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.
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.