Man's physical universe



gain of electrons by the ions of the lower metal in solution to form the

metallic element. This statement will be better understood if you will

recall that ions are formed by the loss of electrons by metals and by the

corresponding gain of electrons by nonmetals. A positive ion therefore

represents an atom or group of atoms (radical) which has lost one or

more electrons.

A metal will displace from its solution any metal lower in the displacement

series. Inasmuch as this process involves a transfer of

electrons, it follows that the elements at the top of the displacement

series are those which lose electrons most readily.

This conclusion is

borne out by other facts; for example, the relative tendencies of metals

to emit electrons when heated or when exposed to radiant energy are

in the same order as the metals appear in the displacement series.

Any two elements in the displacement series would have a difference

of potential set up between them, provided chemical action were made

possible as a result of the difference in their attraction for electrons.

These differences in potential are listed in the displacement series table.

Such a series of elements is generally called an electromotive, or electrochemical,

series. See Unit VII, Section 3, p. 496.

Chemical Cells and Batteries Are Applications of the Differences in the

Tendency of Atoms to Lose or Gain Electrons.

The principle of the chemical cell is very simple.

All that is needed is

one electrode to give up electrons and another electrode to accept electrons

and a suitable conducting medium between them. An electric current

may be produced by placing any two different metals in a solution of an

electrolyte and connecting them with a metallic conductor.

The difference

of potential produced by such a cell depends upon how far apart

the metals are in the electromotive series. It also is possible to obtain

electric currents from cells consisting of two electrodes of the same metal

in two different electrolytes because in this case the difference of potential

between the metal and the ion in solution will be different at each electrode.

Cells may be prepared in which the same metal is placed in

different concentrations of the same electrolyte because in this case the

tendency to lose electrons depends upon the concentration of the ions

in solution.

Many chemical cells are of little value because they become polarized

easily. In the majority of cells hydrogen gas is liberated at one electrode

when hydronium ions, always present in water, accept electrons,

forming hydrogen gas, while the metal of the other electrode goes into

solution to form metallic ions, thus giving up electrons.

This hydrogen

gas which is liberated forms a layer of gas around the cathode, and the

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