When a metal rod (M) is dipped in the solution of its own ions (Mn+), then-
The metal ion (Mn+) may collide with the metal rod and bounce back without any change.
The metal ion (Mn+) may collide with the metal rod, gain ‘n‘ electrons (i.e. get reduced) to give a metal atom i.e.
Mn+ + ne– ———–> M
The metal atom on the rod lose ‘n‘ electrons (i.e. get oxidized) to enter the solution as metal ions.
M ————> Mn+ + ne–
Now, if the metal has a relatively high tendency to lose electrons (i.e. to get oxidized), its atoms form positive ions and pass into the solution and a negative charge is developed on the rod due to the accumulation of electrons lost. This negative charge does not allow the metal rod to continue losing electrons, but re-attract the metal ions from the solution and a state of ≡m will be established between the metal and its ions in solution i.e.
M (Metal Rod) ⇌ Mn+ (Solution) + ne– (In Metal Rod)
Similarly, if the metal ions have a relatively higher tendency to get reduced, they accept electrons at the rod from the metal atoms and so a positive charge is developed on it and ultimately, a similar ≡m is established between the metal ions in solution and the metal atoms i.e.
Mn+ (Solution) + ne– (From Metal Rod) ⇌ M (Metal)
When such an ≡m is established between the metal and its ions in solution a separation of positive and negative charges would occur leading to the development of Potential difference between the metal and its ions in solution. This electrode potential difference set up between the metal and its ions in solution is known as “Half Cell Electrode Potential“. It is the measure of the tendency of an electrode in a half cell to lose or gain electrons. Thus, the tendency of an electrode to lose electrons and get oxidized is called its “Oxidation Potential” and “the tendency of an electrode to gain electrons and get reduced is known as its “Reduction Potential“.
In short, the electrical potential difference set up between the metal and its ions in the solution is called electrode potential or the electrode potential may be simply defined as the tendency of an electrode to lose or gain electrons when it is in contact with the solution of its own ions.