It should be noted that a variation of charge carriers occurs in the conduction process between the metal electrodes and the gas interface. While in the metal the electric current flow is given by electrons, in gas the charge carriers are its ions.
The discharge voltage, ie the voltage to which the conduction is triggered by spark, depends on distance between the electrodes (the greater the distance, the greater the voltage required to trigger the conduction), on pressure, on temperature and on gas type. Experimentally, it is demonstrated that the dependence of the discharge voltage passes for a minimum value at a certain pressure and then rises to very high values for pressures tending to zero ie to approaching the absolute vacuum. Absolute vacuum condition is an extreme situation where the current passage between two metal electrodes can occur only if electrons disengage from the electrode with negative polarity and travel to the other electrode with positive polarity. It should be noted that what is written is true only if the phenomenon of thermoelectric emission is absent or negligible.
Hydrogen is a chemical element that under normal conditions is a biatomic molecule. In the molecule, two hydrogen atoms are joined together in a stable structure composed of two protons and two electrons. The bond that joins the two atoms may break under certain conditions, resulting in the formation of two identical hydrogen atoms made of a single proton and a single electron. This type of structure, that is, the hydrogen atom, is the simplest atomic structure existing in our universe.
Now, if an hydrogen atom acquires an electron, it turns into a H– ion, an entity that, while not being neutral, is still atomic in the sense that it is constituted by a positive nucleus surrounded by a negative electronic cloud. Speaking of size, although it increase, it changes slightly.
Instead, if an hydrogen atom loses an electron transforming into an H+ ion and it becomes an atom without electronic cloud (it becomes a nucleus which in this case is a proton) and therefore no longer belongs to the entities of atomic type and its size collapses in several orders of magnitude (about 30,000 times smaller).