Fundamental understanding of electrode potentials

Question

The electrical double layer, is commonly used to describe the origins of electric potentials that is fundamental to electrochemistry. This model is quite outdated from my understanding, but it is still used inside my textbook.

Right now, my understanding is that when a metal rod is placed within a solution of its respective ions, some of those metals atoms change to metal ions and go into the solution leaving a negative charge at the electrode surface. Hence, the metal cations inside the solution move towards the electrode surface. At this electrolyte - electrode interface there is now a seperation of charge, that gives rise to the phenomenon of electric potential. At least this how my cambridge textbook explains it.

Some answers on stackechange reinforce the theory as stated above. However, my problem lies in understanding why a negative layer of surface charge builds up in the first place. How can metal atoms just convert to metal cations and move into the solution? This doesn't make sense to me. The surface of the metal rod comes into contact with the electrolyte solution, so what is causing the surface to gain this initial negative charge?

The rest seems to make sense, but this part specifically doesn't.

Answer 1

Consider an electrode immersed into an electrolyte, two things can occur.

1. Metal ion $M^{n+}$ from the solution may collide with the electrode, gaining $n$ electrons from it, and convert to metal atoms. This means that the ions are reduced.
2. Metal atom on the surface may lose $n$ electrons to the electrode and enter the solution as the ion $M^{n+}$. This means that the metal atoms are oxidized

Notice that for different elements, the redox equilibrium established is different, either further to the right or further to the left. Hence, depending on the electrode, one of the processes above is going to be more favourable then the other, creating either a positive or negative surface charge.

Answer 2

Negative charge is kept near surface by attraction of excess positive ions in solution. The reason for ions going into solution and electrons staying in metal is electron's De broglie wavelength strongly depending on environment. Creating a bubble larger than intermolecular distance in a liquid made of polar molecules needs energy. That does not happen in the situation stated in the question.

For a nice example of a bubble filled with electron, see https://en.wikipedia.org/wiki/Electron_bubble

There are counterexamples, where both cations and electrons go into solution, such as alkali metals in liquid ammonia. They can do this because in these cases surface electron affinities of the alkali metals are lower.