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 Fig. 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 _K_₁ and the electron _E_₂ then attract each other as do the nucleus _K_₂ and the electron _E_₁. 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.