CHAPTER II. Atoms

Atoms

All matter is composed of atoms, which are very tiny objects. We cannot see them because waves of light wash over them like ocean waves over a pebble. An atom is about as big in comparison to a human cell, which can be clearly seen under an ordinary microscope, as a human cell is in comparison to a billiard ball. Somewhat more precisely, a hundred million atoms laid side by side would be about an inch in length.

Despite its Greek name, which means indivisible, the atom is made up of parts. It consists of a central nucleus, which carries a positive electrical charge, around which one or more negatively charged electrons are distributed. One frequently hears of the electrons revolving in orbits around the nucleus, somewhat as the planets revolve around the sun in our own solar system. This is not quite a correct picture, however. For one thing the electrons are more elusive than the planets. They do not revolve in definite orbits as the planets do. Also the orbits are more delicate. One would destroy the atom by the attempt to find out precisely what the electron orbits are.

The planets do not fly away from the sun because of the gravitational attraction which the sun exerts. The electrons and the nucleus, however, are held together because positive and negative electrical charges attract each other. The gravitational attraction between the electrons and the nucleus is incredibly weak compared to the electrical attraction.

Most of the atom’s weight comes from its nucleus. Even the lightest known nucleus weighs about 1840 times as much as an electron. In spite of this, the nucleus occupies only a tiny portion of the total volume of the atom. In fact, the nucleus is about as big in comparison to the whole atom as the atom is in comparison to the human cell. Twenty thousand nuclei laid side by side would be about equal in length to the diameter of the atom. If matter were composed of nothing but nuclei densely packed together, an object the size of a penny would weigh approximately forty million tons.

Later we are going to see that the size of the nucleus has a great effect upon the ways in which nuclei react with each other. For that very reason the size of the nucleus is a well-defined measurable quantity. It is much harder to say precisely what one means by the size of the electron. It seems acceptable to say that it is somewhat less than the size of the average nucleus. In any case it is certain that both the electrons and the nucleus are small compared to the size of the whole atom. Consequently, the atom must consist mostly of empty space. This means, of course, that when you look at solid matter, what is before your eyes is empty space with a slight addition of real substance. What lends strength to solids is the interplay of electric attractions and repulsions inside atoms and between atoms.

When a charged particle, such as an electron or a nucleus, happens to move through solid matter, it is constantly acted on by large electric forces. To such a particle matter does not seem to be very transparent. But if there were such a thing as an electrically neutral particle, comparable in size to the nucleus, it would be able to move around freely inside matter, without experiencing electric forces, and only now and again bumping into a nucleus or maybe an electron. As a matter of fact, there is such a particle and it can pass right through an inch or two of solid matter without bumping into anything. Later on in this book we shall be very interested in this particle, which is called a neutron.

Although the electrons and the nucleus are charged particles, the atom as a whole is electrically neutral; this means that the positive charge of the nucleus must be equal in magnitude to the total charge of all the negative electrons. All electrons have precisely the same charge, which is the smallest charge that has ever been observed. What is particularly strange and not yet explained is the fact that all other charges are as big as the electron charge, or twice as big, or three times as big, or a million times as big. But we never find a charge which, expressed in terms of the electron charge, is fractional. No object ever carries three and a half electron charges. The electron charge therefore may be used conveniently as a standard unit of charge.

Every atom can be distinguished by the charge of its nucleus. The simplest atom one can imagine would clearly be one with a single electron revolving around a nucleus having one unit of positive charge. Such an atom exists and is called hydrogen. An atom with a nucleus of charge two and two electrons revolving around it, is called helium; three, lithium ... six, seven, eight; carbon, nitrogen, oxygen ... 92, uranium. Atoms with almost all charges from one to 92 are found in nature, and practically none above 92 are found. Some odd charges—43, 61, 85, and 87—are missing. The reason for these missing atoms is connected with the properties of the nucleus. The nucleus will soon become our main object of interest.

The most surprising fact about atoms is their similarity, indeed their identical behavior. If two atoms have the same kind of nucleus and have the same number of electrons revolving around these nuclei, then these two atoms are apt to be encountered in a condition which is most precisely the same for the two. One could imagine that the various component parts of the atom would be arranged in different ways and found in different states of motion, in a variety without limit. Whence the complete similarity? The answer to this question is not only most surprising, but it is even in apparent contradiction to common sense. For this very reason it is difficult to explain. The hardest things to understand are not those which are complicated but those which are unexpected.

Fortunately for our purpose we need not go into this more intricate portion of atomic physics. It is sufficient to say that there is one arrangement or pattern of motion of the electrons which is preferred and which leads to the greatest stability of the atom. If the electrons are in this particular state of motion, which is called the ground state, they have less energy than they would have if they were in any other state of motion. There are other less stable, but not less sharply defined, states of atoms which we call “excited” states. When an atom is in such an excited state, it tends to be unstable and tries to get into the ground state as soon as possible. Since the ground state contains less energy than any other state, the atom must release energy in the process of adjustment. The released energy manifests itself in the form of electromagnetic radiation—often as a little pulse of visible light. The color of this light depends upon the amount of energy released, going progressively through the rainbow from red toward blue as the amount of energy increases.

There are very few states in which the excitation energy is small. But of strongly excited states there is a great abundance. In the region of this high excitation small additional changes are possible. Thus we approach a situation more in accordance with experience and common sense: the pattern of motion can be changed by any small amount.

The description we have just given is of course incomplete. We must avoid here the crucial questions why only some patterns of motion are possible, why one lowest level is stable and why the electrons never descend into decreasing states of energy, following the attraction of the nucleus. At the same time one should emphasize that a complete explanation of these facts has been given. This explanation makes precise predictions about many of the properties of matter, and we can have complete confidence that, but for the involved mathematical procedure, all ordinary properties of materials could be precisely predicted. The atom has been explained as completely as Newton has explained the motion of planets.

To form an idea what an atom is or why two atoms of, let us say, hydrogen are precisely the same, it is not necessary to search for intricate reasons or deep meanings. Two atoms of the same kind are alike as two pawns are for the chess player, except for one little point: in the case of the pawns we do not care about the difference; in the case of the atoms there is no difference. This is a simple statement and it honestly describes a simple situation. The beauty of science is due to the fact that the correct answers to our most interesting questions have turned out to be surprising by their simplicity.

In order to understand an atom one must consider the distribution of electrons around one nucleus. In order to understand a molecule one has to consider the distribution of electrons around two or more nuclei. The chemical behavior of an atom is the manner in which it interacts with other atoms, and that means the precise way in which the electrons rearrange themselves when two or more atoms approach each other. The interaction between atoms occurs mainly between their outermost electrons. It may happen that two quite different atoms, containing nuclei of different charges and different numbers of electrons, may nevertheless be similar in the structure of their outermost electrons. In this case the two atoms exhibit similar chemical properties. Examples are lithium with charge 3 and sodium with charge 11; also helium, charge 2 and neon, charge 10. A most important example for our purpose is the set of three chemically similar atoms: calcium, charge 20, strontium, charge 38; and radium, charge 88.

When two or more atoms approach each other, whether they are similar or different, their electrons—particularly the outermost ones—find new states of motion instead of those that were available to them when there was only one nucleus in the vicinity. It may now happen that amongst these new states of motion there are some that are even more stable than the state of the separated atoms. In this event the atoms will tend to stick together, and the electrons will adopt whatever new state of motion now corresponds to maximum stability. The composite system of the atoms is called a molecule, and its state of maximum stability, the ground state of the molecule.

There are atoms of particularly great stability which cannot increase their stability by combining with other atoms. Examples are helium, neon, and argon. These atoms tend to remain single, retain their independent motion in a rather “permanent” gaseous state, and are generally unsociable. They are called therefore the noble gases.

An especially simple example of the formation of a molecule is the combining of sodium and chlorine to form ordinary table salt. The sodium atom happens to have a rather loosely bound outer electron. The chlorine atom possesses a convenient niche for an extra electron. Consequently the energy spent in prying the outer electron loose from the sodium atom is largely repaid by adding it to the chlorine atom. The remaining sodium “atom,” deprived of one of its electrons, now has a net positive charge. The chlorine “atom” with its extra electron has a net negative charge. The two “atoms” therefore attract each other to make a molecule of sodium chloride. Actually matter will continue to aggregate. A great number of positive sodium “atoms” and negative chlorine “atoms” will arrange themselves into a beautiful and regular lattice which is the sodium chloride crystal.

The simplest molecule which does not tend to grow into a bigger aggregate is made up of two hydrogen atoms. Around two hydrogen nuclei a particularly stable pattern of two electrons can be formed. Because of this fact hydrogen atoms associate pairwise so that this pattern should become possible.

The ways in which atoms can be joined are incredibly manifold. They can form metals in which the outer electrons roam freely and carry electric currents with the greatest of ease. They can form liquids in which atoms or molecules are tied together in a loose and disorderly fashion. They can move independently making occasional encounters, which is what happens in a gas. And they can form long spiraling molecules where groups of atoms are strung together without an apparent simple order, but in a way which is somehow related to the processes of life.

We all know in how many forms matter can appear and how changeable these forms are. That the stone and the spray, the air and an insect, and even the human brain should be composed of the same few kinds of atoms, and that these atoms should be subject to laws which are subtle and simple and precisely described—this certainly is the most remarkable fact that we have learned since Newton proved that the same science applies to the earth and in the heavens.