When titanium chloride reacts with sodium or magnesium, an argon atmosphere is used to prevent the reaction from coming into contact with oxygen or moisture. This is because the reaction is highly exothermic and can easily catch fire if exposed to air. Additionally, the presence of oxygen or moisture can lead to the formation of unwanted byproducts, such as titanium oxide or magnesium hydroxide. By using an argon atmosphere, the reaction can be carried out in a controlled environment, preventing these unwanted side reactions from occurring.