In a chemistry reaction, we begin with: 2 Mg + O2. A moderate amount of energy is gained and it becomes 2 Mg + 2 O. Then a large amount of energy is lost, resulting in MgO + MgO.

To analyze the reaction you've described, let's break it down step by step.

1. Initial Reaction: You start with 2 Magnesium (Mg) atoms and 1 molecule of Oxygen (O2).
2. Intermediate Step: The reaction you mention gaining a moderate amount of energy to yield 2 Mg + 2 O suggests that some energy is being absorbed to break the O2 molecule into two individual oxygen atoms. This step is endothermic because it requires energy to overcome the bond between the oxygen atoms in O2.
3. Final Step: The subsequent reaction where a large amount of energy is lost results in the formation of 2 MgO (magnesium oxide). The formation of MgO from Mg and O (2 Mg + O → 2 MgO) is an exothermic reaction, meaning it releases energy.

To conclude, the overall process involves both an endothermic step (the breaking of O2) and an exothermic step (the formation of MgO). However, since the final step, which produces MgO, releases a significant amount of energy, the overall reaction process is typically considered exothermic in nature due to the stability and lower energy of the MgO products compared to the reactants.

Thus, the formation of magnesium oxide (MgO) from magnesium and oxygen is indeed an exothermic reaction, despite the intermediate step being endothermic.