When the atom has 8 valence electrons.
Nucleous
4
isotope
to become stable, it will gain an electron from a metallic atom otherwise it will mutually share electron(s) with another non - metallic atom, or even with itself
It can do either, depending on it's state previous to losing neutrons. Generally if an atom is shedding neutrons it is doing so to become more stable.
The only way a carbon atom becomes stable is if they gain or lose electrons. Typically carbon will bond with other elements to do this.
It becomes most stable when its nucleus is filled, not when it is filling it.
2
To become stable. When the outermost energy field is filled to maximum electrons, the atom is stable.
the oxidation number
If it is not stable, then the atom will likely either emit alpha radiation or beta radiation in order to become more stable.
Generally by losing the one electron