Q.Using the classical (pre-electronic) idea of oxidation and reduction, explain why the reaction $2\text{Mg}(s) + \text{O}_2(g) \to 2\text{MgO}(s)$ is a redox reaction. Identify which species is oxidized and which is reduced.

[!TLDR] Classically, oxidation = gain of oxygen, reduction = loss of oxygen; in $2\text{Mg}+\text{O}_2\to 2\text{MgO}$, magnesium gains oxygen (oxidized) while the reaction as a whole is a redox process since one species combines with the oxidizing element and one species supplies it. [!ANSWER] Magnesium is oxidized (it combines with oxygen); $\text{O}_2$ is the oxidizing agent that is itself reduced in the process.

By the classical definition, oxidation is the addition of oxygen to a substance and reduction is the removal of oxygen from a substance (or, equivalently for hydrogen-based redox, addition/removal of hydrogen). In $2\text{Mg}(s) + \text{O}_2(g) \to 2\text{MgO}(s)$, magnesium starts as the free, uncombined element and ends up chemically combined with oxygen in magnesium oxide — this is exactly the classical picture of oxidation, so magnesium is oxidized. The oxygen supplied by $\text{O}_2$ is the species being consumed to bring about this oxidation; it is therefore the oxidizing agent. Since oxidation and reduction must occur together in any redox reaction, and the only two species present are $\text{Mg}$ and $\text{O}_2$, this combination reaction is classified as a redox reaction, with magnesium oxidized and oxygen serving as (and, in the modern electronic sense, itself being reduced by accepting electrons from magnesium to form the $\text{O}^{2-}$ ions present in $\text{MgO}$) the oxidizing agent. [!ANSWER] Magnesium is oxidized (it combines with oxygen); $\text{O}_2$ is the oxidizing agent that is itself reduced in the process.

Apply the classical rule (oxidation = addition of oxygen) directly to identify which reactant gains oxygen in the product.

Thinking a combination reaction with only two reactants cannot be redox because 'nothing is removed'; forgetting that the element supplying oxygen is itself called the oxidizing agent.