ChemMatters October 1983 Page 4

© Copyright 1983, American Chemical Society

An Atomic Tour

By Isaac Asimov

Argon

Imagine that we were immersed in a container of argon and magically found ourselves growing smaller and smaller. (Let’s also imagine that we don’t have to breathe!) Originally, the container of gas looks quite empty because argon is as colorless and transparent as air. We would only see the container growing larger and larger, and familiar objects seen through the glass walls would become farther away, growing mistier until they vanished. All we would then have about us would be the argon.

Eventually the argon would seem to become "grainy." It would no longer be smooth nothingness but would consist of tiny, quickly moving objects. These would be the argon atoms. We couldn’t really see the atoms, because the light waves by which we see are larger than atoms and "step over" them, so to speak. Let’s pretend, though, that we can see them and that we shrink until the Argon atoms are as large as billiard balls. They wouldn’t look like billiard balls, however. Their surfaces would be fuzzy and twinkling, for the outside of atoms, the electrons, are so light they look more like flickering waves than like particles. Each argon atom has 18 electrons. Nor are the argon atoms motionless; each of them is moving. (They move so rapidly in comparison to their size that we wouldn’t see them as they sped by us. We have to imagine that we are watching them in slow motion.)

In that case, we would see that they are moving at different speeds. A few are moving quite slowly. A few are moving quite quickly. Most move at intermediate speeds. The average speed of the atoms is related to the temperature of the gas—the hotter the gas, the greater the average speed of the atoms.

Naturally, the atoms collide occasionally. In fact, each atom collides with other atoms hundreds of millions of times a second, but since we are watching in slow motion, we will see it happen just now and then. Two atoms will approach, squeeze together, and bounce off as though they were soft rubber balls. The electrons on each atom all carry negative electric charges, and like charges repel each other. Every time two atoms collide and bounce, their speed is likely to change—one moves faster than it did before, the other slower. On the whole, though, the average speed of the molecules does not change. And so, if not interfered with from outside, the argon gas would go on forever, with its trillions upon trillions of individual atoms each moving in random directions, each undergoing innumerable collisions, changing directions and speed with each collision—and yet the whole sample would, to our normal view, remain motionless and unchanging.

Oxygen

If you could imagine shrinking into a container of oxygen you would again see tiny particles flying about. However, you would not see spherical atoms, as in the case of argon. You would see double atoms, as though two spheres had meshed together and remained in that condition.

The reason for this is that individual oxygen atoms are not stable. Each oxygen atom has eight electrons, whereas it needs 10 to be stable. If two oxygen atoms collide and remain in contact, each one can make use of two electrons of its neighbor. Each oxygen atom would have six electrons of its own and four that it is sharing with its neighbor, giving it 10 all together. Such a combination of atoms is called a molecule, and the oxygen molecule is made up of two oxygen atoms. An oxygen atom is symbolized as O; an oxygen molecule is O2. The oxygen atom is only about two-fifths as massive as the argon atom. The oxygen molecule, which contains two oxygen atoms, is therefore four-fifths as massive as the argon atom. The less massive an atom or molecule, the more rapidly it moves at a given temperature. If oxygen and argon are at the same temperature the oxygen molecules move, on the average, 11% faster than the argon atoms.

Except for that, things are much the same. The oxygen molecules move at different speeds—some quite slowly, some quite quickly, and most at intermediate speeds. They collide continually, and change speeds and direction at each collision. What’s more, the molecules are always spinning and vibrating, and these motions change at each collision.

Air

Now let us imagine our miniature selves surrounded by air. We will encounter familiar two-atom molecules such as those we met in oxygen. If we observe closely, however, we will see that they are not alike at all. Only one in five is an oxygen molecule. The remaining four are nitrogen molecules (N 2), which are slightly smaller. Because the nitrogen molecules are somewhat less massive, they move about 7% faster, on the average, then the oxygen molecules do.

All the molecules move and spin and vibrate at a variety of speeds, collide continually, and change motions with each collision. Whereas argon, oxygen, and nitrogen are all elements made up of only one kind of atom (whether the atoms remain single or combine into molecules), air is a mixture, containing different kinds of molecules.

If you watch, every once in a while you will see a single argon atom move by. There is about one argon atom for every 100 molecules. Occasionally you will see atoms of neon, krypton, or xenon, the noble gas relatives of argon. For every 200,000 molecules that pass by, you will even see a tiny atom of helium. It is the second lightest atom there is and moves, on the average, 3 1/6 times as fast as the argon atom. For every 3000 molecules of oxygen or nitrogen, you will encounter one three-atom molecule moving and spinning along. The three atoms are arranged in a straight line. At each end there is an oxygen atom and in the middle a smaller atom of carbon. It is a carbon dioxide (CO 2) molecule.

Because the CO 2 molecule is made up of more than one kind of atom, it is our first encounter with a compound. (Air, you see, is a rather complicated mixture.)

One other important compound in air is water vapor. It, too, is made up of three atoms—two hydrogen atoms and one oxygen atom. It is present in air in varying amounts. There is usually more present on hot days than on cold days, more on humid days than on dry ones, more over the ocean than over the desert. Let’s examine water vapor separately.

Water vapor

We usually think of water as liquid, but it has a tendency to evaporate and became gaseous. If the water vapor is hot, it is often called steam. If we could shrink into a container of steam without being affected by the temperature we would find ourselves surrounded by the three-atom molecules of water. The shape of the water molecule is not like any we have encountered so far. The two hydrogen atoms are not on opposite sides of the oxygen atom, but both are rather near each other on the same side of the oxygen atom. They are positioned somewhat like eyes on the head of a fly. If you drew lines from the center of each hydrogen atom to the center of the oxygen atom, the two lines would form an angle of about 105 degrees, or just a little more than a right angle. The electrons in the water molecule tend to concentrate slightly on the oxygen atom, rather than on the hydrogen atoms. The oxygen atom thus has a slight excess of negative electric charge (-), while the hydrogen atom has a deficiency, which is the same as having a slight excess of positive charge (+). The water molecule thus has a negative pole where the negative charge is the highest and a positive pole where the positive charge is the highest. For that reason, the water molecules are said to be polar.

All the other atoms and molecules we have considered, the argon and helium atoms, and the oxygen, nitrogen, and carbon dioxide molecules, are nonpolar.

The fact that the water molecules are polar causes them to behave in a special way. In a collision, the two molecules may stick together somewhat before bouncing away. Water molecules are, so to speak, stickier than CO 2 or oxygen molecules.

As the temperature drops, the water molecules move more slowly and collide with each other less forcefully. They have less energy with which to break the attraction of one pole for the opposite pole so that they become steadily more and more sticky. Finally, when the temperature is low enough, the water molecules cling together and have very little tendency to break apart again; the water vapor condenses and becomes liquid. This occurs at 100 °C, the condensation point. If you start with liquid water and heat it, the water boils and becomes gaseous at the same temperature, so it is also the boiling point.

Even nonpolar atoms and molecules tend to stick together very slightly. If the temperature gets low enough, they also become liquid. The temperature must get considerably lower than for the sticky water molecules, however. The other substances in air don’t liquefy until far below zero. Oxygen liquefies at -183 °C, nitrogen at -196 °C.

Liquid water

If we imagine our shrunken selves inside a container of liquid water, we would encounter the same molecules of H 2 O. Now, though, the molecules are in contact with each other. They are not moving freely as they were on the vapor, but are jiggling very rapidly. Because of rapid jiggling, they move over and under and past one another, forever changing their position.

However, the molecules in water aren’t entirely free in their jiggling. They tend to arrange themselves so the negative pole of one is in contact with the positive pole of another. Because the positive pole consists of a hydrogen atom, this particular positive-negative attraction is called a "hydrogen bond" (often represented in drawings by a dashed line). The hydrogen bonds cause the water molecules to clump together in twos, threes, fours, and so on. The clumping isn’t very strong and is easily broken. At the surface of the water, where it meets the air, the water molecules are pulled by the stickiness of the molecules below and to the side, but there are none to pull them above. The water molecules at the surface are limited to their jiggling because of the unbalanced pull, so that the water acts as though there is a "skin" that is stiffer than the rest of it. This is called surface tension. The water molecules jiggle at different speeds, and every once in a while one water molecule happens to jiggle in an upward direction with enough speed to break free. The molecule evaporates. The higher the temperature, the more likely water molecules are to have a great enough speed for this to happen, and the faster the water evaporates. Because of the stickiness of the water molecules, it is harder for water to evaporate than for nonpolar liquids to do so, and it takes more heat to evaporate water. Water also has an unusually high heat capacity, or ability to store heat, which makes it an excellent liquid to use in a hot water bottle.

Ice

If the temperature drops even more, the water molecules jiggle so slowly that they can’t break the sticky hold their polar nature enforces. Every molecule freezes into position, forming solid water—ice. In each water molecule, one hydrogen atom sticks to the oxygen atom of a neighboring molecule, the second hydrogen atom sticks to the oxygen atom of another neighboring molecule, while the oxygen atom of the original molecule sticks to one hydrogen atom of each of the third and fourth molecules. This means that each water molecule is bound to four other water molecules in a particular symmetrical pattern. (See the center of this magazine for a special insert: a mold of an ice crystal.)

In this pattern the water molecules are therefore rather far apart, leaving small holes, or cavities, in the structure. In liquid water, individual molecules fill these cavities. Thus, ice has fewer molecules in a given volume than water does. This explains the well-known fact that water expands when it freezes. Water is unusual in this respect, because most substances contract when they solidify. The expanded solid water is less dense than liquid water, which causes ice to float.

It is unfortunate that we can’t really shrink into the molecules around us like this, for we could quickly solve countless scientific puzzles. As it is, chemists must do it the hard way—by juggling laboratory measurements and scientific theories to figure out the structure of matter.

CAPTIONS

Argon atoms are invisible because they are not large enough to diverse the light waves At 25°C, the average speed of Ar atoms is 460 meters per second (1030 miles per hour) Air contains 0.1% to 2.87. H 2 O vapor Composition of air by volume (except H 2 O) Average speed of oxygen molecules at 25°C: 510 meters per second (1150 miles per hour). The strong attractions between water molecules (dashed lines) are called hydrogen bonds. These attractions cause the vapor to condense into liquid at 100 °C.

Molecules of liquid water attach to their neighbors with hydrogen bonds (top). An instant later (bottom) they form new attachments. In ice the water molecules form a hexagonal pattern, enclosing some empty space.

BIOGRAPHY

Isaac Asimov is well known as a writer of science fiction, but he has also produced many factual science books. He earned a Ph.D. in chemistry at Columbia University and for many years has been professor of biochemistry at the Boston University School of Medicine. He began writing as a teenager and has published over 270 books on science, science fiction, mathematics, humor, literature, and the Bible. His most recent novel, "Foundation’s Edge," is a best seller and an earlier novel, "Fantastic Voyage," became a popular motion picture. Asimov has published books on the discovery of the elements, the chemistry of nitrogen and carbon, and other chemical topics. For a look at how the scientist became a writer, see the Profile column, p. 15.