Why is a bulb containing magnesium is filled with noble gas instead of air?

It is filled with Argon because air will break down the magnesium. Magnesium has two more electrons than it wants to (all atoms want stability). Noble gases all have the perfect number of valence electrons, so are the most stable elements. The argon in the bulb keeps the magnesium from breaking down because the argon has no need for new electrons. Where oxygen, on the other hand, is not a stable element (it wants two more). Argon will reject any electrons from the magnesium so it will just keep its electrons, therefore not breaking down (as quickly).