When a current is passed through the solution of an ionogen, the electrified particles carry their charges to the electrodes (see [p042] below). They are called the ions[64] of the electrolyte; the positively charged ions are distinguished as cations from the negatively charged anions, and the electrode toward which the cations move is called the cathode (negative electrode), and the electrode to which the anions move is called the anode (positive electrode).
The dissociation of hydrogen chloride may be expressed, in the terms of the assumptions made, in the following equation: HCl ⇄ H+ + Cl−; that is, hydrogen chloride is dissociated, to a greater or smaller extent and in reversible fashion, into positively charged hydrogen ions H+, and negatively charged chloride ions Cl−, and the charge on each chloride ion is equal in quantity to the positive charge on each hydrogen ion. Zinc chloride is dissociated according to the equation ZnCl2 ⇄ Zn2+ + 2 Cl−, and, according to (2), the charge on each zinc ion is twice as great in quantity as the charge on each chloride ion, and therefore twice as great also as the charge on each hydrogen ion (see below, p. [58]). It is practically certain, according to more recent results, that the ions are combined with water to form hydrates, such as H+(H2O)x and Cl−(H2O)y.[65] This does not modify, essentially, the fundamental assumptions of the theory, but contributes rather to a satisfactory explanation of the rôle of water as an ionizing agent, a question to which we shall return later.
The Theory of Ionization and the Electron Theory of Electricity and of Matter.
One of the most fundamental and most characteristic properties of elements is considered to be the affinity which their atoms show for electrons; thus, the atoms of metals like sodium and potassium, which are generally called "electropositive" elements,[68] show an enormous tendency to lose one electron each and to form positively charged particles[69] Na-ε (= Na+) and K-ε (= K+).[70] The atoms of strongly electronegative elements, like chlorine, have a tremendous tendency for gaining and holding electrons beyond the number originally in such atoms. Thus, chlorine atoms tend to assume an electron each; they thereby become negatively charged particles, Cl+ε ( = Cl−).
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-εCl+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-εCl+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, Me2+ (e.g. Zn2+, Fe2+, etc.), or trivalent ions, Me3+ (e.g. Bi3+, Fe3+), and so forth. Similarly, atoms of the so-called negative elements may assume two or more electrons, forming bivalent ions, X2− (e.g. S2−), and so forth.
The Validity of the Theory of Ionization.
Ionization and Electrical Conductivity.
Fig. 8.