The gas used in light bulbs that does not react at all is argon (Ar).

Here's why:

* Inertness: Argon is a noble gas, meaning it has a full outer shell of electrons. This makes it extremely unreactive and stable.

* Preventing Oxidation: In a light bulb, the filament (usually tungsten) gets extremely hot. If oxygen were present, it would react with the tungsten, causing it to burn and break. Argon prevents this oxidation.

* Other gases: While some bulbs might contain a small amount of nitrogen (N2), argon is the primary gas used because of its inertness.

Let me know if you have any other questions!