Which gas is commonly filled in electric bulbs to prolong the life of the filament?

Argon is an inert gas that does not react with the hot tungsten filament, reducing its oxidation and evaporation. Oxygen would burn the filament, hydrogen is flammable and chlorine is reactive and corrosive. Nitrogen is sometimes mixed with argon, but pure argon is the standard textbook answer. Argon is cheap and abundant, being nearly one per cent of air, which is why it is preferred to other inert gases.