond is broken, we do not know whether the electron couple is broken or not. There is some indication that catalysts break bonds while preserving electron couples. Professor Boudart has reminded me that catalysts do more than just break one malecule-they break many thousands of molecules one after another.
348 / Journal of Chemical Education
might mention that the electron lost by the molecule does not, initially come out of the carhon-carhon bond, hut from the carhon-oxygen hond. However the missing electron is soon found in the carhon-carbon hond. The subtle manner in which the electrons thus rearrange is one of the problems which make theoretical chemistry so attractive. A second method is to shine light on the molecule in a photochemical experiment. Light acts as a blow on the electron couple, and "punches" one of the electrons into a different wave which is wnerallv an ill-fated one. In acetone we see on our model that one of the two original electrons in the carbon-carbon hond is now in an ill-fated wave. huilt from the egg-shaped waves on the central carhon atom and the oxygen
atom. Again there is only one electron left to hold the two carbon atoms together, and the bond readily hreaks. The illfated electron will rapidly find its way down to an auspicious wave. (One may notice-again t h a t the electron which is "punched" up does not come originally from the carbon-carhon bond; but the terminal situation is still that described above). From the point of view of energy, whereas normally the molecule has to "climb a hill" because of the energy required to hreak the bond, with light the molecule starts high up in energy and proceeds continuously downwards, without any effort. The third method to hreak a couple brings us to the eternal story of the triangle. One can hreak a couple by approaching a bachelor electron-one which has no uartner. Bachelor electrons are notoriously greedy, and they will compete with that electron in a couple which has the same intimate vrooertv as they have-the 'ompetition being for the p a r t i e r . . ~ h k bachelor electron is often successful! We can illustrate this hy approaching a chlorine atom to a hydrogen molecule: the chlorine bachelor electron will couple with one of the hydrogen electrons, and in so doing hreak the molecule (see bottom of page). A new molecule of hydrogen chloride is formed. Our last experiment illustrates simultaneously two of the previous methods for breaking electron couples. M. Coates i- ~13ingt u grneral? a niirture (11 hydroyen molecule< and chlorine moltwles: the latter are fimnrd try two chlorine atom.. linked tonether. 'l'hen he will flash :i light in imnt of the gaseous mixtu;k. The light first breaks t h e electron couple holding the two chlorine atoms together in the chlorine molecules. Chlorine atoms with bachelor electrons are released. Next the chlorine atoms attack the hydrogen molecules and hreak them. You will now see the end result of this rather dangerous experiment: a large amount of energy is released! You have seen that only extreme violence will separate an electron pair. Otherwise, as we have indicated, it is a very faithful couple.
