Final answer:
The reaction 2Mg(s) + O₂(g) → 2MgO(s) is a synthesis reaction because it involves the combination of two reactants to form a new compound, magnesium oxide (MgO).
Step-by-step explanation:
The reaction 2Mg(s) + O₂(g) → 2MgO(s) is classified as a(n) synthesis reaction. This is because it involves the combination of two or more reactants to form a single product. In this case, magnesium (Mg) and oxygen (O₂) combine to form magnesium oxide (MgO), which is the product formed during this reaction. This reaction is not a decomposition reaction because it does not involve breaking a compound into two or more components. Moreover, it isn't a combustion reaction in the strictest sense, which typically involves the burning of a hydrocarbon to form water and carbon dioxide; however, it does release heat and involves oxygen, so it may also be described as a combustion reaction in a broader context.