by Isaac Asimov
Altogether, then, I can list twenty Gases which fall into the following categories:
(1) Five elements made up of single atoms: helium, neon, argon, krypton, and xenon.
(2) Four elements made up of two-atom molecules: hydrogen, nitrogen, oxygen, and fluorine.
(3) One element form made up of three-atom mole cules: ozone (of oxygen).
(4) Ten compounds, with molecules built up of two different elements, at least one of which falls into category (2).
The twenty Gases are listed in order of increasing boil ing point in the accompanying table, and that boiling point is given in both the Celsius scale (' C.) and the Absolute scale (' K.).
The five inert gases on the list are scattered among the fifteen other Gases. To be sure, two of the three lowest 192 boiling Gases are helium and neon, but argon is seventh, krypton is tenth, and xenon is seventeenth. It would not be surprising if all the Gases, then, were as inert as the inert gases.
The Twenty Gases
Substance Fori ula B.P. (C.-) B.P. (K.-)
Helium He -268.9 4.2
Hydrogen H, -252.8 20.3
Neon Ne -245.9 27.2
Nitrogen N, -195.8 77.3 f
Carbon monoxide '-O -192 81
Fluorine F2 -188 85
Argon Ar -185.7 87.4
Oxygen 0, -183.0 90.1
Methane CH4 -161.5 111.6
Krypton Kr -152.9 120.2
Nitrogen oxide NO -151.8 121.3
Oxygen difluoride OF, -144.8 128.3
Carbon tetrafluoride CF, -128 145
Nitrogen trifluoride NF3 -120 153
Ozone 0, -111.9 161.2
Silane SiH, -111.8 161.3
Xenon Xe -107.1 166.0
Ethylene C,H, -103.9 169.2
Boron trifluoride BF, -101 172
Chlorine fluoride CIF -100.8 172.3
Perhaps they might be at that, if the smug, self-sufficient molecules that made them up were permanent, unbreak able affairs, but they are not. All the molecules can be broken down under certain conditions, and the free atoms (those of fluorine and oxygen particularly) are active in deed.
This does not show up in the Gases themselves. Sup pose a fluorine molecule breaks up,into two fluorine atoms, and these find themselves surrounded only by fluorine molecules? The only possible result is the re-formation of a fluorine molecule, and nothing much has happened. If, however, there are molecules other than fluorine present, a new molecular combination of greater stability than F2 is possible (indeed, almost certain in the case of fluorine), and a chemical reaction results.
The fluorine molecule does have a tendency to break apart (to a very small extent) even at ordinary tempera tures, and this is enough. The free fluorine atom will attack virtually anything n.on-fluorine in sight, and the heat of reaction will raise the temperature, which will bring about a more extensive split in fluorine molecules, and so on. The result is that molecular fluorine is the most chemically active of all the Gases (with chlorine fluoride almost on a par with it and ozone making a pretty good third).
The oxygen molecule is torn apart with greater diffi culty and therefore remains intact (and inert) under con ditions where fluorine will not. You may think that oxygen is an active element, but for the most part this is only true under elevated temperatures, where more energy is avail able to tear it apart. After all, we live in a sea of free oxygen without damage. Inanimate substances such as pa per, wood, coal, and gasoline, all considered flammable, can be bathed by oxygen for indefinite periods without perceptible chemical reaction-until heated.
Of course, once heated, oxygen does become active and combines easily with other Gases such as hydrogen, carbon monoxide, and methane which, by that token, can't be considered particularly inert either.
The nitrogen molecule is torn apart with still more diffi culty and, before the discovery of the inert gases, nitrogen was the inert gas par excellence. It and carbon tetrafluoride are the only Gases on the list, other than the inert gases themselves, that are respectably inert, but even they can be torn apart.
Life depends on the fact that-certain bacteria can split the nitrogen molecule; and important industrial processes arise out of the fact that man has learned to do the same thing on a large scale. Once the nitrogen molecule is torn apart, the individual nitrogen atom is quite active, bounces around in all sorts of reactions and- in fact, is the fourth most common atom in living tissue and is essential to all its workings.
In the case of the inert gases, all is different. There are no molecules to pull apart. We are dealing with the self sufficient atom itself, and there seemed little likelihood that combination with any other atom would produce a situa tion of greater stability. Attempts to get inert gases to form compounds, at the time they were discovered, failed, and chemists were quickly satisfied that this made sense.
To be sure, chemists continued to try, now and again, but they also continued to fail. Until 1962, then, the only successes chemists had had in tying the inert.gas atoms to other atoms was in the formation of "clathrates." In a clathrate, the atoms making up a molecule form a cage like structure and, sometimes, an extraneous atom-even an inert gas atom-is trapped within the cage as it forms.
The inert gas is then tied to the substance and cannot be liberated without breaking down the molecule. However, the inert gas atom is only physically confined; it has not formed a chemical bond.
And yet, let's reason things out a bit. The boiling point of helium is 4.2' K.; that of neon is 27.20 K., that of argon 87.4' K., that of krypton 120.2' K., that of xenon 166.0' K. The boiling point of radon, the sixth and last inert gas and the one with the most massive atom, is 211.3- K. (-61.8- C.) Radon is not even a Gas, but merely a gas.
Furthermore, as the mass of the inert gas atoms in creases, the ionization potential (a quantity which meas ures the ease with which an electron can be removed alto gether from a particular atom) decreases. The increasing boiling point and decreasing ionization potential both indi cate that the inert gases become less inert as the mass of the individual atoms rises.
By this reasoning, radon would be the least inert of the inert gases and efforts to form compounds should concen trate upon it as offering the best chance. However, radon is a radioactive element with a half-life of less than four days, and is so excessively rare that it can be worked with only under extremely specialized conditions. The next best bet, then, is xenon. This is very rare, but it is available and it is, at least, stable.
Then, if xenon is to form a chemical bond, with what other atom might it be expected to react? Naturally, the most logical bet would be to choose the most reactive sub stance of all-fluorine or some fluorine-containing com pound. If xenon wouldn't react with that, it wouldn't react with anything.
(This may sound as though I am being terribly wise after the event, and I am. However, there are some who were legitimately wise. I am told that Linus Pauling rea soned thus in 1932, well before the event, and that a gentleman named A. von Antropoff did so in 1924.)
In 1962, Neil Bartlett and others at the University of British Columbia were working with a very unusual com pound, platinum hexafluoride (PtF6). To their surprise, they discovered that it was a particularly active compound.
Naturally, they wanted to see what it'could be made to do, and one of the thoughts that arose was that here might be something that could (just possibly) finally pin down an inert gas atom.
So Bartlett mixed the vapors of PtF6 with xenon and, to his astonishment, obtained a compound which seemed to be XePtFc,, xenon platinum hexafluoride. The announce ment of this result left a certain area of doubt, however.
Platinum hexafluoride was a sufficiently complex compound to make it just barely possible that it had formed a clath rate and trapped the xenon.
A group of chemists at Argonne National Laboratory in Chicago therefore tried the straight xenon-plus-fluorine experiment, heating one part of xenon with five parts of fluorine under pressure at 400' C. in a nickel container.
&nb
sp; They obtained xenon tetrafluoride (XeF4), a straightfor ward compound of an inert gas, with no possibility of a clathrate. (To be sure, this experiment could have been tried years before, but it is no disgrace that it wasn't. Pure xenon is very hard to get and pure fluorine is very danger ous to handle, and no chemist could reasonably have been expected to undergo the expense and the risk for so slim-chanced a catch as an inert gas compound until after Bartlett's experiment had increased that "slim chance" tremendously.)
And once the Argonne results were announced, all Hades broke loose. It.looked as though every inorganic chemist in the world went gibbering into the inert gas field. A whole raft of xenon compounds, including not only XeF4, but also XeF., XeF6, XeOF2, XeOF3, XeOF4, XeO3, H4XeO4, and H,XeO,, have been reported.
Enough radon was scraped together to form radon tetra fluoride (RnF4). Even krypton, which is more inert than xenon, has been tamed, and krypton difluoride (KrF2) and krypton tetrafluoride (KrF4) have been formed.
The remaining three inert gases, argon, neon, and helium (in order of increasing inertness), as yet remain untouched.
They are the last of the bachelors, but the world of chemis try has the sound of wedding bells ringing in its ears, and it is a bad time for bachelors.
As an old (and cautious) married man, I can only say to this-no comment.
16. The Haste-Makers
When I first began writing about science for the general public-far back in medieval times-I coined a neat phrase about the activity of a "light-fingered magical catalyst."
My editor stiffened as he came across that phrase, but not with admiration (as had been my modestly confident expectation). He turned on me severely and said, "Nothing in science is magical. It may be puzzling, mysterious, in expbeable-but it is never magical."
It pained me, as you can well imagine, to have to learn a lesson from an editor, of all people, but the lesson seemed too good to miss and, with many a wry grimace, I learned
That left me, however, with the problem of describing the workings of a catalyst, without calling upon magical power for an explanation.
Thus, one of the first experiments conducted by any beginner in a high school chemistry laboratory is to pre pare oxygen by heating potassium chlorate. If it were only potassium chlorate he were heating, oxygen would be evolved but slowly and only at comparatively high temper atures. So he is instructed to add some manganese dioxide first. When he heats the mixture, oxygen comes off rapidly at comparatively low temperatures.
What does the manganese dioxide do? It contributes no oxygen. At the conclusion of the reaction it 'is all still there, unchanged. Its mere presence seems sufficient to hasten the evolution of oxygen. It is a haste-maker or, more properly, a catalyst.
And how can one explain influence by mere presence?
Is it a kind of molecular action at a distance, an extra sensory perception on the part of potassium chlorate that the influential aura of manganese dioxide is present? Is it telekinesis, a para-natural action at a distance on the part of the manganese dioxide? Is it, in short, magic?
Well, let's see…
To begin at the beginning, as I almost invariably do, the first and most famous catalyst in scientific history never existed.
The alchemists of old sought methods for turning base metals into gold. They failed, and so it seemed to them that some essential ingredient was missing in their recipes. The more imaginative among them conceived of a substance which, if added to the mixture they were heating (or what ever) would bring about the production of gold. A small quantity would suffice to produce a great deal of gold and it could be recovered and used again, no doubt.
No one had ever seen this substance but it was de scribed, for some reason, as a drv, earthy material. The ancient alchemists therefore called it xenon, from a Greek word meaning "dry."
In the eighth century the Arabs took over alchemy and called this gold-making catalyst "the xerion" or, in Arabic, at-iksir. When West Europeans finally learned Arabic alchemy in the thirteenth century, at-iksir became "elixir."
As a further tribute to its supposed dry, earthy prop erties, it was commonly called, in Europe, "the philos opber's stone." (Remember that as late as 1800, a "natural philosopher" was what we would now call a "scientist.")
The amazing elixir was bound to have other marvelous properties as well, and the notion arose that it was a cure for all diseases and might very well confer immortality.
Hence, alchemists began to speak of "the elixir of life."
For centuries, the philosopher's stone and/or the elixir of life was searched for but not found. Then, when finally a catalyst was found, it brought about the formation not of lovely, shiny gold, but messy, dangerous sulfuric acid.
Wouldn't you know?
Before 1740, sulfuric acid was hard to prepare. In the* That's all right, though. Sulfuric acid may not be as costly as gold, but it is conservatively speaking-a trillion times as in trinsically useful.
ory, it was easy. You bum sulfur, combining it with oxygen to form sulfur dioxide (SO2)- You burn sulfur dioxide further to make sulfur trioxide (SO3)- You dissolve sulfur trioxide in water to make sulfuric acid, (H2SO4) - The trick, though, was to make sulfur dioxide combine with oxygen.
That could only be done slowly and with difficulty.
In the 1740s, however, an English sulfuric acid man ufacturer named Joshua Ward must have reasoned that saltpeter (potassium nitrate), though nonflammable itself, caused carbon and sulfur to burn with great avidity. (In fact, carbon plus sulfur plus saltpeter is gunpower.) Con sequently, he added saltpeter to his burning sulfur and found that he now obtained sulfur tri'oxide without much trouble and could make sulfuric acid easily and cheaply.
The most wonderful thing about the process was that, at the end, the saltpeter was still present, unchanged. It could be used over and over again. Ward patented the process and the price of sulfuric acid dropped to 5 per cent of what it was before.
Magic? - Well, no.
In 1806, two French chemists, Charles Bernard Ddsormes and Nicholas C16ment, advanced an explanation that contained a principle which is accepted to this day.
It seems, you see, that when sulfur and saltpeter bum together, sulfur dioxide combines with a portion of the saltpeter molecule to form a complex. The oxygen of the saltpeter portion of the complex transfers to the sulfur dioxide portion, which now breaks away as sulfur tri oxide.
What's left (the saltpeter fragment minus oxygen) pro ceeds to pick up that missing oxygen, very readily, from the atmosphere. The saltpeter fragment, restored again, is ready to combine with an additional molecule of sulfur dioxide and pass along oxygen. It is the saltpeter's task simply to pass oxygen from air to sulfur dioxide as fast as it can. It is a middleman, and of course it remains un changed at the end of the reaction.
In fact, the wonder is not that a catalyst hastens a re action while remaining apparently unchanged, but that anyone should suspect even for a moment that anything "magical" is involved. If we were to come across the same phenomenon in the more ordinary affairs of life, we would certainly not make that mistake of assuming magic.
For instance, consider a half-finished brick wall and, five feet from it, a heap of bricks and some mortar. If that were all, then you would expect no change in the situation between 9 A.m. and 5 P.m. except that the mortar would dry out.
Suppose, however, that at 9 A.M. you observed one fac tor in addition-a man, in overalls, standing quietly be tween the wall and the heap of bricks with his hands empty. You observed matters again at 5 P.m. and the same man is standing there, his hands still empty. He has not changed. However, the brick wall is now completed and' the heap of bricks is gone.
The man clearly fulfills the role of catalyst. A reaction has taken place as a result, apparently, of his mere pres ence and without any visible change of diminution in him.
Yet would we dream for a moment of saying "Magic!"?
We would, instead, take it for granted that had we ob served the ma
n in detail all day, we would have caught him transferring the bricks from the heap to the wall one at a time. And what's not magic for the bricklayer is not magic for the saltpeter, either.
With the birth and progress of the nineteenth century, more examples of this sort of thing were discovered. In 1812, for instance, the Russian chemist Gottlieb Sigis mund Kirchhoff…
And here I break off and begin a longish digression for no other reason than that I want to; relying, as I always do, on the infinite patience and good humor of the Gentle Readers.
It may strike you that in saying "the Russian chemist, Gottlieb Sig7ismund Kirchhoff" I have made a humorous error. Surely no one with a name like Gottlieb Sigismund Kirchhoff can be a Russian! It depends, however, on whether you mean a Russian in an ethnic or in a geographic sense.
To explain what I mean, let's go back to the beginning of the thirteenth century. At that time, the regions of our land and Livonia, along the southeastern shores of the Baltic Sea (the modem Latvia and Estonia) were in habited by virtually the last group of pagans in Europe. It was the time of the Crusades, and the Germans to the southeast felt it a pious duty to slaughter the poorly armed and disorganized pagans for the sake of their souls.
The crusading Germans were of the "Order of the Knights of the Sword" (better known by the shorter and more popular name of "Livonian Knights"). They were joined in 1237 by the Teutonic Knights, who had first established themselves in the Holy Land. By the end of the thirteenth century the Baltic shores had been conquered, with the German expeditionary forces in control.
The Teutonic Knights, as a political organization, did not maintain control for more than a couple of centuries.
They were defeated by the Poles in the 1460s. The Swedes, under Gustavus Adolphus, took over in the 1620s, and in the 1720s the Russians, under Peter the Great, replaced the Swedes.
