What happens on an atomic level when electricity arcs between two contacts?

Question

What actually is blackening and putting of metal

Answer 1

electricity arcs due to the attraction of opposite charges.

  Arcs can happen in a vacuum.

  If the arc happens in air (where there is oxygen) then a small amount of the metal and some oxygen react forming some corrosion which is the blackening of the contacts ...  but I have never heard of putting ... do you mean pitting?  some metal (if the voltage is high can vaporize some of the atoms of the contacts leaving small holes (pits) in the surface of the metal.

Answer 2

Well...

(guessing) I think usually

- there is OXIDATION of the metal

- there is CRYSTALLIZATION of the metal

As you suggest, there is a little "arc" every time the contacts of the switch get close enough together.

If it's hot enough, the metal will oxidize (i.e. the metal will actually burn...a little bit).

Discoloration of the metal can also be caused by the heat crystallizing the metal (which is normally what you're seeing when the metal turns purplish or blue).