These shared electrons fit into the outermost electron shells of each carbon atom that contributes. Each carbon atom has 4 electrons of its own in that outermost shell and 4 electrons contributed (one apiece) by four neighbors.

Now, each carbon atom has the 2,8 configuration of neon, but only at the price of remaining close to its neighbors.

The result is a strong interatomic attraction, even though electrical charge is not involved. Carbon is a solid'and is not a gas until a temperature of 42000 C. is reached.

The atoms of metallic elements also stick together ,strongly, for similar reasons, so that tungsten, for instance, is not a gas until a temperature of 59000 C. is reached.

We cannot, then, expect to have a Gas when atoms achieve stable electron distribution by transferring elec trons in such a manner as to gain an electric charge; or by sharing electrons in so complicated a fashion that vast numbers of atoms stick together in one piece.

What we need is something intermediate. We need a situation where atoms achieve stability by sharing electrons (so that no electric charge arises) but where the total number of atoms involved in the sharing is very small so that only small molecules result. Within the molecules, attractive forces may be large, and the molecules may not be shaken apart without extreme temperature. The attrac tive forces between one molecule and its neighbor, how ever, may be smafl-and that will do.

Let's consider the hydrogen atom, for instance. It has but a single electron. Two hydrogen atoms can each con tribute its single electron to form a shared pool. As long as they stay together, each can count both electrons in its outermost shell and each will have the stable helium configuration. Furthermore, neither hydrogen atom will have any electrons left to form pools with other neighbors, hence the molecule will end there. Hydrogen gas will con sist of two-atom molecules (H2) The attractive force between the atoms in the molecule is large, and it takes temperatures of more than 20001 C. to shake even a small fraction of the hydrogen molecules into single atoms. There will, however, be only weak at tractions among separate hydrogen molecules, each of which, under the new arrangement, will have reached a satisfactory pitch of self-sufficiency. Hydrogen, therefore, will be a Gas not made up of separate atoms as is the case with the inert gases, but of two-atom molecules.

Something similar will be true in the case of fluorine (electronic distribution 2,7), oxygen (2,6) and nitrogen (2,5). The fluorine atom can contribute an electron and form a shared pool of two electrons with a,neighboring fluorine atom which also contributes an electron. Two oxygen atoms can contribute two electrons apiece to form a shared pool of four electrons, and two nitrogen atoms can contribute three electrons each and form a shared pool of six electrons.

I In each case, the atoms will achieve the 2,8 distribution of neon at the cost of forining paired molecules. As a result, enough stability is achieved so that fluorine (F2). oxygen (02), and nitrogen (N2) are all Gases.

The oxygen atom can also form a shared pool of two electrons with each of two neighbors, and those two neigh bors can form another shared pool of two electrons among themselves. The result is a combination of three oxygen atoms (O:j), each with a neon configuration. This com bination, 03, is called ozone, and it is a Gas too.

Oxygen, nitrogen, and fluorine can form mixed mole cules, too. For instance, a nitrogen and an oxygen atom can combine to achieve the necessary stability for each.

Nitrogen may also form shared pools of two electrons with each of three fluorine atoms, while oxygen may do so with each of two. The resulting compounds: nitrogen oxide (NO), nitroen trifluoride (NF3), and oxygen di fluoride (OF2) are all Gases.

Atoms which, by themselves, will not form Gases may do so if combined with either hydrogen, oxygen, nitrogen, or fluorine. For instance, two chlorine atoms (2,8,7, re member) will form a shared pool of two electrons so that ,both achieve the 2,8,8 argon configuration. Chlorine (CI2) is therefore a gas at room temperature-with intermolecu lar attractions, however, large enough to keep it from be ing a Gas, Yet if a chlorine atom forms a shared pool of two electrons with a fluorine atom, the result, chlorine fluoride (CIF), is a.Gas.

The boron atom (2,3) can form a shared pool of two electrons with each of three fluorine atoms, and the carbon atom a shared pool of two electrons with each of four fluorine atoms. The resulting compounds, boron trifluoride (BF3) and carbon tetrafluoride (CF4), are Gases.

A carbon atom can form a shared pool of two elec trons with each of four hydrogen atoms, or a shared pool of four electrons with an oxygen atom, and the resulting compounds, methane (CH-4) and carbon monoxide (CO), are gases. A two-carbon combination may set up a shared pool of two electrons with each of four hydrogen atoms (and a shared pool of four electrons with one another); a silicon atom may setup a shared pool of two electrons with each of four hydrogen atoms. The compounds, ethylene (C2H4) and silane (SiH4), are Gases.

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.