The atom and the Bohr theory of its structure

by Helge Holst; Hendrik Anthony Kramers

Introduction.

We have hitherto restricted ourselves mainly to those applications of the Bohr theory which have a direct connection with the processes of radiation. We have shown how fertile the theory has proved to be, how many problems, previously inexplicable, have been solved, and what exact agreement has been established between experiment and theory in this comprehensive field. We may now ask how the theory accounts for the chemical behaviour of the different elements. As early as 1913, Bohr, in connection with his researches on spectral phenomena, had considered the chemical properties of the elements and had pointed out interesting possibilities.

Fig. 33.—Early representation of the formation
of a hydrogen molecule (Bohr, 1913).

The Combination of Atoms into Molecules.

In his discussion of hydrogen, Bohr suggested a model for the structure of its molecule, which we shall give here, because, by a simple example, it illustrates how two neutral atoms may form a molecule ([cf. p. 13]). In F[ig. 33], a, K₁E₁ and K₂E₂ are two neutral hydrogen atoms which are approaching each other with the orbits of the two electrons parallel. The nucleus K₁ and the electron E₂ then attract each other as do the nucleus K₂ and the electron E₁. The two electrons repel each other as do the two nuclei; but when the electrons are in opposite positions of their orbits, the forces of attraction outweigh the effect of the forces of repulsion. Calculation shows that when the atoms are allowed to approach each other the positions of the atoms will be as is shown in [Fig. 33, b], where the orbits are closer to each other than the nuclei. Finally, for small distances of the two nuclei, the two orbits will be merged into one, as is shown in [Fig. 33, c]. This orbit will be slightly larger than the original ones. The two hydrogen atoms may, in this way, combine into one molecule. In fact, an equilibrium position can be found for which the nuclei are held together, in spite of their mutual repulsion, by the attractive forces existing between them and the electrons which are moving in their common orbit. It must, however, be assumed, for reasons which cannot be given here, that the hydrogen molecule is, in reality, constructed somewhat differently; probably the orbits of the electrons make an angle with each other.

The formation of a hydrogen molecule may also be supposed to occur when a positive hydrogen ion, i.e., a hydrogen nucleus, and a negative hydrogen ion, i.e., a hydrogen nucleus with two electrons, are drawn together by their mutual attractions. The forces of attraction would be much stronger than in the first example given, and the formation of a neutral molecule would not take place in the same way. More energy would also be released, but the final result will be the same.

Just as in the case of the hydrogen molecule, other molecules may be formed from atoms belonging to the same or to different elements. The method of formation of molecules varies according as it is a union of neutral molecules in the normal state, or a union of positive and negative ions. Conversely, by chemical decomposition a molecule can be separated either into neutral parts or into ions. If, for instance, common salt (sodium chloride, NaCl) is dissolved in water, the salt molecules are, under the influence of the water molecules, decomposed in Na-ions with one positive charge and Cl-ions with one negative charge, corresponding to the monovalent electropositive character of sodium and the monovalent electronegative character of chlorine.

The possibilities are, however, far from being exhausted by these two methods of composition and decomposition. An atom may exist not only in the normal state where it has its complete number of electrons collected as tightly as possible about the nucleus, and in the ionized state with one or more too many or too few electrons; but in a neutral atom one or more of the outer electrons may be in a stationary orbit at a greater distance from the nucleus than corresponds to the normal stationary state. It is easy to understand that an atom in such an “excited” or (as it is called in chemistry) active state often finds it easier to act in concert with other atoms than when it is in the normal state; in this latter state the atom is often more like a little compact lump of neutral substance than in the active state.

It will, in any case, be understood that the interplay between the atoms, which reveals itself in the chemical processes or reactions between different elements, offers many opportunities for the Bohr theory to give in the future a more detailed explanation than was possible to the earlier theories of chemistry. We must also mention the fact that it has become possible to elucidate in main features the phenomena, hitherto unexplained, of the chemical effects of light, as on a photographic plate (photochemistry) and of catalysis, which consists in bringing about, or accelerating, the chemical interaction between two substances by the presence of a third substance which does not itself enter in the compound, and often needs only to be present in very small quantities. It must, however, be emphasized that at present we do not yet possess a detailed theory of molecular constitution comparable with our knowledge of the structure of the atoms.