[vmelkon]

I was wondering what exactly is meant by a reducing atmosphere
http://en.wikipedia.org/wiki/Reducing_atmosphere

If you have a atmosphere of N2, H2, NH3, CO and CO2 and perhaps some H2S and SO2, then already, some of those molecules have atoms in some oxidation state (NH3, CO, CO2, H2S and SO2).

It seems to refer to O2 being an oxidizer. Is this because of the electronegativity of oxygen  is high?

In case anyone is wondering,  "reducing atmosphere" comes from the Miller–Urey experiment.

I imagine that if you had SO2 then fluorine would be an oxidizer in this case. You would end up with SF6 and OF2 perhaps.

[Schrödinger]

Fluorine is the best oxidizing agent, even better than oxygen, after which the term was coined O2 being as oxidizer because the oxidation state of O here is 0, but the most stable/preferred state is -2. Hence, O2 -> O2- is spontaneous in situations where possible (look up electrochemical series for more on what reduces/oxidizes what in different situations)

[vmelkon]

By electrochemical, you mean the table of half reactions right and emf values?

But they call it a reducing atmosphere (Miller–Urey experiment) in the sense that no O2 is present I guess. If there was any O2 at all, it would have reacted with H2 to make H2O, it would have reacted with CH4 and also NH3. The high excess of H2, CO, CH4, NH3 would remain in the atmosphere.

Even in present day, I read that CH4 in the atmosphere doesn't remain for too long since it oxidizes because of UV light.