[qfruit]

Why in a redox equation, when you're identifying which chemical species is reduced, a way to tell is by seeing which lost its oxygen?

I know oxidation is loss of electrons, gain of oxygen & loss of hydrogen, while
reduction is gain of electrons, loss of oxygen & gain of hydrogen. I don't understand the part about the oxygens and hydrogens.

e.g. 2MnO4^-  -->  2Mn^2+

Isn't this losing electrons? The answer says that it's gaining.

I have a feeling it's very obvious and that I will feel dumb when I find out the answer...
Thanks in advance.

[Borek]

I know oxidation is loss of electrons, gain of oxygen & loss of hydrogen, while
reduction is gain of electrons, loss of oxygen & gain of hydrogen. I don't understand the part about the oxygens and hydrogens.

And don't bother with this part. Gain or loss of electrons is what counts.

Hydrogen and oxygen will be in most cases get or lost the way you described but it is possible to show counterexamples, so these are hardly rules describing redox reactions.

e.g. 2MnO4^-  -->  2Mn^2+

Isn't this losing electrons? The answer says that it's gaining.

You have to look at whole halfreaction:

MnO₄^- + 8H⁺ + 5e^- -> Mn²⁺ + 4H₂O

Electrons must be added on the left side of the reaction to balance charge. (When balancing reaction you have to balance not only atoms, but also charge - just like atoms can not appear from nowhere, charge can't appear nor disappear).

And in halfreaction it is obvious where do the electrons go.