* Argon is an inert gas: Argon belongs to Group 18 of the periodic table, also known as the noble gases. These elements are very unreactive due to their full outer electron shells. They are very stable and don't readily form bonds with other elements.
* Magnesium is a reactive metal: Magnesium is a highly reactive metal. It readily reacts with nonmetals like oxygen to form magnesium oxide, which is a stable compound. However, it doesn't react with inert gases like argon.
In essence: Since argon is inert and does not readily form bonds, combining it with magnesium would not result in a chemical reaction. Instead, you would simply have a mixture of magnesium and argon, with no new substance being created.
