Final answer:
The species undergoing oxidation in the reaction Fe(s) + O2(g) → Fe2O3(s) is Fe, as it loses electrons and its oxidation state increases from 0 to +3.
Step-by-step explanation:
In the reaction Fe(s) + O2(g) → Fe2O3(s), the species undergoing oxidation is Fe (Iron). During the process of forming Fe2O3, or iron(III) oxide, the iron atoms lose electrons and their oxidation state changes from 0 to +3. This increase in oxidation state signifies that oxidation is occurring. Oxygen, on the other hand, is being reduced as its oxidation state changes from 0 in O2 to -2 in Fe2O3, making oxygen the species that is undergoing reduction.
Oxidation and reduction are complimentary processes; they occur simultaneously in what is known as a redox (reduction-oxidation) reaction. The iron starts out in its elemental form, Fe(s), with an oxidation number of 0 and becomes oxidized to Fe3+ in Fe2O3(s), which is an increase in its oxidation state.