Atoms and electrons

§ 1. The Outer Electrons

ANY theory of the atom which is to secure our assent must, in broad lines if not in detail, account for the remarkable periodicity in the properties of the chemical elements. We have already shown that this periodicity, the recurrence of similar physical and chemical properties, led Mendeléev to construct the Periodic Table. In each of the columns of the diagram in Chapter IV the elements run through a cycle of chemical properties which is approximately repeated in the next column. This repetition of properties is not perfectly regular. The columns are of unequal length; some contain 8, some 18, and some 32 elements. But the periodicity, although not perfectly simple, is quite unmistakable, and is one of the most important and outstanding properties of the chemical elements. Instead of arranging the atoms by their general chemical properties we may arrange them according to some specific property. We may, for instance, arrange them in accordance with what is called their “atomic volumes.” If we do this we again get a periodic relation. Still other properties, such as “compressibility,” “expansion coefficient,” etc., show the same intriguing peculiarity. All these properties seem to be connected with the actual amount of space taken up by the atom. They are not properties of the nucleus; they are dependent upon the outer electrons. And also, if we study the visible spectra of the various elements, we find the same curious recurrence. All those elements which are called Alkalis, for example, have spectra which seem to be constructed on the same ground plan. The different alkalis differ enormously from one another in the complication of their structure; their atomic numbers are 3, 11, 19, 37, 55, so that we pass from a system containing three circulating electrons to one containing fifty-five. And yet their spectra are fundamentally the same. Here, also, we are concerned with the outermost electrons. The nuclei of these atoms are not concerned in their visible spectra. The nuclei of the various atoms, arranged according to their atomic numbers, simply show a straightforward advance from complexity to complexity. There is no kind of repetition or recurrence in the properties which depend on the nucleus. But the way in which the outer electrons are arranged does show a recurrence, and all those physical and chemical properties which show a recurrence may be assumed to depend on the outer electrons. Thus we may say that the chemical properties of an atom depend not on its nucleus, but on its outer electrons.

The visible spectrum, as we have said, depends upon the arrangement of the outer electrons. But X-rays also possess a spectrum. The X-rays emitted from any source are not all of the same wave-length and these waves, by a method of which we shall learn more later, can be arranged in order like those of visible light. Now the X-ray spectra of the elements do not manifest a recurrence. They advance, in a straightforward way, with the atomic number. They depend upon the inner part of the atom and not upon the outer electrons. And it is because they originate in the neighbourhood of the nucleus, where the atomic forces are most intense, that the X-rays possess their great penetrative power. Periodicity is not an inner, but only an outer, property of the atom.

The great dominating factor which governs the properties of an atom is the charge on its nucleus. We see this very clearly in the case of isotopes. Two isotopic varieties of an element cannot be distinguished from one another by their chemical properties. The outer electrons are arranged in the same way in the two varieties of atoms, and it is this arrangement which determines the chemical properties. Their visible spectra are also the same, and so are the spectra in the ultra-violet region. This fact furnishes an even more exact proof that their outer electrons are arranged in the same way than does the identity of their chemical properties. Two isotopic elements also have the same X-ray spectrum; therefore the inner structure of the atoms, also, is the same in the two cases. The whole structure of the atom is evidently dependent on the charge carried by the nucleus; where this charge is the same the atomic structure is the same.

The general question of how the electrons in a heavy atom are to be supposed to be arranged is one of great difficulty, and no perfectly precise answer can yet be given. There are certain general considerations, however, which enable us to give a partial answer to the question. In the periodic table each column ends with what is called an “inert gas.” These elements are so called because they possess great stability; they are not in the least eager to enter into combination with other elements. Let us consider the element argon, for instance. It is an inert gas, and occurs at the end of the second group of eight in the periodic table. Its atomic number is 18. It therefore contains 18 electrons rotating round the nucleus. Owing to the marked stability of argon we must suppose that these 18 electrons are arranged in some peculiarly stable configuration. The natural ideal of every atom would be to reach so stable a condition. It is a state to which every atom aspires. The atom of chlorine, which just precedes argon in the table, and therefore possesses only 17 electrons, shows a marked disposition to capture one additional electron. It is striving towards the perfect state of possessing 18 electrons—not because it requires the extra electron to become electrically neutral, of course, but because the extra electron gives it greater mechanical stability. On the other hand, the element potassium, which immediately follows argon in the table, shows a marked tendency to get rid of one of its 19 electrons—again in order to attain the perfect state of possessing 18 electrons. If we go back to sulphur, which has 16 electrons, or forward to calcium, which has 20 electrons, the same tendency manifests itself. Sulphur has a tendency to capture two electrons and calcium has a tendency to lose two electrons. We can understand, therefore, why the Germans call the inert gases, like argon, the “noble” gases. It is not only that they are sublimely inactive, but their condition is that to which all the others aspire.

The great stability of the inert gases, and the fact that they occur at the end of each period in the periodic table, so that, immediately after each inert gas, the whole cycle of chemical properties begins again, show us that they are, as it were, the natural terminations of the building schemes which led up to them. After each inert gas a fresh building scheme has to be adopted for the next group of atoms. We have seen that each step along the periodic table means the addition of a fresh electron. We can imagine the outer electrons of an atom to be arranged in a ring, or on the surface of a sphere, or in what configuration we like. When a fresh electron has to be added to produce the atom one step on in the periodic table, we may imagine that, in general, this new electron joins the rest. It takes its place in the ring or on the sphere or whatever it may be. But there will come a moment when the addition of a fresh electron will spoil the stability of the whole structure. There will be no place for it in the ring, and it will have to start a new ring, by itself, outside the existing one. When yet another electron joins up, it will help the first one in establishing the new ring. Presently the new ring will itself have all the members it can stably hold, and further electrons will have to build up yet another ring. The process will not be quite so simple as this, for as outer rings continue to be built they will so influence the inner rings that these will be able to take more members than they could originally accommodate. But, in the broadest outlines, the process is as we have described. Now the inert gases form such points of departure. By the time an inert gas is reached the system which led up to it has done all it could; it has fulfilled itself in producing an inert gas. If atom building is to continue, it must be on a different system, although the system of the new atom will, of course, resemble the system of the atom to which it is connected by a line in the periodic table. In just the same way, all the different inert gases have systems which resemble one another.

The inert gas which precedes argon is neon, an element possessing ten electrons. There is reason to suppose that it contains 2 inner and 8 outer electrons. The inert gas preceding neon is helium, the first of the inert gases, and helium has 2 electrons. With the obvious exception of helium it is supposed that the chemical similarity of all the inert gases is due to their possessing 8 outer electrons, however many groups of inner electrons they may have. The following table, showing the number and arrangement of the electrons in successive groups, going outwards from the nucleus, has been proposed by Bohr.

How are we to consider these different groups of electrons to be arranged? No precise answer can yet be given to this question, but the whole trend of the most modern speculations is to emphasise the fact that it is not sufficient to regard the different groups as lying in plane rings. It is necessary to investigate the spatial configuration of the electronic orbits. It is very probable that different electrons move in orbits which are inclined at various angles to one another. Even in the solar system, the planets do not all rotate in precisely the same plane. In the electronic orbits, we must imagine these differences to be much greater. It has even been suggested that the number 8, which occurs with such frequency in electronic groups, may indicate that these groups of eight are arranged like a cube, one electron being at each corner. The idea has something to recommend it, although it appears that such an arrangement cannot be explained by the known forces within the atom. But it serves as an indication of the direction in which a solution is being sought. We shall now deal with this question in more detail.

§ 2. Hydrogen and Helium

Hydrogen and helium are the two members of the first group in the periodic table, and we shall now proceed to examine their atomic structure. We have already described the structure of the hydrogen atom in some detail. We know that it consists of a single electron rotating about a nucleus. The electron can circulate on a number of different orbits, but its most stable state is obtained when it circulates on the first orbit, the one nearest the nucleus.

When we come to the helium atom the question is much more complicated. A helium atom which had but one electron would be essentially similar to a hydrogen atom, with the difference that the nucleus would carry two positive charges instead of one. The real problem of the helium atom is to determine the way in which the second electron enters into its constitution. A spectrum of helium consists of two complete series of lines, and for this reason helium was supposed to consist of two different gases, called “orthohelium” and “parhelium.” But it is now known that these two series of lines arise from the fact that the second electron can enter into the constitution of the helium atom in two different ways. The two electrons may be describing orbits of the same kind, but inclined at an angle to one another, or they may be describing orbits of different kinds, one outside the other. The first case gives the most stable state for the atom, and in reaching it the atom emits the spectrum which used to be referred to “parhelium.” The second case is less stable and the process of reaching it gives the “orthohelium” spectrum. This state was produced experimentally by bombarding helium atoms with electrons. Such a bombardment could produce what was called a “metastable” condition of the helium atom, and it was found that the atom could not return to its normal condition merely by radiating energy. The bombardment had caused the second electron to move in an orbit outside that of the first electron—an orbit of a different kind. And, having once done this, the second electron could not make a jump back to its original orbit. Before it could return to normal the “metastable” atom had to interact with atoms of other elements—it had to go through a sort of chemical reaction. In its normal state, then, the helium atom may be said to consist of two electrons moving round the nucleus in similar circles, these two circles being inclined at an angle of 120° to one another. And owing to the interaction between the two electrons the planes of these two circles are slowly moving. So that already, and when we are dealing with an atom containing only two electrons, we are in the presence of very considerable complications. The detailed working out of the constitution of more complicated atoms would obviously be a task of immense difficulty. Bohr has been able, however, to say something about the broad lines of their structure.

§ 3. Lithium—Neon

We now come to the second group of the periodic table, a group possessing eight members. We begin with lithium. Its atomic number is 3, and therefore an atom of lithium consists of 3 electrons revolving about a nucleus. We shall assume that two of these electrons move in orbits similar to those characteristic of the normal helium atom, that is, in orbits which are not in the same plane but which are otherwise similar. This assumption is very natural, for the normal structure of a helium atom is a very stable condition. It is distinctly more stable, for instance, than the structure of the hydrogen atom. Helium is the first of the inert gases. We assume, then, that two of the electrons in a lithium atom move as do the two electrons of the helium atom. How are we to suppose the third electron to move? The spectrum of lithium shows us that the third electron moves in orbits which are altogether outside the region containing the first two electrons. The spectrum also shows that the third electron sometimes moves in orbits which, although they lie outside the region of the first two electrons for the greater part of their length, yet, at their nearest point to the nucleus, approach it as closely as do the first two electrons. These are the orbits characteristic of the lithium atom in its normal state. The firmness with which the outer electron is held in these orbits is only about one-third of that with which the electron in a hydrogen atom is held, and only about one-fifth of that with which the helium electrons are held. The chemical properties of these three elements, therefore, depending, as they do, on the outer electrons, should be very different, as, in fact, they are.

We may assume that, in any atom, the third electron moves in the same kind of orbit as does the third electron of the lithium atom. This orbit is, as we have said, very excentric. It may be regarded as markedly elliptical. That part of it nearest the nucleus is within the region in which the two inner electrons move. The rest of it extends far beyond this region. We may imagine that the fourth, fifth, and sixth electrons move in similar orbits. There is reason to suppose that the four electrons, from the third to the sixth inclusive, which move in these excentric orbits, are so distributed as to form an exceptionally symmetrical configuration. Each of these outer electrons penetrates to the region occupied by the inner electrons, but not at the same moment. Bohr supposes that the outer electrons reach their nearest point to the nucleus separately at equal intervals of time.

This structure carries us as far as carbon, which has six electrons. Lithium has one outer electron, beryllium two, boron three and carbon four. Each of these outer electrons moves in very excentric orbits which enclose and partly penetrate the approximately circular orbits within which the two inner electrons move. This method of building reaches completion in the carbon atom. If yet another electron were added to the four outer electrons of carbon the symmetry of the arrangement would be destroyed. There is, as it were, no room for five such orbits. Also, the fact that the elements in the second half of this group in the periodic table have very different properties from those in the first half suggests that a new system of building comes into existence directly we proceed beyond carbon. Bohr supposes that in nitrogen, an element possessing seven electrons, the seventh electron moves in a large and approximately circular orbit. It lies completely outside the two inner electrons, although the outermost parts of the excentric orbits on which the other four outer electrons lie extend beyond it. The eighth, ninth and tenth electrons also move in large circular orbits of this kind. The great stability of the last element reached in this way, the inactive gas neon, suggests that the final arrangement possesses great symmetry. We must suppose that, with this element, the four large circular orbits are not only symmetrical amongst themselves, but also in relation to the four elliptical orbits.

§ 4. Sodium—Argon

We have, so far, considered two types of orbits, the approximately circular, and the markedly elliptical. For the first two electrons we assume circular motion. For the next four we assume elliptical motion, and for the next four we again assume circular motion. In this way we have got as far as neon, and we now begin another group of the periodic table. Acting on the same general principles, we shall assume that the eleventh electron inaugurates a new era of elliptical orbits. These orbits are very elliptical. For the most part they are well outside the orbits of the first ten electrons, but for part of their course they, like the first group of elliptical orbits, penetrate even closer to the nucleus than do the two innermost electrons. The existence of such markedly elliptical orbits, passing, during part of their course, so close to the nucleus, greatly helps the stability of the atom. In the distribution of the twelfth, thirteenth, and fourteenth electrons we meet conditions similar to those we encountered when considering the fourth, fifth, and sixth electrons. There seems to be an exception in the case of aluminium, the element whose atom contains 13 electrons. In this case the thirteenth electron seems to move in a less markedly elliptical orbit. But Bohr does not regard this behaviour as typical for the thirteenth electron in all atoms; it is peculiar to aluminium, where the thirteenth electron is also the last electron. By the time we get to silicon, containing 14 electrons, we find the thirteenth electron, like the eleventh, twelfth, and fourteenth, moving in the markedly elliptical type of orbit inaugurated by the eleventh electron. As in the preceding cases, we suppose this type of construction to be completed when we have four electrons describing these new orbits. The fifteenth electron introduces a new system, just as the seventh electron did. But whereas the seventh electron introduced an almost circular type of orbit, the fifteenth electron continues with the excentric type of orbit, although the excentricity is not so marked as in the case of the orbits we have just left. These new orbits are excentric enough to penetrate closer to the nucleus than do the circular orbits inaugurated by the seventh electron, but they do not reach the region of the two innermost electrons. These orbits will accommodate four members, so that this type of construction will carry us up to the atom possessing 18 electrons, i. e., up to the element argon. Thus we again reach an inert gas, and, allowing for the greater complexity, we see that its symmetrical properties closely correspond to those of the preceding inert gas, neon.

§ 5. The Remaining Elements

The development we have been describing hitherto is straightforward in the sense that fresh groups of electrons have been regarded as possessing fresh types of orbits which are, as it were, independent of those previously existing. We have not considered the later electrons as causing any development of the inner groups of electrons. That there must be some interaction is obvious, but we have not found it necessary to assume that the interaction is sufficient to cause any fundamental modification of the orbits already established. When we come to the fourth period of the periodic table, however, matters are different. As we see from the diagram in Chapter IV, the fourth period contains 18 elements. At the beginning of this period the atom continues to develop in a way analogous to that we have already studied. The first two elements of this period are, as shown by the connecting lines, analogous to the first two elements of the third period. But then occurs a group of eight elements which do not correspond, in our diagram, to anything in the third period. And after this group occur two elements which are again analogous to the first two elements of the third period. Why is it that we have this interregnum, as it were, lasting over eight elements? Bohr’s answer is that we are here concerned with the development of one of the inner groups of electrons. The normal system of atom building, as we have sketched it, cannot now proceed. The later electrons captured will now be concerned in the internal rearrangement, and only when this is completed will the normal process be able to proceed.

This theory gives an interesting explanation of the facts that many elements of the fourth period differ markedly from the elements of the preceding periods in their magnetic properties and also in the characteristic colours of their compounds. That highly magnetic substance, iron, for instance, occurs in the fourth period. To understand the explanation, offered by the theory, of the magnetic properties of these elements, we must revert to the familiar fact that an electric current is always attended by magnetic force. Electric currents are constituted by the movements of electrons, and the moving electrons within an atom will give rise to their appropriate magnetic forces. Now we may imagine that, in any thoroughly symmetrical arrangement of the electrons within an atom, these magnetic forces form a closed system within the atom, so that no resultant external effects are manifested. With any markedly unsymmetrical arrangement of the electrons, however, we may expect appreciable external magnetic effects to manifest themselves. Bohr supposes, therefore, that the process of reorganisation within the atom which characterises a group of elements in the fourth period, is attended by the lack of symmetry which would result in magnetic forces being exhibited. Where the symmetry is at last restored the magnetic effects cease. In Bohr’s words: “On the whole a consideration of the magnetic properties of the elements within the fourth period gives us a vivid impression of how a wound in the otherwise symmetrical inner structure is first developed and then healed as we pass from element to element.”

The characteristic colours to which we have alluded also find an explanation on this theory. These colours are due, of course, to the absorption of light, and they are thus evidence that energy changes are going on comparable with those giving visible spectra. This is in contrast to the elements of the earlier periods, where the electrons are more firmly held and where the less rigid conditions, due to the development of an internal group of orbits, do not occur.

Table showing distribution of Electrons in the Inert Gases, including a
hypothetical element of atomic number 118.

The building up of the rest of the elements, up to and including the seventh period, may be supposed to take place on the broad lines we have now laid down. The process is a double one. New groups of outer orbits will be formed, and also there will be a development of groups of inner orbits. The whole process is very complex, and no attempt has yet been made to examine it in detail.

The seventh period ends abruptly with uranium, whose atomic number is 92. The last elements in this period are all radioactive, and, as we have said before, it seems probable that an element of higher atomic number than 92 would be too unstable to exist. Nevertheless, on the principles we have followed hitherto we can construct theoretically, and in its main lines, the structure of an atom having a higher atomic number than 92. The last inert gas known to us, niton, has an atomic number 86. The next inert gas, if it existed, would have an atomic number 118. Bohr gives a table, which we reproduce, showing in some detail the number of electrons and the characters of their orbits for the six inert gases. He includes, as a seventh, the imaginary gas having an atomic number 118 and shows its hypothetical construction.

Chapter VII: The Inner Regions