On the basis of these views, we have in sodium chloride NaCl a substance, whose molecules contain an atom, Na, with a tremendous ‹tendency to lose an electron›, and an atom, Cl, which has a tremendous ‹affinity for an electron›. It is natural to suppose, then, that ‹both tendencies will be satisfied by the passage of an electron from the sodium to the chlorine atom›, NaCl → Na^{-ε}Cl^{+ε}. Or, if we use the sign + to designate the positive charge produced on an atom by the loss of an electron and the sign − to indicate the charge gained through the assumption of an electron, we have[71]: NaCl is Na^{+}Cl^{−}. Similarly we have in hydrogen chloride H^{-ε}Cl^{+ε} or H^{+}Cl^{−}. It is altogether likely, therefore, that the atoms in a molecule of sodium chloride or of hydrogen chloride already possess electric charges,[72] so that, even while combined, [p044] their tendencies to lose or gain electrons are satisfied. It is also possible that the atoms are held together in the molecule by the electrical attraction of the opposite charges.[73] The force with which opposite electrical charges attract each other depends, as is well known, on the nature of the ‹surrounding medium›. Now, when molecular sodium chloride or hydrogen chloride is dissolved in water (a favorable medium), a decided decrease in the attraction (see p. 62), between the charged atoms within the molecules is brought about, and a process of ‹ionization› results: H^{+}Cl^{−} ⇄ H^{+} + Cl^{−}. The charged particles are called ‹ions› only after they have separated from one another and have become independent molecules, capable, for example, of moving in ‹opposite› directions. While the atoms of some metallic elements tend to lose a single electron and form ions Me^{+} (‹e.g.› Na^{+}, K^{+}), the atoms of other elements tend to lose two or more electrons, forming bivalent ions, Me^{2+} (‹e.g.› Zn^{2+}, Fe^{2+}, etc.), or trivalent ions, Me^{3+} (‹e.g.› Bi^{3+}, Fe^{3+}), and so forth. Similarly, atoms of the so-called negative elements may assume two or more electrons, forming bivalent ions, X^{2−} (‹e.g.› S^{2−}), and so forth. «The Validity of the Theory of Ionization.»—In determining the validity of the theory of ionization, we may consider, first, the sufficiency of the explanations which it offers for observed facts and important phenomena. We may then weigh, more critically, by any evidence offering itself, the facts which will enable us to decide between this theory and the older theory of ionization, that of Clausius (p. 51). The latter, although displaced, is still often revived by opponents of the modern theory. Such facts as we will consider are found, first, in the domain of ‹conductivity phenomena›, next, in the ‹osmotic pressure› and related properties of solutions, and, finally, in the study of the ‹chemical activity› of electrolytes (see Chapter V).